ArticlePDF Available

Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

June 1999
Monthly Notices of the Royal Astronomical Society 309(3)

June 1999
309(3)

DOI:10.1046/j.1365-8711.1999.02879.x

Source
arXiv

Authors:

Andrew Blain

University of Leicester

Ian Smail

Durham University

Malcolm Longair

University of Cambridge

Show all 6 authorsHide

A large fraction of the luminous distant submm-wave galaxies recently detected using SCUBA on the JCMT appear to be associated with interacting optical counterparts. We investigate these systems using a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a burst of either star formation activity or the fuelling of an active galactic nucleus (AGN). The models are well constrained by the observed spectrum of the FIR/submm background radiation and the 60-micron counts of low-z IRAS galaxies. The ratio between the total amount of energy released during mergers and the mass of dark matter involved must increase sharply with z at z<1, and then decrease at higher z, independent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter - the reciprocal of the product of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter - is introduced, to allow the relationship between merging dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by a factor of about 5 and 100 at redshifts of 1 and 3 respectively, compared with the present epoch. Hence, if mergers account for the SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of galaxies and the intensity of background radiation in the optical/near-IR wavebands in the context of these hierarchical models, and thus investigate the relationship between the populations of SCUBA and Lyman-break galaxies. Comment: 18 pages, 12 figures, in press at MNRAS. Final proof version

The intensity of background radiation in the millimetre, submillimetre and far-infrared wavebands, as deduced by: Puget et al. (1996)– thin solid lines with error bars at the ends; Fixsen et al. (1998)– thin dotted line that ends within the frame; Schlegel et al. (1998)– stars; Hauser et al. (1998) and Dwek et al. (1998)– thick solid crosses. The diagonal crosses represent lower limits to the background intensity inferred from source counts. From left to right: the 850-μm count of Blain et al. (1999b); the 450-μm count of Smail et al. (1997), as updated to include more recent information from Ivison et al. (1999); and the 175- and 95-μm counts of Kawara et al. (1998) and Puget et al. (1999). The background spectrum predicted in the 35-, 40-, 45- and 50-K models (Table 1) are represented by solid, dashed, dot-dashed and dotted lines respectively, and are plotted across the entire abscissa. In a recent paper, Lagache et al. (1999) claim to have detected a warm diffuse Galactic dust component that accounts for about 50 per cent of the isotropic DIRBE signal attributed to the extragalactic background intensity by Hauser et al. (1998) and Schlegel et al. (1998).

…

The redshift distributions of 850-μm galaxies in the Smail et al. (1998) sample predicted in the 35-, 40-, 45- and 50-K hierarchical models listed in Table 1 (thick smooth lines; solid, dashed, dot-dashed and dotted respectively), the Gaussian model (thin smooth solid line; Blain et al. 1999c) and the modified Gaussian model (thin dotted line; Barger et al. 1999b). The modified Gaussian model is fitted to the median redshift of the preliminary redshift distribution determined by Barger et al. (1999b) for the optical counterparts to submillimetre-selected galaxies (Smail et al. 1998), which is shown by the thinner jagged solid line. Sources with no optical counterparts are assumed to be at z=4. Using subsequent K-band observations, Smail et al. (1999a) have modified the identifications of the counterparts of the two galaxies at z<1 in the sample, from low-redshift spirals to high-redshift EROs. After this modification, the revised redshift distribution is shown by the thicker jagged solid line, if the EROs are both assumed to lie at z=3.

…

Figures - uploaded by Ian Smail

Content may be subject to copyright.

Content uploaded by Ian Smail

Content may be subject to copyright.

arXiv:astro-ph/9906311v2 1 Sep 1999

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 1 February 2008 (MN L

X style ﬁle v1.4)

Dust-obscured star formation and AGN fuelling in

hierarchical models of galaxy evolution

A. W. Blain,

1,2

Allon Jameson,

Ian Smail,

M. S. Longair,

J.-P. Kneib

and R. J. Ivison

Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK.

Observatoire Midi-Pyr´en´ees, 14 Avenue E. Belin, 31400 Toulouse, France.

Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK.

Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK.

1 February 2008

ABSTRACT

A large fraction of the luminous distant submillimetre-wave galaxies recently detected

using the Submillimetre Common- Use r Bolometer Array (SCUBA) camera on the

James Clerk Ma xwell Telesc ope appear to be associated with interacting optical coun-

terparts. We investigate the nature of these systems using a simple hierarchical clus-

tering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies

is assumed to be gener ated at the epoch of galaxy mergers in a burst of either star

formation activity or the fuelling of an active galactic nucleus (AGN). The models

are well constrained by the observed spectrum of the far- infrared/submillimetre-wave

background radiation and the 60-µm counts o f low-redshift IRAS galaxies. The ratio

between the total amount of energy released during mergers and the mass of dark

matter involved must increase sharply with redshift z at z

∼

1, and then decrease

at greater redshifts. This result is independent of the fraction of the luminosity of

mergers that is produced by starbursts and AGN. One additional parameter – the

reciprocal of the product of the duration of the enhanced luminosity produced by the

merger and the fraction of merger s tha t induce an enhanced luminosity, which we call

the activity parameter – is introduced, to allow the relationship between merging dark

matter haloes and the observed counts of distant dusty galaxies to be investigated. The

observed counts can only be reproduced if the activity parameter is greater by a factor

of about 5 and 100 at redshifts of 1 and 3 respectively, as compared with the present

epoch. Hence, if merging galaxies account for the population of SCUBA galaxies, then

the merger process must have been much more viole nt at high redshifts. We discuss

the counts of gala xies and the intensity of background radiation in the optical/near-

infrared wavebands in the context of these hierarchical models, and thus investigate

the relationship between the populations of submillimetre-selected and Lyman-break

galaxies.

Key words: galaxies: evolution – galaxies: formation – cosmology: observations –

cosmology: theory – diﬀuse r adiation – infrared: galaxies

1 INTRODUCTION

The history of star formation in dusty galaxies was recently

discussed by Blain et al. (1999c), who assumed that the dis-

tant galaxies recently detected using the 450/850-µm Su b-

millimetre Common-User Bolometer Array (SCUBA) cam-

era (Holland et al. 1999) were the high-redshift counterparts

of local ultraluminous IRAS galaxies. The global star for-

mation rate (SFR) in dust obscured galaxies was inferred to

be signiﬁcantly greater than that of optically selected high-

redshift galaxies (Steidel et al. 1996a,b, 1999), subject to the

uncertain fraction of the luminosity of the submillimetre-

selected samples of galaxies ( Smail, Ivison & Blain 1997;

Barger et al. 1998; Hughes et al. 1998; Barger et al. 1999a;

Blain et al. 1999b; Eales et al. 1999) that is produced by

accretion processes in active galactic nuclei (AGN). A frac-

 0000 RAS

2 A. W. Blain et al.

tion of at most 30 per cent, and more likely 10–20 per cent,

is suggested by both follow-up observations ( Frayer et al.

1998; Ivison et al. 1998; Smail et al. 1998; Barger et al.

1999b; Frayer et al. 1999; Lilly et al. 1999), and informa-

tion derived in other wavebands; see section 5.4 of Blain et

al. (1999c), Almaini, Lawrence & Boyle (1999) and Gunn

& Shanks (1999). Using a diﬀerent app roach, in which the

high-redshift SCUBA population is decoupled from the lo-

cal infrared-luminous galaxies, Trentham, Blain & Goldader

(1999) were able to reconcile the SCUBA counts with a less

dramatic amount of obscured star-formation activity. Use

another empirical approach, Tan, Silk & Balland (1999) de-

rived results somewhere between the two. A summary of the

existing data on the history of star formation is presented

in Fig. 1.

Although well constrained, and in accord with the avail-

able observational data, the models in Blain et al. (1999c)

and Trentham et al. (1999) included few details of the phys-

ical origin of the large luminosity of SCUBA galaxies. Semi-

analytic models of hierarchical galaxy formation, in which

galaxies assemble by the merger of progressively larger sub-

units (Cole et al. 1994; Baugh et al. 1998; Kauﬀmann &

Charlot 1998; Somerville, Primack & Faber 1999) have been

used to account for a wide range of observations in the

optical and near-infrared wavebands, and have been ex-

tended into the far-infrared and submillimetre wavebands

by Gu iderdoni et al. (1998). These models involve a large

number of free parameters, and the interplay between them

can make it diﬃcult to identify the most important physics

responsible for a p articular observation. In this paper we de-

velop a model of infrared-luminous galaxies in a simple ver-

sion of such a scenario (Blain & Longair 1993a,b; Jameson,

Longair & Blain 1999), which includes many fewer parame-

ters and hopefully makes the astrophysics more transparent.

We attempt to reproduce the SCUBA counts by invoking

bright dust-enshrouded bursts of either star formation ac-

tivity or AGN fuelling at the epochs of mergers.

Motivation for considering the SCUBA galaxies as lumi-

nous mergers is provided by both the optical identiﬁcations

of the Smail et al. (1998) sample, which appear to contain a

large fraction of interacting galaxies, and th e gas consump-

tion rate that is inferred from observations of CO emission of

two submillimetre-selected galaxies made using the Owens

Valley Millimeter Array (Frayer et al. 1998, 1999), which

cannot be sustained for more than a few 10

yr. Even the

faint and compact counterparts listed in Smail et al. (1998)

could be merging galaxies, b ut too faint to identify as such;

see th e simulations of the appearance of high-redshift merg-

ers in Bekki, Shioya & Tanaka (1999). If the SCUBA galaxies

are the high-redshift counterparts of the low-redshift ultra-

luminous infrared galaxies, which are predominantly merg-

ing systems, then this also oﬀers support for modeling the

SCUBA galaxies as mergers. Using our simple model, we

emphasise the most important features and the underlying

physics of the evolution of submillimetre-selected galaxies

and their relationship to the population of quiescent galax-

ies.

In Section 2 we describe the details of the model, and

investigate the constraints imposed by the intensity of the

far-infrared and submillimetre-wave background radiation

and the counts of low-redshift IRAS galaxies. We discuss

the evolution of the luminosity density in the model and

compare the models with observations in the same way as

the evolving IRAS luminosity function models discussed by

Blain et al. (1999c). In Section 3 we discuss the predic-

tions in the context of source counts in the submillimetre

and far-infrared wavebands, and investigate whether the

SCUBA galaxies can easily be explained in an hierarchi-

cal picture of galaxy formation and evolution. In Section

4 the corresponding background radiation intensities and

galaxy counts in the near-infrared and optical wavebands

are discussed. In Section 5 we review the parameters we

have introduced to describe the models. We present our

main conclusions in Section 6. A value of Hubble’s constant

= 100h km s

−1

Mpc

−1

, with h = 0.5, a density parameter

Ω

= 1 and a cosmological constant Ω

= 0 are assumed.

2 AN ANALYTIC HIERARCHICAL PICTURE

The evolution of galaxy-scale structures under gravity ac-

cording to hierarchical clustering models can be analysed

using the Press–Schechter formalism (Press & Schechter

1974), which describes the time-d epen dent mass spectrum of

bound objects. The analytic results of the Press–Schechter

formalism are in quite acceptable agreement with those of

N-body simulations (Brainerd & Villumsen 1992). The for-

malism can be extended to yield a very straightforward semi-

analytic merger rate, under the single assumption that the

process of halo mergers is independent of mass (Blain &

Longair 1993a,b).

2.1 The Press–Schechter Formalism

According to the Press–Schechter prescription, the mass

spectrum of bound objects with masses between M and

M + dM is

(M, z) =

¯ρ

√



∗



γ/2

exp

−



∗



, (1)

in which ¯ρ is the smoothed comoving density of the Uni-

verse, d ominated by dark matter, γ = (3 + n)/3, where n

is the power-law index of primordial density ﬂuctuations,

and M

∗

(z) is a parameter which d escribes the evolution of

density ﬂuctuations as a function of redshift z:

∗

(z) = M

∗

(0)



δ(z)

δ(0)



2/γ

. (2)

δ(z) is the function describing the growth of perturbations

in a general cosmology, and is derived from the equation,

δ + 2

δ −

4πG¯ρ

δ = 0, (3)

in which R is the scalefactor of the Universe. In an Ein-

stein de-Sitter model, t he growing mode has δ ∝ (1 + z)

−1

∗

(0) is the typical mass of bound objects at z = 0. In-

homogeneities do not grow if γ ≤ 0, that is if n ≤ −3.

For scale-invariant density ﬂuctuations, n = 1 or γ = 4/3.

This is close to the value observed on the largest scales from

the cosmic microwave background radiation (CMBR). Ob-

servations of large-scale structure indicate that n ≃ −1.5,

or γ ≃ 1/2, on t he smallest scales (Peacock & Dodd s 1994),

which can be associated with the transfer function between

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 3

Figure 1. The SFR as a function of redshift, τ

SFR

(z), as inferred from ultraviolet/optical/near-infrared observations, in order

of increasing redshift, by Gallego et al. (1996; ﬁl led triangle), Gronwall (1999; thin diagonal cross), Treyer et al. (1998; open

triangle), Tresse & Maddox (1998; empty circle), Lilly et al. (1996; ﬁlled stars), C owie, Songaila & Barger (1999; small ﬁlled

circles); Glazebrook et al. (1999; bent square at z = 0.9), Connolly et al. (1997; ﬁlled squares), Yan et al. (1999; e mpty lozenge),

Madau et al. (1996; large ﬁlled circles), and Pettini et al. (1998a,b; empty squares). Flores et al. (1999; empty stars) and Pettini

et al. (1998a,b) have corrected the Lilly et al. and Madau et al. results respectively for observed dust extinction. The high-redshift

points are derived from analyses of the Hubble Deep Field (HDF). A recent ground-based survey of a wider area by Steidel et al.

(1999) increased the estimated high-redshift SFR, as shown by the thick diagonal crosses. No extinction correction is applied to

these points; Steidel et al. (1999) estimate that the extinction-corrected SFR is greater by a factor of about 5. Other analyses of the

HDF data by Sawicki, Lin & Yee (1997) and Pascarelle, Lanzetta & Fern´andez-Soto (1998) yielded s imilar results. Cowie, Songaila

& Barger (1999) have argued recently that the low-redshift SFR increases more gradually, as (1 + z)

with γ ≃ 1.5, in contrast

with the usual γ ≃ 4 (Lilly et al. 1996). At longer wavelengths, SFRs have been derived from mid-infrared observations by Flores

et al. (1999) and from submillimetre-wave observations by Blain et al. (1999c; lines) and Hughes et al. (1998; empty circle wi th

upward pointing arrow). Cram (1998) investigated the star-formation history at z

∼

1 using radio observations, deriving a rate

of 2.5 × 10

−2

⊙

−1

Mpc

−3

at z = 0, which increases by an order of magnitude at z ≃ 1. The submillimetre-based Gaussian

and modiﬁed Gaussian SFR models (see Blain et al. 1999c and Barger et al. 1999b) ar e represented by the solid and dotted lines

resp ectively. A better ﬁt to the prelim inary redshift distribution of submillimetre-selected galaxies (Barger et al. 1999b) is provided

by the modiﬁed model, in which the parameters H, σ and z

(Blain et al. 1999c) take the values 70, 1 Gyr and 1.7 respectively, as

compared with 95, 0.8 Gyr and 2.1 in the original model. The total amount of star formation/AGN fuelli ng activity taking place in

the modiﬁed Gaussian model is 90 per cent of that in the original Gaussian model.

a primordial n = 1 spectrum and the spectrum after recom-

bination.

Using the Tully–Fisher relation (Hudson et al. 1998),

Blain, M¨oller & Maller (1999) obtained a value M

∗

(0) =

3.6 ×10

⊙

. The ex act value of M

∗

(0) is not very impor-

tant here, as a mass-to-light ratio is introduced to convert

the mass spectrum into a luminosity function.

2.2 Deriving a merger rate

Working from the mass spectrum N

(M, z) , Blain & Lon-

gair (1993a,b) showed that a formation rate of bound objects

form

(M, z) in galaxy halo mergers can be constructed if the

mass distribution of the components involved in a statisti-

cal sample of merger events is assumed to b e independent

of mass. In this case, the merger rate can be represented

accurately by the fun ction

form

+ φ

∗

exp

(1 − α)



∗



, (4)

where

= γ

∗

h

∗



−

. (5)

φ and α are numerical constants, typically about 1.7 and 1.4;

their exact values depend on the assumed mass distribution

of merging components (Blain & Longair 1993a), but have

little eﬀect on the results. The values of both φ and α are

weak functions of γ and depend on the world model param-

eters, but the form φ/

√

α t hat appears in the calculations

of the background radiation intensity and metal abundance

is almost ind ependent of the value of γ.

 0000 RAS, MNRAS 000, 000–000

4 A. W. Blain et al.

2.3 Deriving observable quantities

The merger rate as a function of mass

form

can b e readily

used to estimate a number of observable quantities, starting

with the luminosity density (or volume emissivity),

(z) = 0.007c

x(z)

1 − f

form

dM, (6)

in which x(z) is the ratio of the mass of baryonic matter con-

verted into metals by nucleosynthesis in a merger-induced

starburst to the total dark mass involved in the merger.

The rationale behind t his form of relation is given by Lon-

gair (1998). The factor of 0.007 is the approximate eﬃciency

of conversion of mass into energy in stellar nucleosynt hesis.

The parameter f

< 1 describes the fraction of the total lu-

minosity of merging galaxies that is attributable to accretion

in AGN, and is expected to lie in the range 0.1 ≤ f

≤ 0.3

(Genzel et al. 1998; Lutz et al. 1998; Almaini et al. 1999;

Barger et al. 1999b; Gunn & Shanks 1999). The parameter

x(z) is expected to vary with redshift z. Blain & Longair

(1993b) predicted a ﬂat background spectrum in the sub-

millimetre and far-infrared wavebands, assuming a constant

value of x. Subsequent observations (e.g. Fixsen et al. 1998)

demand a redshift-dependent form of x(z), as discussed by

Blain et al. (1999c).

By evaluating the integral in equation (6), the luminos-

ity density can be expressed as

(z) = 0.007c

x(z)

1 − f

¯ρ

√

∗

. (7)

Interestingly,

∗

δ(z)

, (8)

and so, b ecause the density contrast δ(z) is not a function of

the perturbation spectral index γ, the γ dependence in this

term is just a simple scaling. Thus the eﬀect of the value

of γ on the background radiation intensity can be studied

or removed very easily. In an Einstein–de Sitter model t he

luminosity density ǫ

∝ x(z)(1 + z)

3/2

The comoving density of metals produced in starbursts

between a redshift z

, at which star formation activity be-

gins, and z, is

(z) = ¯ρ

√

x(z)

(1 + z)

∗

dz, (9)

where r is the radial comoving distance coordinate. N ote

that t his result depends on the merger eﬃciency parame-

ter x but not on the AGN fraction f

, as metals are only

generated in merger-induced starbursts and not in AGN fu-

elling events. Similarly, the background radiation intensity

per unit solid angle emitted by these galaxies, which have a

spectral energy distribution (SED) f

, is

4π

(z)

1 + z

ν(1+z)

′

dν

′

dz. (10)

If the form of ǫ

(equation 7) is included explicitly, then

0.007c

¯ρ

4π(1 − f

)

√

x(z)

1 + z

∗

ν(1+z)

′

dν

′

dz. (11)

Details of the assumed dust SED can be found in Blain et

al. (1999c). The mid-infrared SED is assumed to take the

form f

∝ ν

−1.7

None of the quantities listed above are aﬀected by the

time dependence of the release of energy during merging

events; however, the source count requires the time proﬁle

of the merger induced starbu rst/AGN to be included. For

simplicity, this proﬁle is assumed to have a top-hat form with

duration σ. The time proﬁle of the luminosity generated in a

detailed simulation of the merger of gas-rich galaxies is dis-

cussed by Mihos & Hernquist (1996), Bekki et al. (1999) and

Mihos (1999). The typical duration of AGN fuelling events

and starbursts may diﬀer; for example, a lower limit to the

duration of a starburst is set by the lifetime of the highest

mass stars, but there is no lower limit to the duration of

an AGN fuelling event. However, to avoid introdu cing an

unnecessarily complicated model, the time-scale of a merger

induced luminous phase is assumed to be independ ent of

its origin. In addition, because not all the mergers of dark

matter haloes that take place at each epoch need induce

a starburst/AGN, a fraction F ≤ 1 is assumed. Again, this

fraction could diﬀer for starbursts and AGN, but for simplic-

ity it is assumed not to. The luminosity of a typical merger

induced starburst/AGN of mass M is thus

L(M, z) = 0.007c

x(z)

1 − f

F σ

M. (12)

The source count N of galaxies per unit solid angle brighter

than a ﬂux density S

N(S

) =

∞

min

F σ

form

(M, z) dM D

(z)

dz, (13)

where D(z) is the comoving distance parameter. The mini-

mum mass merger visible at a ﬂux density S

and redshift

z is

min

4πD

(1 + z)S

0.007c

F σ

x(z)

(1 − f

)

′

dν

′

ν(1+z)

. (14)

The time-scale and bursting fraction parameters, σ and F ,

always appear together in calculations, and thus the sin-

gle parameter F σ is constrained by observations. We de-

ﬁne (F σ)

−1

to be the activity parameter, which is large in

violent starbursts/AGN and free to vary as a function of

redshift. The rate of energy release within each individual

starburst/AGN is controlled by the value of the activity pa-

rameter. Within a representative cosmological volume the

presence of a population of either rare long-lived or com-

mon short-lived starbursts/AGN cannot be distinguished.

This is why the time-scale and bursting fraction parameters

σ and F are bound together in the activity parameter.

Incorporating redshift evolution of the activity param-

eter introduces another degree of freedom into the count

model, in addition to that provided by the redshift evolu-

tion of t he star formation/AGN fuelling eﬃciency parameter

x. Of course, x, F and the time-scale σ are also free to vary

as a function of mass. At present, we ﬁnd no compelling

reason to incorporate this additional complication into the

model.

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 5

(a) (b) (c)

Figure 2. The results of ﬁtting (a) the background radiation s pectrum, (b) the shape of the IRAS 60-µm counts (see Fig. 4a for

references to the data) and (c) both sets of data. The contours are dr awn 1, 2, 3 and 5σ away from the peak probability, w hich is

marked by a cross. This example is calculated for T

= 45 K.

Table 1. Values of the parameters p and z

max

required in the

expression for the merger eﬃciency x(z) (equation 16) to ﬁt the

submillimetre/far-infrared background radiation spectrum and

the low-redshift 60-µm IRAS galaxy counts as a function of as-

sumed dust temperature T

. The corresponding values of x

, F σ

and Ω

are also listed. F ≤ 1. The star formation histories asso-

ciated with these models are shown i n Fig. 7. Note that the most

intense star formation activity takes place at a redshift z ≃ 5z

max

/K p z

max

/γ(1 − f

) (F σ/γ)/Gyr Ω

/10

−3

(at z = 0) ×(1 − f

)

35 4.4 0.44 4.32 × 10

−5

0.040 1.2

40 4.0 0.55 4.08 × 10

−5

0.041 1.3

45 3.3 0.73 4.68 × 10

−5

0.063 1.4

50 2.7 0.96 6.36 × 10

−5

0.141 1.7

2.4 Constraining the parameters

The background radiation intensities and source counts cal-

culated from the equations derived above depend on a range

of parameters: the world model, deﬁned by H

, Ω

and Ω

;

the perturbation spectral index n and the value of M

∗

(0);

the constants φ and α in the merger rate; the merger eﬃ-

ciency x(z); the AGN fraction f

; the fraction of mergers

that lead to a starburst/AGN F ; their duration σ; and their

SED f

Blain et al. (1999c) used the low-redshift 60-µm IRAS

source count and the 175-µm ISO counts to constrain their

models; here, however, we use the bright 60-µm counts and

the form of the far-infrared/submillimetre background spec-

trum, the two best determined observables, to constrain the

parameters that describe the merger eﬃciency x(z). If the

SED does not d epend on the mass of the merging galaxies,

then the background spectrum (equation 10) is determined

entirely by the form of the merger eﬃciency x(z),

∝

√

x(z)

1 − f

δ(z)

1 + z

ν(1+z)

′

dν

′

dz. (15)

Note that the dependence of I

on the perturbation index

n through γ is completely separate from t he dependence on

the world model. Thus the background spectrum determined

by Puget et al. (1996), Guiderdoni et al. (1997), Dwek et al.

(1998), Fixsen et al. (1998), Hauser et al. (1998), Schlegel,

Finkbeiner & Davis (1998) and Lagache et al. (1999) can

always be used to constrain the form of the merger eﬃciency

x(z). We adopt a form of x(z) identical to the ‘peak model’

described in Blain et al. (1999c):

⋆

x(z) = 2x

1 + exp

max

−1

(1 + z)

p+(2z

max

)

−1

. (16)

This is not a uniquely appropriate functional form of x(z).

It was originally chosen to allow the star-formation history

derived by Madau et al. (1996) to be ﬁtted. Its three param-

eters can be manipulated to produce a wide range of plausi-

ble star formation histories. The three parameters are: p, th e

asymptotic low-redshift slope of the merger eﬃciency x(z) in

(1+z); z

max

, the redshift above which the high-redshift expo-

nential cut-oﬀ starts to take eﬀect; and x

the value of x(0).

The epoch of most intense star-formation/AGN-fuelling cor-

respond s to a redshift z ≃ 5z

max

in these models.

In Figs 2(a) and (b ) the probabilities of ﬁtting the

background radiation spectrum and the slope of the 60-µm

counts predicted from the merger eﬃciency x(z), deﬁned in

equation (16), to observations are shown as a function of

the key parameters p and z

max

, as an example for a dust

temperature of 45 K. In Fig. 2(c) the joint probability of ﬁt-

ting both sets of data is shown. Note that a constant dust

temperature is assumed. The value of f

does not aﬀect the

results. If diﬀerent dust temperatures are assumed, then the

position of maximum probability moves around the p–z

max

plane. However, when the form of the evolution of luminos-

ity density is calculated for each temperature, the curve has

a similar form. The best-ﬁtting values of p and z

max

, and the

corresponding values of the merger eﬃciency x

, the activity

parameter (F σ)

−1

and the density parameter in metals at

z = 0, Ω

, are presented in Table 1 for four plausible values

of the dust temperature: 35, 40, 45 and 50 K. Note that the

⋆

Note that the form of this equation published in Blain et al.

(1999c) contained a typographical error in the index of (1 + z).

 0000 RAS, MNRAS 000, 000–000

6 A. W. Blain et al.

Figure 3. The intensity of background radiation in the millime-

tre, submillimetre and far-infrared wavebands, as deduced by:

Puget et al. (1996) – thin s ol id lines with error bars at the ends;

Fixsen et al. (1998) – thin dotted line that ends within the frame;

Schlegel et al. (1998) – stars; Hauser et al. (1998) and Dwek e t al.

(1998) – thick solid crosses. The diagonal crosses represent lower

limits to the background intensity inferred from source counts.

From left to right: the 850-µm count of Blain et al. (1999b); the

450-µm count of Smail et al. (1997), as updated to include more

recent information from Ivison et al. (1999); and the 175- and

95-µm counts of Kawara et al. (1998) and Puget et al. (1999).

The background spectrum predicted in the 35-, 40-, 45- and 50-

K m odels (Table 1) are represented by solid, dashed, dot-dashed

and dotted lines respectively, and are plotted across the entire

abscissa. In a recent paper, Lagache et al. (1999) claim to have

detected a warm diﬀuse Galactic dust component that account s

for about 50 per cent of the isotropic DIRBE signal attributed

to the extragalactic background intensity by Hauser et al. (1998)

and Schlegel et al. (1998).

best ﬁtt ing values of p and z

max

depend only weakly on the

world model assumed.

The physical processes that demand a form of luminos-

ity density which rises steeply with increasing redshift before

turning over have not been considered here in any detail. It

seems likely, however, that the steep decline in the star for-

mation rate to the present day is related to the declining

gas content of galaxies at z

∼

1, and that t he behaviour at

high redshifts could be attributable to relatively ineﬃcient

cooling of gas and thus of star formation in the mergers of

metal-poor high-redshift systems (see Pei & Fall 1995 and

Pei, Fall & Hauser 1999 for discussions of gas and dust evo-

lution in the Universe). We discuss these issues further in

Jameson, Blain & Longair (2000).

The bright low-redshift 60-µm count,

∝

(F σ)

(1 − f

)

−3/2

, (17)

is independent of the cosmological model. At S

= 10 Jy,

= 19 ± 2 sr

−1

(Saunders et al. 1990). A value of

/(F σ)

(1 − f

)

= ( 11 ± 2)γ × 10

−7

Gyr

−1/2

provides

a good ﬁt for any dust temperature between 30 and 60 K.

The values of t he normalization of the luminosity density

/(1 − f

) and the activity parameter (F σ)

−1

at z ≃ 0

that are required to ﬁt the background spectrum and 60-

µm counts depend on the ﬂuctuation index γ as γ

−1

and γ

respectively. Thus the mass-to-light ratio of merging galax-

ies (equation 12) is expected to be independent of the value

of the perturbation index. The values of F σ listed in Table 1

are lower limits to the time-scale of the starburst/AGN σ,

as F ≤ 1. They are generally consistent with the starburst

time-scales of order 5×10

yr derived by Mihos & Hernquist

(1996) from hydrodynamical simulations of galaxy mergers.

The background radiation spectra and 60-µm counts

corresponding to the models listed in Table 1 are shown in

Figs 3 and 4(a) resp ectively. Note that the inclusion of pos-

itive evolution in the eﬃciency parameter x( z) overcomes

the underprediction of th e 60-µm counts in the constant-x

model of Blain & Longair (1996); see their ﬁg. 6. The slope

of the 60-µm counts in the new model also more adequately

represents the data than the predictions of Guiderdoni et

al. (1998). The luminosity function of nearby d usty galaxies

at 60-µm (Saunders et al. 1990; Soifer & Neugebauer 1991)

predicted in the 40-K model, which is calculated at z = 0

by evaluating F σ

form

at the mass corresponding to a lumi-

nosity L in equation (12), is shown in Fig. 4(b). The form of

the luminosity function depends on the value of the pertur-

bation index γ, even though the 60-µm counts are indepen-

dent of γ. The best representation of the observed function

is provided by a value of γ = 2/3, or n ≃ −1. A similar high-

luminosity slope could be achieved by modifying the form

of equation (12) so t hat L ∝ M

, where β > 1. However,

to match the observations with a scale-independent value

of n = 1, a rather extreme value of β = 2 is required. In

the work that follows n is assumed to take the value −1,

similar to the value found for this range of masses by Pea-

cock & Dodds (1994). In this case, the faint-end slop e of

the low-redshift 60-µm luminosity function is equivalent to

a Schechter function parameter α = −5/3 ( Schechter 1976).

This is the same as the faint-end slope α = −1.60 ± 0.13 of

the optically selected luminosity function derived by Steidel

et al. (1999) at z ≃ 3, which describes the most numerous

population of h igh-redshift galaxies that are actively form-

ing stars. At 0.75 < z < 1, the luminosity function of the

blue star-forming galaxies in the CFRS survey also has a

faint end slope α = −1.56 (Lilly et al. 1995). Steep faint-end

slopes with indices of about −1.8 are expected for the lu-

minosity functions of dwarf, irregular and infrared-luminous

galaxies, as discussed by Hogg & Phinney (1997).

2.5 Self-consistency

In section 4 of Blain et al. (1999c) the self-consistency of

models of du st-obscured galaxy formation was discussed.

We demanded that a suﬃciently large mass of metals, and

associated d ust, was required to be generated by nucleosyn-

thesis at each epoch to account for the far-infrared emission

predicted by the model. As the mass of du st required to

generate a given far-infrared luminosity depends strongly

on the dust temperature, this consistency condition is most

easily expressed as a minimum dust temperature at each red-

shift. In the cases of the models listed in Table 1, this lower

limit t o the dust temperature is presented in Fig. 5, both

with and without an assumed high-redshift Population III

to generate dust. If 2 per cent of the total star formation

activity takes place in a high-redshift Population III, then

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 7

(a) (b)

Figure 4. (a) Observed 60-µm counts of IRAS galaxies and the best-ﬁtting models li sted in Table 1, plotted in the format used

by Oliver, Rowan-Robinson & Saunders (1992). The data are taken from Hacking & Houck (1987; crosses), Rowan-Robinson et al.

(1990; empty triangles), Saunders et al. (1990; ﬁlled triangles) and Gregorich et al. (1995; circles): see al so Bertin, Dennefeld &

Moshir (1997). (b) The 60-µm luminosity function predicted in the 40-K m odel, for four diﬀerent values of the primordial ﬂuctuation

index n. The results are compared with the luminosity function of Saunders et al. (1990) and Soifer & Neugebauer (1991). The

luminosity functions derived for the other assumed dust temperatures are very similar.

the self-consistency limits are always satisﬁed. The details

of the calculations can be found in Blain et al. (1999c). It is

assumed that all the dust that is generated prior to a partic-

ular redshift still existed at that redshift and was available

to absorb and reprocess the light from young hot stars and

AGN. However, in an hierarchical model, only a small frac-

tion of all the d ust generated by any epoch is found in galax-

ies actively involved in a merger at th at epoch; t his fraction

increases from about 8 t o 25 per cent progressively from the

35-K to 50-K model listed in Table 1. Thus, because most

of the dust will be found in q uiescent objects, the condition

in Fig. 5 is less severe than it might be. However, even if

only 10 per cent of all dust is involved in a luminous dust

enshrouded merger, then the lower limit to the temperature

increases only by about 50 per cent, and so this consistency

requirement is not diﬃcult to satisfy. The same correction

is required if 90 per cent of the energy generated in merg-

ing galaxies is attributable to accretion onto AGN, that is,

if the AGN fraction f

= 0.9. Hence, the models are read-

ily self-consistent if high-redshift Population-III stars exist

to generate early metals. More sophisticated models of the

coupled evolution of dust, gas and stars have been presented

by Pei & Fall (1995), Eales & Edmunds (1996) and Pei et

al. (1999).

2.6 Metal enrichment and the production of

low-mass stars

At the present epoch the density of metals formed by nu-

cleosynthesis in stars Ω

(0) (equation 9) is expected to be

about 10

−3

in the models presented in Table 1. I f solar metal-

licity, about 2.5 per cent by mass (S avage & Sembach 1996),

is typical of the Universe as a whole, and the density param-

eter in baryons Ω

= 0.019 (Burles & Tytler 1998), then

Ω

≃ 1.9×10

−3

if h = 0.5. Thus all the mo dels listed in Ta-

ble 1 are consistent with this limit, even if the AGN fraction

Figure 5. The lower limits to the dust temperature as a func-

tion of redshift required by the consistency argument discussed

in Section 2.5. The dust temperatures in the 35, 40, 45, and 50-K

models are shown by the hori zontal lines, and the lower limit im-

posed by the CMBR temperature T

CMBR

is shown by the lower

solid curve. In a self-consistent model, the lower limit to the tem-

perature must be less than the plotted temperature limits at all

epochs. The line styles are the same as those in Figs 3 and 4(a).

The four thin lines show the lower limits if no Population III stars

are included. The four thick lines show the l ower limits if 2 per

cent of all star formation activity takes place in a hi gh-redshift

Population III. z

= 20 in all the models.

= 0 and all the luminosity of merging galaxies is due to

star formation activity that generates heavy elements.

The density parameter in the form of stars at the

present epoch Ω

∗

(0) = (5.9 ± 2.3) × 10

−3

(Gnedin & Os-

triker 1992). Observations of L yman-α absorbers along the

line of sight to distant quasars allow the evolution of the

mass of neutral gas and the typical metallicity in the Uni-

verse to be traced as a function of epoch (Storrie-Lombardi,

 0000 RAS, MNRAS 000, 000–000

8 A. W. Blain et al.

(a) (b)

Figure 6. (a) The density parameter of gas and stars as a function of redshift. The line styles represent the same models as

those in Figs. 3, 4(a) and 5. The curves represent the density parameter of material processed into stars in the models listed in

Table 1, assuming a Salpeter IMF with mass lim its of 0.07 and 100 M

⊙

. The data for the total density of stars at the present epoch

(shaded region) was obtained by Gnedin & Ostriker (1992). The data points represent the density parameter in neutral hydrogen

(Storrie-Lombardi et al. 1996). (b) The rate of increase of metallicity expected in the same models, with data from Pettini et al.

(1997).

McMahon & Irwin 1996; Pettini et al. 1997). In Fig. 6(a)

the mass of material that has been processed into stars is

derived as a function of epoch in each of the star-formation

histories listed in Table 1, assuming that t he AGN fraction

= 0 and a Salpeter initial mass function ( IMF) with

a lower mass limit of 0.07 M

⊙

. In this case about 65–70 per

cent of all stars formed are still burning at the present epoch.

The values of Ω

∗

(0) predicted are thus about 3 times larger

than the observed value, but are comparable with the values

derived in our earlier models (Blain et al. 1999c). In order to

account for this diﬀerence, either a lower mass limit to the

IMF of about 1 M

⊙

or a value of the AGN fraction f

≃ 0.75

is required. This high-mass IMF would be compatible with

the inferred lower limit to the IMF of 3 M

⊙

required by Zepf

& Silk (1996) to explain the mass-to-light ratios of elliptical

galaxies, and by Rieke et al. (1993) to interpret observations

of M82. Stars with masses less t han 3 M

⊙

appear to be less

numerous than expected from a Salpeter IMF in recent ob-

servations of the low-redshift starburst galaxy R136 (N ota

et al. 1998). Goldader et al. (1997) report that the results

of near-infrared spectroscopy of nearby IRAS galaxies with

luminosities between 10

and 10

⊙

support a deﬁcit of

stars with masses less than 1 M

⊙

in these systems. More de-

tails about variations in the high-redshift IMF are discussed

by Larson (1998).

Metals appear to be overproduced by about a factor of

5 at redshifts of 2 and 3 in the hierarchical models, as shown

in Fig. 6(b ), but again these results can be reconciled with

the observations if a signiﬁcant fraction of the luminosity

of dusty galaxies is being powered by accretion on to AGN.

Note, however, that the observations of metallicity could

be biased against metal-rich regions of the Universe, either

because of their small physical size (Ferguson, Gallagher &

Wyse 1998) or because of the complete obscuration of a

fraction of background QSOs (Fall & Pei 1993). ASCA X-

ray observations of signiﬁcant enrichment in intracluster gas

(Mushotzky & Loewenstein 1997; Gibson, Loewenstein &

Figure 7. The luminosity density predicted by the 35-, 40-, 45-

and 50-K models (thick solid, dashed, dot-dashed and dotted lines

resp ectively) listed in Table 1. The values are converted into a star

formation rate assuming the same conversion rate as Blain et al.

(1999c). References to the points on the plot are listed in the

caption of Fig. 1. The thin sol id and dotted lines trace the star

formation his tories derived in Blain et al. (1999c) and Barger et

al. (1999b), which were shown in Fig. 1.

Mushotzky 1998), could indicate that this is the case, at

least in high-density environments.

The observed turn-over in the n eutral gas fraction and

the maximum rate of star formation shown in Fig. 6(a) are

approximately coincident in redshift, and the rate of enrich-

ment in the hierarchical models is broadly consistent with

the slope interpolated between the three highest redshift

data points plotted in Fig. 6(b).

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 9

2.7 The history of star-formation/AGN fuelling

In Fig. 7 the form of evolution of the luminosity density is

shown as a function of redshift in the models listed in Ta-

ble 1. All the models predict curves with a rather similar

form at z

∼

2. The transformation between luminosity and

star formation rate is the same as that assumed by Blain

et al. (1999c): that is, a SFR of 1 M

⊙

−1

is equivalent to

a luminosity of 2.2 × 10

⊙

. At low redshifts the evolution

of luminosity density is consistent with optical and near-

infrared observations, and with the results presented in ou r

earlier paper.

3 SOURCE COUNTS

3.1 Fitting the available data

The models presented in Table 1 were constrained using the

properties of the counts of the low-redshift 60-µm IRAS

galaxies. The same formalism can be used to determine

the counts of more distant dusty galaxies in t he mid-/far-

infrared and millimetre/submillimetre wavebands, where a

large amount of additional information about the surface

density of more distant dusty galaxies is available. There is

an upper limit to the surface density of sources at 2.8 mm

(Wilner & Wright 1997); counts at 850 µm (Smail et al. 1997;

Barger et al. 1998; Holland et al. 1998; Hughes et al. 1998;

Barger et al. 1999a; Blain et al. 1999b; Eales et al. 1999); up-

per limits (Smail et al. 1997; Barger et al. 1998), and a new

count (Blain et al. 2000) at 450 µm; 175-µm ISO counts from

Kawara et al. (1998) and Puget et al. (1999); 95-µm counts

from Kawara et al. (1998); and 7- and 15-µm counts from an

extremely deep ISO image of Abell 2390 (Altieri et al. 1999),

which yields counts th at are even deeper than th ose deter-

mined in blank-ﬁeld surveys by Oliver et al. (1997), Aussel

et al. (1999) and Flores et al. (1999).

If the values of the activity parameter at redshift zero,

(F σ)

−1

, listed in Table 1 are used to estimate the counts of

galaxies at 850 and 175 µm, then the results underpredict

the observed counts by a large factor. The form of evolu-

tion of the merger eﬃciency x(z) is ﬁxed by the observed

background radiation intensity, and so, keeping within the

framework of our well-constrained models, the value of the

activity parameter (F σ)

−1

at high redshift must be allowed

to increase above its value at redshift zero in order to ac-

count for the observations. This has the eﬀect of increas-

ing the luminosity of high-redshift mergers, thus increasing

the 175- and 850-µm counts. However, the background ra-

diation intensity and the low-redshift 60-µm counts remain

unchanged.

The form of evolution of the activity parameter (F σ)

−1

that is required to explain the data is illustrated in Fig. 8.

In Fig. 8(a) the ratio of the model predictions and t he ob-

served counts at wavelengths of 175 and 850 µm (Kawara et

al. 1998; Blain et al. 1999b respectively) are compared as a

function of the activity parameter in the four models listed

in Table 1. The same value of the activity parameter cannot

account for the observed counts at both wavelengths simul-

taneously, and the value required to explain the low-redshift

60-µm counts is diﬀerent from either. The value of the activ-

ity parameter required to ﬁt th e 60-, 175- and 850-µm counts

increases monotonically. Because the median redshift of the

galaxies contributing to the counts at these redshifts is ex-

pected to increase monotonically, in Fig. 8(b) we present the

ratio of the mod el predictions and t he observed counts as a

function of a parameter p

that describes a simple form of

exponential redshift evolution of the activity parameter,

(F σ)

−1

= (F σ)

−1

exp p

z. (18)

The exponential form provides a reasonable ﬁt to th e data,

but is only one example of a whole family of p otential func-

tions. The important feature is that the function chosen to

represent the activity parameter (F σ)

−1

increases rapidly

with increasing red sh ift.

The zero-redshift value of the activity parameter

(F σ)

−1

is ﬁxed by requiring that the low-redshift 60-µm

count prediction is in agreement with observations; see Ta-

ble 1. The values of the evolution parameter p

that cor-

respond to th e most reasonable ﬁt for assumed single dust

temperatures of 35, 40, 45 and 50 K are about 1.5, 1.5, 2.0

and 2.3 respectively. If the speciﬁc form of the red sh ift evo-

lution of the activity parameter ( F σ)

−1

shown in equation

(18) is assumed, then a dust temperature of 35 or 40 K is

most consistent with the data, the same temperature that

was required for consistency by both Blain et al. (1999c)

and Trentham et al. (1999), and is in agreement with the

dust temperatures derived for high-redshift QSOs by Ben-

ford et al. (1999).

The increase in t he value of the activity parameter

(F σ)

−1

as a fun ction of redshift can be interpreted in terms

of two extreme scenarios, or as a combination of both. In

the ﬁrst scenario, the fraction of dark halo mergers that

lead to a luminous phase in a dusty galaxy F is ﬁxed, but

that the duration of t he luminous phase σ is less at high

redshifts. This is plausible, b ased on the results of simula-

tions of galaxy mergers (e.g. Mihos 1999; Bekki et al. 1999);

on average, the typical mass of a merging pair of galaxies is

expected to be less at high redshifts in an hierarchical sce-

nario of galaxy evolution, and the gas content of the galaxies

is expected to be greater. As a result, the dynamical time

of a merger would be expected to decrease with increasing

redshift, and the viscosity of the ISM would be expected to

increase. Both of these factors might be expected to increase

the star formation eﬃciency of a merger with increasing red-

shift. In the second scenario, the duration of the luminous

phase associated with a merger σ is independent of redshift,

but the fraction of mergers that induce such a phase F is re-

duced as redshift increases. It is perhaps more plausible that

the second of these scenarios could produce the large change

in the activity parameter (F σ)

−1

, by a factor of about 100

from z = 0 to z = 3 that is required to ﬁt the data. This

is because the duration of the luminous phase of a merger-

induced starburst σ must exceed the lifetime of a reasonably

massive star, i.e. σ

∼

yr. If star formation activity pow-

ers a signiﬁcant fraction of the SCUBA galaxies, as seems

reasonable, then this limit to the value of the merger dura-

tion σ is constrained to be greater than about 10

yr, only a

few times less than the values of σ at z = 0 listed in Table 1.

Thus it seems likely that a large fraction of the increase in

the value of the activity parameter (F σ)

−1

, which is requ ired

at high redshifts to account for the observed counts, should

be attribu ted to a reduction in the fraction F of dark halo

mergers that generate a luminous galaxy. We speculate that

this may be connected with the lower typical metallicity ex-

 0000 RAS, MNRAS 000, 000–000

10 A. W. Blain et al.

(a) (b)

Figure 8. The ratio of the counts predicted by the four models listed in Table 1, and the observed counts, at 175 and 850 µm

(Kawara et al. 1998; Blain et al . 1999b respectively). The upper limits to the counts at 450 µm and 2.8 mm are never viol ated. In

(a) the ratio is shown as a function of the reciprocal of the activity parameter F σ, which is ﬁxed as a function of redshift; in (b)

the ratio is shown as a function of p

, the exponent in equation (18), which describes the exponential evolution of the activity

parameter (F σ)

−1

∝ (F σ)

−1

exp p

z. Clearly, strong redshift evolution of the activity parameter in merging galaxies is required in

order to ﬁt the counts at long wavelengths.

pected at higher redshifts. In a lower metallicity system the

cooling of dense gas would be ex pected to b e less eﬃcient,

and so a large amount of high-mass star formation may be

unable to take place during the short merger process.

3.2 Predicted counts of dusty galaxies

Counts predicted by the four models listed in Table 1, em-

ploying the values of p

listed above, are compared with

observations at wavelengths of 15, 60, 175, 450, 850, 1300

and 2800 µm in Fig. 9. While these models do not present

unique solutions, fewer parameters are involved in the model

than the number of separate pieces of constraining data. In

future, by comparing the predictions of t he models with ob-

servations, especially with the redshift distributions of th e

SCUBA galaxies (Barger et al. 1999b; Lilly et al. 1999; Smail

et al. 1999, in preparation), the models can be developed to

account more accurately for the increasing amount of avail-

able data.

3.3 The corresponding radio counts

There is a tight correlation between the ﬂux densities of

low-redshift galaxies in the radio and far-infrared wavebands

(see the review by Condon 1992). Thus the counts of faint

galaxies observed in the radio waveband should not be over-

produced when the SEDs of the galaxies in the 35-, 40-, 45-

and 50-K models presented here are extended into t he radio

waveband using this correlation. It is permissible to under-

predict the counts, as there will be a contribution from AGN

to the faint counts, which need not be associated with power-

ful restframe far-infrared emission from dust. Partridge et al.

(1997) report a 8.4-GHz galaxy count of 1.0 ± 0.1 arcmin

−2

brighter than a ﬂux density of 10 µJy. The correspondings

count predicted by the 35-, 40-, 45- and 50-K models are 0.6,

0.7, 1.0 and 0.6 arcmin

−2

respectively. Thus all the mo dels

discussed here are consistent with the deep radio observa-

tions. For comparison, a count of 0.8 arcmin

−2

is predicted

by the modiﬁed Gaussian model discussed in Barger et al.

(1999b), which is modiﬁed from the results of t he simple lu-

minosity evolution models presented by Blain et al. (1999c).

3.4 Redshift distributions

The redshift distributions of submillimetre-selected sources

at, or just below, the ﬂux density limits of current surveys

have been discussed by Blain et al. (1999c) in the context

of models of a strongly evolving population of distant du sty

galaxies, based on the low-redshift IRAS galaxy luminosity

function. The ﬁrst spectroscopic observations of a large frac-

tion of the potential optical counterparts to SCUBA galax-

ies identiﬁed in deep multicolour optical images (Smail et al.

1998) have been made by Barger et al. (1999b) (see Fig. 10).

This redshift distribution is consistent with the optical iden-

tiﬁcations made by Lilly et al. (1999) in a SCUBA survey

of Canada–France R edshift Survey (CFRS) ﬁelds. The dis-

tribution shown in Fig. 10 is, however, subject to potential

misidentiﬁcations of SCUBA galaxies. For example, recent

deep near-infrared images show that two of the Smail et

al. (1998) SCUBA galaxies, which were originally identiﬁed

with low-redshift spiral galaxies, can more plausibly be asso-

ciated with extremely red objects (EROs) that were uniden-

tiﬁed in optical images (Smail et al. 1999). In two other

cases, at z = 2.55 (Ivison et al. 1999) and z = 2.81 (Ivi-

son et al. 1998), the identiﬁcations have been conﬁrmed by

detections of redshifted CO emission (Frayer et al. 1998;

1999), and in another case spectroscopy and ISO observa-

tions (Soucail et al. 1999) strongly support the identiﬁcation

of a ring galaxy at z = 1.06.

The preliminary redshift distribution of SCUBA galax-

ies, shown in Fig. 10, is broadly consistent with the predic-

tions of t he Gaussian model of Blain et al. (1999c). A mod-

iﬁed Gaussian model, as shown in Fig. 1, was described by

Barger et al. (1999b); the values of the evolution parame-

ters in the modiﬁed Gaussian model were explicitly ﬁtted

both to the background radiation intensity and count data

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 11

(a) (b)

Figure 9. Counts predicted by the models li sted in Table 1, compared with available data. The 60-µm counts are shown in (a),

those at 15 and 175 µm are s hown in (b), those at 850 µm and 2. 8 mm are shown in (c), and those at 450 µm and 1.3 mm are shown

in (d). The r ef erences to the data in (a) are given in the caption of Fig. 4(a); in (b), (c) and (d) they are written adjacent to the

data points.

and to the observed median redshift. In Fig. 10 the observed

redshift distribution is compared with the redshift distri-

butions predicted in the Gaussian and modiﬁed Gaussian

models, and with the p redictions of the hierarchical models

developed here (see Table 1). Median redshifts of about 2.2,

2.7, 3.2 and 3.5 are expected in the 35-, 40-, 45- and 50-K

hierarchical models respectively.

The redshift distributions predicted by th e hierarchical

models have m edian redshifts greater than that in the mod-

iﬁed Gaussian model, but less than those in either the oth er

models presented by Blain et al. (1999c) or the hierarchical

model E from Guiderdoni et al. (1998), all of which provide

a reasonable ﬁt to both the background radiation intensity

and source counts in the far-infrared/submillimetre wave-

band. Based on these results, the coolest 35-K mod el seems

to be in best agreement with the available data. A model in

which the single-temperature dust clouds discussed here are

replaced by a temperature distribution will probably be re-

quired to account for the redshift distribution of the SCUBA

galaxies. When the two spiral galaxies at z < 0.5 are re-

placed by EROs at z > 1 (S mail et al. 1999), the agreement

between t he 35-K hierarchical prediction and the observed

redshift distribution is rather satisfactory.

In all the hierarchical models, despite strong negative

evolution of the mass-to-light ratio of mergers with increas-

ing redshift, most of the detected galaxies are expected to

lie at redshifts less than 5, and so will be accessible to multi-

waveband study using 8-m class telescopes. When ﬁnal reli-

able identiﬁcations and redshifts for submillimetre-selected

galaxies are available, this information will be crucial for

reﬁning the hierarchical model.

4 OPTICAL BACKGROUNDS AND

COUNTERPARTS

The discussion has so far centred on the properties of merg-

ing galaxies as observed through their dust emission in

the mid-infrared, far-infrared and millimetre/submillimetre

wavebands. Here we assume the same forms of evolution

of both the merger eﬃciency parameter x and the activ-

ity parameter (F σ)

−1

that were required to account for the

data in th e far-infrared and submillimetre wavebands in the

previous section, bu t make predictions in the near-infrared,

optical and ultraviolet wavebands. In particular, we inves-

tigate the 35-K model, in which the redshift distribution of

SCUBA galaxies is in best agreement with observations.

Subject to the uncertain fraction of the luminosity of

 0000 RAS, MNRAS 000, 000–000

12 A. W. Blain et al.

Figure 10. The redshift distributions of 850-µm galaxies in the

Smail et al. (1998) sample predicted in the 35-, 40-, 45- and 50-K

hierarchical models l isted in Table 1 (thick smo oth lines; solid,

dashed, dot-dashed and dotted respectively), the Gaussian model

(thin smooth solid line; Blain et al. 1999c) and the modiﬁed Gaus-

sian model (thin dotted l ine; Barger et al. 1999). The modiﬁed

Gaussian model is ﬁtted to the median redshift of the preliminary

redshift distribution determined by Barger et al. (1999b) for the

optical counterparts to submillimetre-selected galaxies (Smail et

al. 1998), which is shown by the thinner jagged solid line. So urces

with no optical counterparts are assumed to be at z = 4. Using

subsequent K-band observations, Smail et al. (1999) have modi-

ﬁed the identiﬁcations of the counterparts of the two galaxies at

z < 1 in the sample, from low-redshift spirals to high-redshift

EROs. After this modiﬁcation, the revised redshift distribution

is shown by the thicker jagged solid line, if the EROs are both

assumed to lie at z = 3.

these galaxies t hat is assumed to be powered by star forma-

tion activity, we predict th e integrated background radiation

intensity from the near-infrared to ionizing ultraviolet wave-

bands, and the counts of galaxies with SEDs that are domi-

nated by evolved stars in the near-infrared K-band, and by

young stars in the optical B-band. By requiring th at the K-

and B-band counts are reproduced accurately, we estimate

both the fraction of all energy released in mergers that is

reprocessed by dust A and the normalization of the activity

parameter at z = 0, (F σ)

−1

in the optical waveband. For a

discussion of the evolution of faint galaxies and t heir stellar

populations see Ellis (1997).

4.1 K-band counts

The counts of galaxies in the K-band at a ﬂux density S

can be predicted by assuming the forms of the merger eﬃ-

ciency x(z), as listed in Table 1, an SED typical of evolved

stars f

(Charlot, Worthey & Bressan 1996), the Press–

Schechter mass function (equation 1) and th e mass-to-light

ratio of evolved stellar populations R

. The SED was cal-

culated using a 9.25-Gyr old Bruzual–Charlot instantaneous

burst model with a Salpeter IMF. Upper and lower mass lim-

its of 0.1 and 125 M

⊙

were assumed for the IMF. Note that

the form of the evolved stellar spectrum derived is almost

independent of the exact values of the upper and lower mass

limits assumed. The K-band count

) =

∞

(z)

(M) dM D(z)

dz, (19)

with

(z) = 4πD

(1 + z)S

(z)

′

dν

′

(1+z)

. (20)

By ensuring that the predicted counts match the observed

K-band counts, a suitable form of the mass-to-light ratio

is determined as a function of redshift. The mass in

this ratio is deﬁned as the mass of the dark matter haloes of

galaxies, taken from the Press–Schechter function (eq uation

1), and the luminosity is the bolometric luminosity of the

evolved stellar population in the galaxies.

In order to reproduce the observed B-band counts, the

counts derived for the evolved and merging components are

added together, as shown in Fig. 11(b). The redshift depen-

dence of the mass-to-light ratio is the same as that of the

luminosity density of evolved stars,

(z) ∝

1 + z

x(z

′

)

1+z

′

x(z

′

)

1+z

′

, (21)

which depends on the SFR at all earlier epochs. A factor of

1 + z is included in the denominator to mimic the eﬀects of

passive stellar evolution (Longair 1998). At z = 0 a form of

the mass-to-light ratio,

⊙

−1

⊙



80, if L

≥ 2;

117L

−0.55

, otherwise,

(22)

where L

= L/10

⊙

, is required to match the observed

K-band counts (Fig. 11a) and the faint-end slop e of the ob-

served K- band luminosity function (Gardner et al. 1996;

Szokoly et al. 1998).

The well ﬁtt ing K-band count that is derived from

the model with this form of the mass-to-light ratio R

is shown, along with the observational data, in Fig. 11(a).

4.2 B-band counts

Both passive evolved galaxies and luminous merging galaxies

make a contribution to the B-band counts. The evolved con-

tribution is predicted by evaluating the function that pro-

duces the K-band count at the frequency of t he B-band.

The extrapolation is made using the model SED described

above. An additional population of merging galaxies is also

included. Their counts are determined using equation (13)

directly, with a value of

min

1 − A

4πD

(1 + z)S

0.007c

F σ

x(z)

(1 − f

)

′

dν

′

(1+z)

. (23)

A is the fraction of the total energy released in a merger

that is reprocessed into the far-infrared waveband, and f

is the SED of a ﬂat star-forming young stellar spectrum at

frequencies less than the Lyman limit frequen cy ν

= 3.3×

Hz. f

∝ ν

if ν ≤ ν

and zero otherwise. The blue

power-law SED expected from an AGN will be described

reasonably well by this SED at ν < ν

The faint counts at B > 21, which are dominated by

merging galaxies, can be reproduced in the model only if

the dust absorption fraction A ≃ 0.8 and the zero-redshift

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 13

(a) (b)

Figure 11. (a) Near-infrared K-band and (b) optical B-band counts predicted in the 35-K hierarchical model (Table 1). The K-

band counts are produced almost entirely by quiescent non-merging galaxies (solid line). If the ﬂat spectra of the merging galaxies

are extrapolated into the K band, as shown by the dotted line, then the counts would be expected to increase by about 10 per cent

at K = 20.5 and by 30 per cent at K = 23. In the B-band, the total counts (solid line) are dominated at the bright end by the small

hot stellar component of the quiescent galaxies (dashed line), and at the faint end by the young stellar populations of ﬂat-spectrum

merger induced starbursts/AGN (dotted line). Data points obtained using ground-based telescopes are shown by open circles; data

points from the H ubble Space Telescope are shown by ﬁl led stars. The data are taken from the compilation of Metcalfe et al. (1996).

activity parameter (F σ)

−1

= 2.5 Gyr

−1

. The activity pa-

rameter incorporated in the model evolves with redshift as

shown in equation (18), with the value of p

= 1.5 that

is appropriate in the 35-K model. Note that this value of

the activity p arameter is less t han that required to account

for the observed submillimetre-wave counts, and that the

ratio of energy emitted in the restframe ultraviolet and far-

infrared wavebands is 1:4. This ratio is equivalent to 1.75

magnitudes of extinction when integrated over the optical

and ultraviolet wavebands.

The redshift distribution of faint galaxies at z

∼

derived in the hierarchical model is in good agreement

with that observed for galaxies with B < 24 (Cowie, Hu

& Songaila 1995); these details are discussed more exten-

sively elsewhere (Jameson et al. 1999, in preparation). Note

that the dependence of the submillimetre and faint B-band

counts on the merging eﬃciency parameter x(z) and the

AGN fraction f

is identical, and so the value of the AGN

fraction does not aﬀect the resulting counts.

The most numerous population of faint high-redshift

optically selected galaxies known are the Lyman-break

galaxies (Steidel et al. 1996a, 1999) at 2.5 ≥ z ≥ 4.5, which

have apparent SFRs of a few 10 M

⊙

−1

. The surface den-

sity of detected Lyman-break galaxies is about an order

of magnitude greater than that of submillimetre-selected

galaxies, while their SFRs are typically about an order of

magnitude less.

The violence of a typical merger-induced star-

burst/AGN, and thus its detectability, is determined by

the product of the merging eﬃciency and activity param-

eters, x(F σ)

−1

, in the hierarchical model. The value of this

composite parameter that is required to ﬁt the observed

submillimetre-wave counts is about 40 times greater than

that required to ﬁt the faint B-b and counts. The value of

the activity parameter (F σ)

−1

that is required to ﬁt the

submillimetre-wave counts is a factor of about 10 times

greater t han that required to ﬁt the B-band counts. These

diﬀerences are t hus comparable with the observed ratios of

the surface densities and luminosities of typical galaxies in

the submillimetre-selected and Lyman- break samples.

Based on these diﬀerences, we suggest a scenario in

which the optically selected Lyman- break galaxies and the

submillimetre-selected SCUBA galaxies are drawn from the

same underlying population of luminous galaxy merger

events, but are distinguished by being observed during two

distinct phases of the merger pro cess. We associate one

phase, which is very luminous, short-lived and heavily dust

enshrouded, with the SCUBA galaxies, and the other, which

is less luminous and relatively lightly obscured, with the

Lyman-break galaxies. During the ﬁrst phase, which is about

 0000 RAS, MNRAS 000, 000–000

14 A. W. Blain et al.

Figure 12. The background radiation intensity in the hierarchi cal models from the millimetre to the ultraviolet waveband.

The thick solid line shows the total background radiation intensity predicted in the 35-K model (Table 1). In the near-

infrared/optical/ultraviolet waveband the separate contributions to the background intensity from the old stellar population (dashed

line) and the young stellar population/AGN (dotted line) are also shown. At long wavelengths the data points are identical to those

plotted in Fig. 3. Other measurements are numbered: 1. Stanev & Franceschini (1998); 2. Altieri et al. (1999); 3. Dwek & Arendt

(1998); 4. Pozzetti et al. (1998); 5. Toller, Tanabe & Weinberg (1987); 6. Armand, Milliard & Deharveng (1994); 7. Lampton,

Bowyer & Deharveng (1990); and 8. Murthy et al. (1999); see also Bernstein, Freedman & Madore (1999), who determine a greater

absolute background radiation intensity in the optical waveband.

40 times more luminous, but only about a tenth of t he dura-

tion as compared with the second, most of the activity in t he

merger will be almost completely obscured from view in the

optical waveband, but extremely bright in the submillimetre

waveband. This phase is consistent with the extremely com-

pact nuclear starburst/AGN activity observed by Downes &

Solomon (1998) on sub-kpc scales in nearby ultraluminous

IRAS galaxies. During the second phase, less intense star

formation activity would probably b e distributed through-

out the ISM of both merging galaxies. A short lived ultra-

luminous phase and a longer-lived less intense burst of star

formation activity during a merger are consistent with the

results of hydrodynamic models of galaxy mergers by Mihos

& Hernquist (1996) and Bekki et al. (1999).

However, while plausible, this scenario is not necessar-

ily correct. The faint counts in the submillimetre and optical

waveband could simply be drawn from two distinct popula-

tions. The questions of whether and how ultraluminous dust-

enshrouded mergers are connected with the Lyman-break

galaxies can only be answered by making multiwaveband ob-

servations of large samples of submillimetre-selected galaxies

in order to observe a time sequ ence of merging galaxies and

to investigate the merger process in detail. Observations of

the Lyman-break population in the submillimetre waveband

(Chapman et al. 1999) will also help to address these ques-

tions.

4.3 Integrated background light

The global luminosity density predicted by the hierarchical

models is based on the evolution of the merging eﬃciency

parameter x(z). By making minor modiﬁcations to the for-

malism presented in Section 2.3, the models listed in Table 1

can b e used to predict the background radiation intensity.

The near-infrared/optical/ultraviolet background radi-

ation intensity produced by merging galaxies can be calcu-

lated as shown in equation (11), if the fraction of energy

generated by a merger that is absorbed by dust A is in-

cluded. Thus

opt

∝ (1−A)

√

x(z)

1 − f

δ(z)

ν(1+z)

′

dν

′

1 + z

.(24)

The background radiation intensity due to the evolved stel-

lar population is not given directly by the expression for the

far-infrared background in equation (11). In that case the

volume emissivity is given by equations (21) and (22). By

integrating this emissivity of evolved objects over redshift,

assuming the SED f

introduced above, the background ra-

diation intensity produced by evolved non-merging galaxies

can be calculated. When it is added to the background ra-

diation spectrum produced in merging galaxies, a complete

prediction of th e background intensity between the near-

infrared and near-ultraviolet wavebands is obtained.

The background radiation intensity predicted using the

35-K model, which can explain the K- and B-band galaxy

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 15

counts, is shown across the millimetre to the ultraviolet

wavebands in Fig. 12. The background intensity is in agree-

ment with almost all the observed limits and detections. The

only spectral region in which the background radiation in-

tensity is still not very well deﬁned is between about 3 and

7 µm. At these wavelengths, the dominant source of back-

ground radiation switches from dust emission to starlight.

There is almost certainly an additional, and perhaps domi-

nant, contribution to the background radiation intensity in

the mid-infrared waveband from very hot dust grains in the

central regions of AGN, which are not modelled here. The

model curves shown Fig. 12 are not extrapolated into this

region from the well-determined populations of galaxies at

60-µm and in the K band.

5 OVERVIEW OF MODEL PARAMETERS

We have introduced a series of parameters and functions to

account for diﬀerent observations section by section through

the paper. Excluding the world model parameters H

, Ω

and Ω

, ﬁve parameters are required to deﬁne the merger

rate of dark matter haloes. Five further independ ent pa-

rameters [x

, p, z

max

, (F σ)

−1

and p

] and a SED for d usty

galaxies, are required to ﬁt the counts at wavelengths of 15,

60, 175, 450 and 850 µm and the submillimetre-wave/far-

infrared background radiation spectrum. The most impor-

tant element of the models are the two functions that d e-

scribe the merger eﬃciency parameter x(z) and the activity

parameter (F σ)

−1

. We present appropriate forms for these

functions in equations (16) and (18) respectively, b ut stress

that these forms are not unique. As more data becomes avail-

able, other functional forms or free-form ﬁtting functions

may be more appropriate. By modifying the activity pa-

rameter (F σ)

−1

, introdu cing the total fraction of luminosity

absorbed by dust A, and including a template SED for star-

forming galaxies in the rest frame ultraviolet waveband, the

B-band counts can also be rep rodu ced. In Table 2 we sum-

marize these parameters and the most important pieces of

data used to constrain them. The values of the p arameters

required to ﬁt the data in the 35-K mo del are also listed.

6 CONCLUSIONS

(i) We have p resented a simple model of hierarchical

galaxy formation which incorporates the eﬀects of obscu-

ration by dust, in which the galaxies that are detected in

submillimetre-wave surveys are observed during a merger-

induced episode of star formation or AGN fuelling. The aim

of this model is to elucidate the most important physical

processes that could be at work in luminous dusty galaxies,

rather than t o provide a detailed quantitative description.

(ii) The model is constrained primarily by the intensity

of background radiation in the far-infrared/submillimetre

waveband. From these data alone, the luminosity density

from high-redshift galaxies is inferred to exceed that de-

duced from observations in the rest frame ultraviolet and op-

tical wavebands by up to an order of magnitude. The source

counts and background radiation intensity in the submil-

limetre, far-, mid- and near-infrared, and optical wavebands

are reproduced adequately in the model without introducing

a large number of parameters.

(iii) The counts of galaxies detected in t he far-

infrared/submillimetre and optical wavebands, and th e asso-

ciated background radiation intensities in th ese wavebands

are consistent if about 4 times more energy is emitted by

galaxies after being repro cessed into the far-infrared wave-

band by interstellar dust as is radiated directly in the opti-

cal/ultraviolet waveband.

(iv) In order to account for the observed abundance of

distant galaxies detected at 175 and 850 µm using ISO and

SCUBA, the mass-to-light ratio of a typical galaxy merger

must decrease with redshift, by a factor of about 10 and

200 at z = 1 and 3 respectively. Thus high-redsh ift merg-

ers must be typically more violent as compared with their

low-redshift counterparts. We suggest two possible physical

explanations. First, that gas is converted into stars/feeds

an AGN uniformly more eﬃciently and rapidly in all merg-

ing galaxies as redshift increases, perhaps due to a lower

bulge-to-disk ratio, which makes disk instabilities grow more

quickly (Mihos & Hernquist 1996). Secondly, that a decreas-

ing fraction of dark matter halo mergers are associated with

an eﬃcient mod e of star formation/AGN fuelling as redshift

increases.

(v) In the context of galaxy formation within merging

dark matter haloes, we have described how the physical

processes that convert merging mass into visible radiation

must evolve with redshift in order to account for the data

in the far-infrared and submillimetre wavebands. This has

previously been discussed by Guiderdoni et al. (1998), in the

conventional context of semi-analytic models, where gas is

assumed to cool into dark matter haloes and form stars on

galactic scales. In order to account for the observations, an

additional p opulation of ultraluminous galaxies was incor-

porated arbitrarily into their models. We have improved our

previous models (Blain et al. 1999c) signiﬁcantly, by includ-

ing some astrophysics and not simply invoking an empirical

form of the evolution of a low-redshift luminosity function

to ﬁt the data. By assuming only a single population of

luminous merging galaxies we are able to account for all

the data in the far-infrared and submillimetre wavebands.

Clear forms of evolution of both the eﬃciency with which

luminosity is generated by a galaxy merger as a function of

redshift, and of a function that connects the duration of t he

luminous phase and the fraction of dark matter halo mergers

that generate a luminous event are required to reproduce the

results of observations. The way in which gas is processed in

the sub-kpc core regions of galaxy mergers to reproduce the

necessary high eﬃciency and short time-scale of luminous

events must be investigated in future work.

(vi) We ﬁnd that the observed counts of both

submillimetre-selected galaxies and Lyman-break galaxies

can be accounted for in terms of merger events in an hi-

erarchical model of galaxy formation, which include iden-

tical forms of evolution with redshift, but with diﬀerent

absolute normalisations. We ﬁnd that 80 per cent of the

total amount of energy generated in merger-induced star-

bursts/AGN is liberated in the far-infrared waveband. It is

plausible that the submillimetre-selected galaxies and the

Lyman-break galaxies are associated with temporally dis-

tinct phases of a common population of merging dark mat-

ter haloes. A scenario in which a short-lived, highly obscured

 0000 RAS, MNRAS 000, 000–000

16 A. W. Blain et al.

Table 2. A summary of the parameters and functions that have been introduced in this paper, generally listed in order of their

ﬁrst appearance. The values of these parameters in the best-ﬁtting 35-K model, which reproduces the current redshift distribution of

submillimetre-selected galaxies adequately, are also listed. An Einstein–de Sitter world model with h = 0.5 is assumed throughout.

Name Symbol Value in the 35-K model Constrained by

Smoothed density ¯ρ N/A Assumed equivalent to Ω

= 1

Fluctuation index n n ≃ −1 60-µm luminosity function

γ = 1 + (n/3)

Fluctuation mass (z = 0) M

∗

3.6 × 10

⊙

Tully–Fisher relation

Merger rate constants φ, α 1.7, 1.4 Self-similar merging process

Far-infrared SED f

N/A 4 temperatures assumed

Fraction of luminous f

= 0 (generally assumed) see Section 2.3

mergers powered by AGN

Timescale of mergers σ N/A see activity parameter

Fraction of mergers that F N/A see activity parameter

yield luminous events

Merger star-formation x(z) x

≃ 10

−4

(γ

F σ)

1/3

(1 − f

) Gyr

−1/3

Bright 60-µm counts

eﬃciency (equation 16) p = 4.4 Far-infrared background

max

= 0.44 Far-infrared background

Activity parameter (F σ)

−1

(z) (F σ)

−1

= 18.8 Gyr

−1

Bright 60-µm counts

(submillimeter/far-inf rared) (equation 18) p

≃ 1.5 175- and 850-µm counts

Activity parameter (F σ)

−1

= 2.5 Gyr

−1

Faint B-band counts

(B-band) p

≃ 1.5 From submillimetre results

Fraction of energy that A 0.8 Faint B-band counts

is reprocessed by dust

B- and K-band SEDs f

, f

N/A Spectral synthesis models

Mass-to-light ratio of R

See equation (22) K-band counts

evolved galaxies

far-infrared starburst/AGN p hase dominates the integrated

luminosity of the merger and is surrounded in time by a less

luminous, more lightly obscured phase that lasts about 10

times longer is consistent with the data. In this scenario, the

merger would be classiﬁed as a SCUBA galaxy if it was ob-

served during the short-lived phase, and as a L yman-break

galaxy during the long-lived phase.

(vii) The results presented here provide excellent oppor-

tunities for further study. Two key scientiﬁc questions re-

main unanswered. First, what are the physical processes

that are responsible for the evolution of both the star-

formation/AGN- fuelling eﬃciency and the activity param-

eter in galaxy mergers as a function of redshift? Secondly,

what is the relationship between samples of faint galaxies

selected in the optical waveband and submillimetre-selected

galaxies? Larger samples of submillimetre-selected galaxies

and more comprehensive multiwaveband follow-up observa-

tions will allow these questions to be answered.

ACKNOWLEDGEMENTS

We thank Nigel Metcalfe for providing a comprehensive list

of optical count data, and Chris Mihos, Priya Natarajan,

Kate Quirk, Chuck Steidel and Neil Trentham for providing

useful comments on the manu script. Thanks are also due

to an anonymous referee for helpful suggestions and prompt

reading of the manuscript. AWB, A J and RJI acknowledge

PPARC, IS thanks the Royal Society, and JPK thanks the

CNRS for support. In addition, AWB thanks MENRT for

support while in Toulouse, and the Caltech AY visitors pro-

gram for support while this work was completed.

REFERENCES

Almaini O., Lawrence A., Boyle B. J., 1999, MNRAS, 305, L59

Altieri B. et al., 1999, A&A, 343, L65

Armand C., Milliard B., Deharveng J. M., 1994, A&A, 284, 12

Aussel H. , Cesarsky C. J., Elbaz D., Starck J.-L., 1999, A&A, 342,

313

Barger A. J., Cowie L. L., Sanders D. B., Fulton E., Taniguchi Y.,

Sato Y., Kawara K., Okuda H., 1998, Nat, 394, 248

Barger A. J., Cowie L. L., Sanders D. B. , 1999a, ApJ, 518, L5

Barger A. J., Cowie L. L., Smai l I., Ivison R. J., Blain A. W.,

Kneib J.-P., 1999b, AJ, 117, 2656

Baugh C. M., Cole S. M., Frenk C. S., Lacey C.G., 1998, ApJ, 498,

504

Bekki K., Shioya Y., Tanaka I., 1999, ApJ, 520, L99

Benford D. J., Cox P., Omont A., Phillips T. G., 1999, ApJ, 518,

L65

Bernstein R., Freedman W., Madore B., 1999, ApJ, submitted

Bertin E., Dennefeld M., Moshir M., 1997, A&A, 323, 685

Blain A. W., Longair M. S., 1993a, MNRAS, 264, 509

Blain A. W., Longair M. S., 1993b, MNRAS, 265, L21

Blain A. W., Longair M. S., 1996, MNRAS, 279, 847

Blain A. W., M¨oller O., Maller A. H., 1999a, MNRAS, 302, 423

Blain A. W., Kneib J.-P., Ivison R. J., Smail I., 1999b, ApJ, 512,

L87

Blain A. W., Smail I., Ivison R. J., Kneib J.-P., 1999c, MNRAS,

302, 632

Blain A. W., Ivison R. J., Kneib J.-P., Smail I., 2000, in Bunker

 0000 RAS, MNRAS 000, 000–000

Hierarchical star formation/AGN fuelling 17

A., van Breughel W. eds, The Hy-Redshift Universe, Astron.

Soc. Pac., San Francisco, in press (astro-ph/9908024)

Brainerd T. G., Villumsen J. V., 1992, ApJ, 394, 409

Burles S., Tytler D., 1998, ApJ, 499, 699

Chapman S. C. et al., 1999, MN RAS, submitted

Charlot S., Worthey G., Bressan A., 1996, ApJ, 457, 625

Cole S. M., Arag´on-Salamanca A., Frenk C. S. , Navarro J. F.,

Zepf S. E., 1994, MNRAS, 271, 781.

Condon J. J., 1992, ARA&A, 30, 575

Connolly A. J., Szalay A. S., Dickinson M., Subbarao M. U., Brun-

ner R. J., 1997, ApJ, 486, L11

Cowie L. L., Hu E . M., Songaila A., 1995, Nat, 377, 603

Cowie L. L., Songaila A., Barger A. J., 1999, ApJ, in press (astro-

ph/9904345)

Cram L. E., 1998, ApJ, 508, L85

Downes D., Solomon P., 1998, ApJ, 507, 615

Dwek E. et al., 1998, ApJ, 508, 106

Dwek E., Arendt R., 1998, ApJ, 508, L9

Eales S. A., Edmunds M. G., 1996, MNRAS, 280, 1167

Eales S. A., Lilly S. J., Gear W. K., Dunne L., Bond J. R., Ham-

mer F., Le F`evre O., Crampton D., 1999, ApJ, 515, 518

Ellis R. S., 1997, AR A&A, 35, 389

Fall S. M., Pei Y. C., 1993, ApJ, 402, 479

Ferguson A. M. N., Gallagher J. , Wyse R. F. G. , 1998, AJ, 116,

673

Fixsen D . J., Dwek E., Mather J. C., Bennett C. L. , Shafer R. A.,

1998, ApJ, 508, 123

Flores H. et al. 1999, ApJ, 517, 148

Frayer D. T., Ivison R. J., Scoville N. Z., Yun M. S., Evans A. S.,

Smail I., Blain A. W., Kneib J.-P., 1998, ApJ, 506, L7

Frayer D. T. et al., 1999, ApJ, 514, L13

Gallego J., Zamorano J., Arag´on-Salamanca A., Rego M., 1996,

ApJ, 459, L43

Gardner J. P., Sharples R. M., Carrasco B. E., Frenk C. S., 1996,

MNRAS, 282, L1

Genzel R. et al., 1998, ApJ, 498, 579

Gibson B. K., Loewenstein M., Mushotzky R. F., 1998, MNRAS,

290, 623

Glazebrook K., B lake C., Economou F., Lilly S., Colless M., 1999,

MNRAS, 306, 843

Gnedin N. Y., Ostriker J. P., 1992, ApJ, 400, 1

Goldader J. D., Joseph R. D., Doyon R., Sanders D. B., 1997, ApJ,

474, 104

Gregorich D. T., Neugebauer G., Soifer B. T., Gunn J. E.,

Hertler T. L., 1995, A J, 110, 259

Gronwall C., 1999, in Holt S., Smith E. eds, After the dark ages:

when galaxies were young. Am. Inst. Phys. Pr ess, Woodbury,

NY, p. 335

Guiderdoni B., Bouchet F. R., Puget J.-L., Lagache G., Hivon E.,

1997, Nat, 390, 257

Guiderdoni B., Hivon E., Bouchet F. R., Maﬀei B., 1998, MNRAS,

295, 877

Gunn K. F., Shanks T., 1999, MNRAS, s ubmitted

Hacking P. B., Houck J. R., 1987, ApJS, 63, 311

Hauser M. G. et al., 1998, ApJ, 508, 25

Hogg D. W., Phinney E. S. , 1997, A pJ, 488, L95

Holland W. S. et al., 1998, Nat, 392, 788

Holland W. S. et al., 1999, MNRAS, 303, 659

Hudson M . J., Gwyn S. D. J., Dahle H., Kaiser N., 1998, ApJ, 503,

531

Hughes D. et al., 1998, Nat, 394, 241

Ivison R. J., Smail I., Le Borgne J.-F., Blain A. W., Kneib J.-P.,

B´ezecourt J., Kerr T. H., Davies J. K., 1998, MNRAS, 298,

583

Ivison R. J., Smail I., Barger A. J., Kneib J.-P., Blain A. W.,

Owen F. N., Kerr T. H., Cowie L. L., 1999, MNRAS, submit-

ted

Jameson A ., Longair M. S., Blain A. W., in Gibson B. K., Axel-

rod T. S., Putman M. E. eds., The third Stromlo symposium:

The Galactic Halo. Astron. Soc. Paciﬁc conf. series. vol. 165,

Astron. Soc. Pac., San Francisco, p. 346

Jameson A., Blain A. W., Longair M. S., 2000, MNRAS, submit-

ted

Kauﬀmann G., Charlot S., 1998, MNRAS, 294, 705

Kawara K. et al., 1998, A&A, 336, L9

Lagache G., Abergel A., Boulanger F. , Desert F.-X., Puget J.-L.,

1999, 344, 322

Lampton M., Bowyer S., Deharveng J. M., 1990, in Bowyer S.,

Leinert C. eds., The galactic and extragalactic background

radiation, Proc. IAU 139, Kluwer, p. 449

Larson R. B., 1998, MN R AS, 301, 569

Lilly S. J., Tresse L., H amm er F., Crampton D., Le F`evre O.,

1995, ApJ, 455, 108

Lilly S. J., Le F`evre O., Hammer F., Crampton D., 1996, ApJ,

460, L1

Lilly S. J., Eales S. A., Gear W. K. P., Hammer F., Le Fevre O. ,

Crampton D., Bond J. R., Dunne L., 1999, ApJ, 518, 641

Longair M. S., 1998, Galaxy formation. Springer, Berlin

Lutz D., Spoon H. H. W., Rigopoulou D., Mo orwood A. F. M.,

Genzel R., 1998, ApJ, 505, L103

Madau P., Ferguson H. C., Dickinson M. E., Gi avalisco M., Stei-

del C. C., Fruchter A., 1996, MNRAS, 283, 1388

Metcalfe N., Shanks T., Campos A., Fong R., Gardner J. P., 1996,

Nat, 294, 147

Mihos C., 1999, in Tacconi L., Lutz D. eds. ULIRG 98, Ap&SS,

in press (astro-ph/9903115)

Mihos J. C., Hernquist L., 1996, ApJ, 464, 641

Murthy J., Hall D., Earl M., Henry R. C., Holby J. B., 1999, ApJ,

in press

Mushotzky R. F., Loewenstein M., 1997, ApJ, 481, L63

Nota A., Sirianni M., Leitherer C., De Marchi G., Clampin M.,

1998, BAAS, 192, 84.01

Oliver S. J., Rowan-Robinson M., Saunders W., 1992, MNRAS,

256, 15P

Oliver S. J., et al. 1997, MNRAS, 289, 471

Partridge R. B., Richards E. A., Fomalont E. B., Kellermann K. I.,

Windhorst R. A., 1997, ApJ, 483, 38

Pascarelle S. M., Lanzetta K. M. , Fern´andez-Soto A., 1998, ApJ,

508, L1

Peacock J. A., Dodds S. J., 1994, MNRAS, 267, 1020

Pei Y. C., Fall S. M., 1995, ApJ, 454, 69

Pei Y. C., Fall S. M., Hauser M. G., 1999, ApJ, in press (astro-

ph/9812182)

Pettini M., Smith L. J., King D. L., Hunstead R. W., 1997, ApJ,

486, 665

Pettini M., Kellogg M., Steidel C. C., Dickinson M., Adel-

berger K. L., Giavalisco M., 1998a, ApJ, 508, 539

Pettini M., Steidel C. C., Adelberger K. L., Kellogg M., Dickin-

son M., Giavalisco M., 1998b, in Shull J. M., Woodward C. E.,

Thronson H. A. eds, Cosmic Origins: evolution of galaxies,

stars, planets and life. Astr. Soc. Pac., San Francisco, p. 67

Pozzetti L., Madau P., Zamorani G., Ferguson H., Bruzual A.,

1998, MNRAS, 298, 1133

Press W. H., Schechter P., 1974, ApJ, 187, 425

Puget J.-L., Abergel A., Bernard J.-P., Boulanger F., Bur-

ton W. B., D´esert F.-X., Hartmann D., 1996, A&A, 308, L5

Puget J.-L. et al., 1999, A&A, 345, 29

Rieke G. H., Loken K., Rieke M. J., Tamblyn P., 1993, ApJ, 412,

Rowan-Robinson M., Hughes J., Vedi K., Walker D. W., 1990,

MNRAS, 246, 273

Saunders W., Rowan-Robinson M., Lawrence A., Efstathiou G.,

Kaiser N., Ellis R. S., Frenk C. S., 1990, M NRAS, 242, 318

Savage B. D., Sembach K. R., 1996, ARA&A, 34, 279

Sawicki M. J., Lin H. , Yee H. K. C., 1997, AJ, 113, 1

Schechter P., 1976, ApJ, 203, 297

 0000 RAS, MNRAS 000, 000–000

18 A. W. Blain et al.

Schlegel D. J., Finkbeiner D. P., Davis M., 1998, ApJ, 500, 525

Smail I., Ivison R. J., Blain A. W., 1997, ApJ, 490, L5

Smail I., Ivison R. J., Blain A. W., Kneib J.-P., 1998, ApJ, 507,

L21

Smail I., Ivison R. J., Kneib J.-P., Cowie L. L., Blain A. W.,

Barger A. J., Owen F. N., Morrison G., 1999, MNRAS, in

press (astro-ph/9905246)

Smail I., Ivison R. J., Owen F. N., Blain A . W., Kneib J.-P., 2000,

ApJ, in press (astro-ph/9907083)

Soifer B., Neugebauer G., 1991, AJ, 101, 354

Somerville R. S., Primack J. R., Faber S. M., 1999, MNRAS, in

press (astro-ph/9806228)

Soucail G., Kneib J.-P., B´ezecourt J., Metcalfe L., Altieri B.,

Le Borgne J.-F., 1999, A&A, 343, L70

Stanev T., Franceschini A., 1998, ApJ, 494, L159

Steidel C. C., Giavalisco M., Dickinson M., Adelberger K. L.,

1996a, AJ, 112, 352

Steidel C. C., Giavalisco M., Dickinson M., Pettini M., Dickin-

son M., Adelberger K. L., 1996b, ApJ, 462, L17

Steidel C. C., Adelberger K. L., Dickinson M., Giavalisco M., Pet-

tini M., 1999, ApJ, 519, 1

Storrie-Lombardi L. J., M cMahon R. G., Irwin M. J., 1996, MN-

RAS, 283, L79

Szokol y G. P., Subbarao M. U., Connolly A. J., Mobasher B., 1998,

ApJ, 492, 452

Tan J. C., Silk J., Balland C., ApJ, in pr ess (astro-ph/9904004)

Toller G., Tanabe H., Weinberg J. L., 1987, A&A, 188, 24

Trentham N., Blain A. W., Goldader J., 1999, MNRAS, 305, 61

Tresse L., Maddox S. J., 1998, ApJ, 495, 691

Treyer M . A., Ellis R. S., Mi lliard B., Donas J., Bridges T. J., 1998,

MNRAS, 300, 303

Wilner D. J., Wright M. C. H., 1997, ApJ, 488, L67

Yan L., McCarthy P. J., Freudling W., Teplitz H. I., Malu-

muth E. M., Weymann R. J., Malkan M. A., 1999, ApJ, 519,

L47

Zepf S. E., Silk J. E., 1996, ApJ, 466, 114

 0000 RAS, MNRAS 000, 000–000

High-redshift star formation in the Atacama large millimetre/submillimetre array era

Article

Full-text available

Dec 2020

The Atacama Large Millimetre/submillimetre Array (ALMA) is currently in the process of transforming our view of star-forming galaxies in the distant ( z ≳ 1 ) universe. Before ALMA, most of what we knew about dust-obscured star formation in distant galaxies was limited to the brightest submillimetre sources-the so-called submillimetre galaxies (SMGs)-and even the information on those sources was sparse, with resolved (i.e. sub-galactic) observations of the obscured star formation and gas reservoirs typically restricted to the most extreme and/or strongly lensed sources. Starting with the beginning of early science operations in 2011, the last 9 years of ALMA observations have ushered in a new era for studies of high-redshift star formation. With its long baselines, ALMA has allowed observations of distant dust-obscured star formation with angular resolutions comparable to-or even far surpassing-the best current optical telescopes. With its bandwidth and frequency coverage, it has provided an unprecedented look at the associated molecular and atomic gas in these distant galaxies through targeted follow-up and serendipitous detections/blind line scans. Finally, with its leap in sensitivity compared to previous (sub-)millimetre arrays, it has enabled the detection of these powerful dust/gas tracers much further down the luminosity function through both statistical studies of colour/mass-selected galaxy populations and dedicated deep fields. We review the main advances ALMA has helped bring about in our understanding of the dust and gas properties of high-redshift ( z ≳ 1 ) star-forming galaxies during these first 9 years of its science operations, and we highlight the interesting questions that may be answered by ALMA in the years to come.

Optical Confirmation of X-Ray-selected Galaxy Clusters from the Swift AGN and Cluster Survey with MDM and Pan-STARRS Data. III

Article

Full-text available

Feb 2022

To understand structure formation in the universe and impose stronger constraints on the cluster mass function and cosmological models, it is important to have large galaxy cluster catalogs. The Swift AGN and Cluster Survey is a serendipitous X-ray survey aimed at building a large statistically selected X-ray cluster catalog with 442 cluster candidates in its first release. Our initial SDSS follow-up study confirmed 50% of clusters in the Sloan Digital Sky Survey footprint as z < 0.5 clusters. Here we present further optical follow-up analysis of 248 (out of 442) cluster candidates from the Swift cluster catalog using multiband imaging from the MDM 2.4 m telescope and the Pan-STARRS survey. We report the optical confirmation of 55 clusters with >3 σ galaxy overdensities and detectable red sequences in the color–magnitude space. The majority of these confirmed clusters have redshifts z < 0.6. The remaining candidates are potentially higher-redshift clusters that are excellent targets for infrared observations. We report the X-ray luminosity and the optical richness for these confirmed clusters. We also discuss the distinction between X-ray and optical observables for the detected and nondetected cluster candidates.

Multiband Optical and Near-Infrared Properties of Faint Submillimeter Galaxies with Serendipitous ALMA Detections

Article

Jan 2019
ASTROPHYS J

We present a catalog of 26 faint submillimeter galaxies (SMGs) in the XMM Large Scale Structure (XMM-LSS) field identified by cross-matching serendipitously detected sources in archival pre–Atacama Large Millimeter Array (ALMA) band 6 and 7 data with multiband near-infrared (NIR) and optical data from the Spitzer Extragalactic Representative Volume Survey, the VISTA Deep Extragalactic Survey, the Canada–France–Hawaii Telescope Legacy Large Survey, and the Hyper Suprime-Cam Subaru Strategic Program. Of the 26 SMGs in our sample, 15 are identified here for the first time. The majority of the sources in our sample (16/26) have faint submillimeter fluxes (0.1 mJy < S_(1mm) < 1 mJy). In addition to the 26 SMGs with multiband optical and NIR detections, there are 60 highly reliable (>5σ) ALMA sources with no counterpart in any other band down to an Infrared Array Camera (IRAC) [4.5] AB magnitude of ≈23.7. To further characterize the 26 galaxies with both ALMA and optical/NIR counterparts, we provide 13-band forced photometry for the entire catalog using the Tractor and calculate photometric redshifts and rest-frame colors. The median redshift of our sample is 〈z〉 = 2.66. We find that our sample galaxies have bluer colors compared to bright SMGs, and the UVJ color plot indicates that their colors are consistent with main-sequence star-forming galaxies. Our results provide new insights into the nature of the faint population of SMGs and also highlight opportunities for galaxy evolution studies based on archival ALMA data.

Properties of the SCUBA-2 850 μm sources in the XMM-LSS field

Article

Feb 2017
J KOREAN ASTRON SOC

We carry out the study of 850 μm sources in a part of the XMM-LSS field. The 850 μm imaging data were obtained by the SCUBA-2 on the James Clerk Maxwell Telescope (JCMT) for three days in July 2015 with an integration time of 6.1 hours, covering a circular area with a radius of 15′. We choose the central area up to a radius of 9.′15 for the study, where the noise distribution is relatively uniform. The root mean square (rms) noise at the center is 2.7 mJy. We identify 17 sources with S/N > 3.5. Differential number count is estimated in flux range between 3.5 and 9.0 mJy after applying various corrections derived by imaging simulations, which is consistent with previous studies. For detailed study on the individual sources, we select three sources with more reliable measurements (S/N > 4.5), and construct their spectral energy distributions (SEDs) from optical to far-infrared band. Redshift distribution of the sources ranges from 0.36 to 3.28, and their physical parameters are extracted using MAGPHYS model, which yield infrared luminosity LIR = 1011.3−1013.4 L⊙, star formation rate SFR = 101.3−103.2 M⊙yr⁻¹ and dust temperature TD = 30−53 K. We investigate the correlation between LIR and TD, which appears to be consistent with previous studies.

Simulating the dust content of galaxies: successes and failures

Article

Jun 2016
MON NOT R ASTRON SOC

We present full volume cosmological simulations using the moving-mesh code AREPO to study the coevolution of dust and galaxies. We extend the dust model in AREPO to include thermal sputtering of grains and investigate the evolution of the dust mass function, the cosmic distribution of dust beyond the interstellar medium, and the dependence of dust-to-stellar mass ratio on galactic properties. The simulated dust mass function is well-described by a Schechter fit and lies closest to observations at $z = 0$. The radial scaling of projected dust surface density out to distances of $10 \, \text{Mpc}$ around galaxies with magnitudes $17 < i < 21$ is similar to that seen in Sloan Digital Sky Survey data. At $z = 0$, the predicted dust density of $\Omega_\text{dust} \approx 1.9 \times 10^{-6}$ lies in the range of $\Omega_\text{dust}$ values seen in low-redshift observations. We find that dust-to-stellar mass ratio anti-correlates with stellar mass for galaxies living along the star formation main sequence. Moreover, we estimate the $850 \, \mu\text{m}$ and $1.1 \, \text{mm}$ number density functions for simulated galaxies at $z = 1$ and analyse the relation between dust-to-stellar flux and mass ratios at $z = 0$. At high redshift, our model fails to produce enough dust-rich galaxies, and this tension is not alleviated by adopting a top-heavy initial mass function. We do not capture a decline in $\Omega_\text{dust}$ from $z = 2$ to $z = 0$, which suggests that dust production mechanisms more strongly dependent on star formation may help to produce the observed number of dusty galaxies near the peak of cosmic star formation.

Prospects for future far-infrared/submillimeter studies of the high-redshift universe

Conference Paper

Jan 2002

Andrew Blain

Observations made using COBS, SCUBA, ISO and MAMBO have provided a reasonable working knowledge of both the intensity of the submm and far-infrared background radiation and the source counts of luminous high-redshift dusty galaxies. However, because there are uncertainties in the background intensity determinations, the samples of detected galaxies are small, and most importantly, their redshift distributions are very incomplete, details of the evolution of dusty galaxies remain unresolved. The next steps forward in the field will be the launches of SIRTF and ASTRO-F, the commissioning of SOFIA and new, more capable ground-based mm/submm-wave cameras - BOLOCAM, SHARC-II and SCUBA-II - the use of ultra-long duration balloon experiments, such as BLAST, the construction of ALMA and the arrival of FIRST, and ultimately the advent of space-borne far-infrared interferometers, such as SPECS. There are also exciting prospects for direct mm/submm-wave CO-line redshift surveys using wide-band spectrographs. Using these new facilities, the number of high-redshift dusty galaxies known will be increased dramatically. Spectroscopy using SIRTF, SOFIA and FIRST will probe the astrophysical processes within these sources in detail, hopefully addressing the open question of the fraction of the counts and background radiation that is generated by the formation of high-mass stars and by active galactic nuclei (AGNs). The spatial and spectral structure of distant dusty galaxies will finally be resolved in detail using ALMA and SPECS.

Multiwavelength search for the first blazars

Article

Jan 2007

High redshift massive protogalaxies, the sites of early star formation in the Universe, provided a suitable environment for the formation of the first supermassive black holes. Mass accretion causes the black hole to grow and become a gamma-ray blazar for those observers privileged by a line of sight parallel to the jet. Possible evolutionary sequences are explored in terms of their visibility in the millimeter, optical and gamma-ray regimes.

Cosmic history of integrated galactic stellar initial mass function : a simulation study

Article

Full-text available

Nov 2014
ASTROPHYS J

Theoretical as well as observational studies suggest that the stellar initial mass function (IMF) might become top heavy with increasing redshift. Embedded cluster mass function is a power law having index $\beta$, whose value still remains controversial. In the present work, we investigate the effect of evolving IMF and varying indices of $\beta$ for the integrated galactic initial mass function, in relation to several measures of star formation rates of galaxies at various redshifts by random simulation. The resulting IGIMF is segmented power law at various redshifts having slopes $\alpha_{1,IGIMF}$ and $\alpha_{2,IGIMF}$ with a turnover at a characteristic mass $m_{c^{'}}$. These differ from the stellar initial mass functions with slopes $\alpha_{1,IMF}$, $\alpha_{2,IMF}$, and characteristic masses $m_{c}$ for different values of redshift z, $\beta$, minimum and maximum masses of the embedded clusters.

Exploiting magnification bias in ultradeep submillimetre-wave surveys using ALMA

Article

Feb 2002

Andrew Blain

The surface density of populations of galaxies with steep/shallow source counts is increased/decreased by gravitational lensing magnification. These effects are usually called ‘magnification bias’ and ‘depletion’, respectively. However, if sources are demagnified by lensing, then the situation is reversed, and the detectable surface density of galaxies with a shallow source count, as expected at the faintest flux densities, is increased. In general, demagnified sources are difficult to detect and study: exquisite subarcsec angular resolution and surface brightness sensitivity are required, and emission from the lensing object must not dominate the image. These unusual conditions are expected to be satisfied for observations made of the dense swarm of demagnified images that could form very close to the line of sight through the centre of a rich cluster of galaxies using the forthcoming submillimetre-wave Atacama Large Millimeter Array (ALMA) interferometer. The demagnified images of most of the background galaxies lying within about 1 arcmin of a rich cluster of galaxies could be detected in a single 18-arcsec-diameter ALMA field centred on the cluster core, providing an effective increase in the ALMA field of view. This technique could allow a representative sample of faint, 10–100 μJy submillimetre galaxies to be detected several times more rapidly than in a blank field.

Simulated observations of sub-millimetre galaxies: The impact of single-dish resolution and field variance

Article

Full-text available

Jun 2014
MON NOT R ASTRON SOC

Recent observational evidence suggests that the coarse angular resolution [∼20 arcsec full width at half maximum (FWHM)] of single-dish telescopes at sub-mm wavelengths has biased the observed galaxy number counts by blending together the sub-mm emission from multiple sub-mm galaxies (SMGs). We use lightcones computed from an updated implementation of the galform semi-analytic model to generate 50 mock sub-mm surveys of 0.5 deg2 at 850 μm, taking into account the effects of the finite single-dish beam in a more accurate way than has been done previously. We find that blending of SMGs does lead to an enhancement of source extracted number counts at bright fluxes (S850 μm ≳ 1 mJy). Typically, three to six galaxies contribute 90 per cent of the flux of an S850 μm = 5 mJy source and these blended galaxies are physically unassociated. We find that field-to-field variations are comparable to Poisson fluctuations for our S850 μm > 5 mJy SMG population, which has a median redshift z50 = 2.0, but are greater than Poisson for the S850 μm > 1 mJy population (z50 = 2.8). In a detailed comparison to a recent interferometric survey targeted at single-dish detected sources, we reproduce the difference between single-dish and interferometer number counts and find a median redshift (z50 = 2.5) in excellent agreement with the observed value (z50 = 2.5 ± 0.2). We also present predictions for single-dish survey number counts at 450 and 1100 μm, which show good agreement with observational data.

Observations of the Hubble Deep Field with the Infrared Space Observatory - III. Source counts and P(D) analysis

Article

Full-text available

Aug 1997
MON NOT R ASTRON SOC

We present source counts at 6.7 and 15 μm from our maps of the Hubble Deep Field (HDF) region, reaching 38.6 μJy at 6.7 μm and 255 μJy at 15 μm. These are the first ever extragalactic number counts to be presented at 6.7 μm, and are three decades fainter than IRAS at 12 μm. Both source counts and a P(D) analysis suggest that we have reached the Infrared Space Observatory (ISO) confusion limit at 15 μrn; this will have important implications for future space missions. These data provide an excellent reference point for other ongoing ISO surveys. A no-evolution model at 15 μrn is ruled out at > 3σ, while two models which fit the steep IRAS 60-μm counts are acceptable. This provides important confirmation of the strong evolution seen in IRAS surveys. One of these models can then be ruled out from the 6.7-μm data.

Molecular Gas in the z = 2.8 Submillimeter Galaxy SMM 02399–0136

Article

Full-text available

Sep 1998
ASTROPHYS J

We report the detection of CO (3→2) emission from the submillimeter-selected hyperluminous galaxy SMM 02399-0136. This galaxy is the brightest source detected in the recent Submillimeter Common-User Bolometer Array surveys of the submillimeter sky. The optical counterpart of the submillimeter source has been identified as a narrow-line active galactic nucleus/starburst galaxy at z=2.8. The CO emission is unresolved, θ5'', and is coincident in redshift and position with the optical counterpart. The molecular gas mass derived from the CO observations is 8×1010h−275M, after correcting for a lensing amplification factor of 2.5. The large CO luminosity suggests that a significant fraction of the infrared luminosity of SMM 02399-0136 arises from star formation. The high inferred star formation rate of 103M☉ yr-1 and the large gaseous reservoir may suggest that we are seeing the formation phase of a massive galaxy. Future CO observations of other galaxies detected in deep submillimeter surveys will test the generality of these conclusions for the bulk of the faint submillimeter population.

Molecular Gas in the z = 2.565 Submillimeter Galaxy SMM J14011+0252

Article

Full-text available

Feb 1999
ASTROPHYS J

We report the detection of CO (3→2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5×1010h−275M☉, after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter-selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas, which is consistent with the inferred high rates of star formation (103M☉ yr-1). These two systems appear to be associated with merger events which may evolve into present-day luminous elliptical galaxies.

15 Micron Infrared Space Observatory* Observations of the 1415+52 Canada-France Redshift Survey Field: The Cosmic Star Formation Rate as Derived from Deep Ultraviolet, Optical, Mid-Infrared, and Radio Photometry

Article

Full-text available

May 1999

The Canada-France Redshift Survey 1452+52 field has been deeply imaged with the Infrared Space Observatory using ISOCAM through the LW3 filter (12-18 μm). Careful data analysis and comparison with deep optical and radio data have allowed us to generate a catalog of 78 15 μm sources with both radio and optical identifications. They are redder and lie at higher redshift than I-band-selected galaxies, with most of them being star-forming galaxies. We have considered the galaxies detected at radio and 15 μm wavelengths, which potentially include all strong and heavily extincted starbursts, up to z=1. Spectral energy distributions (SEDs) for each of the sources have been derived using deep radio, mid-IR, near-IR, optical, and UV photometry. The sources were then spectrally classified by comparing with SEDs of well-known nearby galaxies. By deriving their far-IR luminosities by interpolation, we can estimate their star formation rate (SFR) in a way that does not depend sensitively on the extinction. Between 35% and 85% of the star formation at z≤1 is related to IR emission, and the global extinction is in the range AV=0.5-0.85. While heavily extincted starbursts with SFRs in excess of 100 M☉ yr-1 constitute less than 1% of all galaxies, they contribute about 18% of the SFR density out to z=1. Their morphologies range from S0 to Sab, and more than a third are interacting systems. The SFR derived by far-IR fluxes is likely to be ~2.9 times higher than those previously estimated from UV fluxes. The derived stellar mass formed since the redshift of 1 could be too high when compared with the present-day stellar mass density. This might be due to an initial mass function in distant star-forming galaxies different from the solar neighborhood one or an underestimate of the local stellar mass density.

A tentative detection of the cosmic infrared background at 3.5 μm from COBE/DIREE observations

Article

Full-text available

Oct 1998
ASTROPHYS J

Foreground emission and scattered light from interplanetary dust (IPD) particles and emission from Galactic stellar sources are the greatest obstacles for determining the cosmic infrared background (CIB) from diffuse sky measurements in the ~1 to 5 mum range. We describe a new method for deriving the CIB at near infrared wavelengths which reduces the uncertainties associated with the removal of the Galactic stellar emission component from the sky maps. The method produces positive residuals at 3.5 and 4.9 mum, of which only the 3.5 mum residual is nearly isotropic. We consider our result as a tentative detection of the CIB at this wavelength.

Interstellar Abundances from Absorption-Line Observations with the Hubble Space Telescope

Article

Sep 1996

The Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) has yielded precision abundance results for a range of interstellar environments, including gas in the local medium, in the warm neutral medium, in cold diffuse clouds, and in distant halo clouds. Through GHRS studies, investigators have determined the abundances of elements such as C, N, 0, Mg, Si, S, and Fe in individual interstellar clouds. These studies have provided new information about the composition of interstellar dust gains, the origin of the Galactic high-velocity cloud system, and the processes that transport gas between the disk and the halo. Precision measurements of the interstellar D to H ratio and of the abundances of r- and s-process elements have also provided fiducial reference values for cosmological and stellar evolutionary observations and theoretical models.

Small-Scale Cosmic Microwave Background Observations at 8.4 GHz

Article

Jul 1997

We have used the Very Large Array (VLA) of the National Radio Astronomy Observatory at 8.44 GHz to image a ~40 arcmin2 field with an unprecedented rms sensitivity of 1.5 mu Jy. After correcting for the effects of discrete foreground radio sources, we examined this most sensitive microwave image of the sky for fluctuations in the cosmic microwave background radiation (CBR). At the 6" resolution of our VLA map, Delta T/T ~ (0.7 +/- 0.8) x 10-4, with an upper limit of 1.3 x 10-4 at 95% confidence. At 1' resolution, we measure a fluctuation amplitude of Delta T/T = (1.2 +/- 1.4) x 10-5. We also report on our observations of the linear and circular polarization of the CBR for which we derive upper limits (at 95% confidence) of Delta T/T

The star formation rate density of the local universe from the KPNO international spectroscopic survey

Article

Apr 1999

Caryl Gronwall

Understanding the nature and future evolution of star-forming galaxies found at 2<z<5 requires a thorough knowledge of the star formation process locally. The KPNO International Spectroscopic Survey (KISS) is a wide-field survey for extragalactic emission-line objects being carried out with the Burrell Schmidt at Kitt Peak. We have discovered approximately 1100 emission-line galaxies (ELGs) in a survey area of 68 sq. degrees (16.6 galaxies per square degree). These ELGs were identified via their Hα flux and have been used to measure the star formation rate (SFR) density of the local universe. We find a SFR density approximately equal to that found by Gallego et al. (1995). We note, however, that our survey is incomplete for galaxies with EW ( H α)<25 Å implying that the total local SFR density is higher than this value.

Cosmic Histories of Stars, Gas, Heavy Elements, and Dust in Galaxies

Article

Sep 1999

We investigate a set of coupled equations that relates the stellar, gaseous, chemical, and radiation contents of the universe averaged over the whole population of galaxies. Using as input the available data from quasar absorption-line surveys, optical imaging and redshift surveys, and the COBE DIRBE and FIRAS extragalactic infrared background measurements, we obtain solutions for the cosmic histories of stars, interstellar gas, heavy elements, dust, and radiation from stars and dust in galaxies. Our solutions reproduce remarkably well a wide variety of observations that were not used as input. These include the integrated background light from galaxy counts from near-ultraviolet to near-infrared wavelengths, the rest-frame optical and near-infrared emissivities at various redshifts from surveys of galaxies, the mid-infrared and far-infrared emissivities of the local universe from the IRAS survey, the mean abundance of heavy elements at various epochs from surveys of damped Lyalpha systems, and the global star formation rates at several redshifts from Halpha, mid-infrared, and submillimeter observations. The chemical enrichment history of the intergalactic medium implied by our models is also consistent with the observed mean metal content of the Lyalpha forest at high redshifts. We infer that the dust associated with star-forming regions is highly inhomogeneous and absorbs a significant fraction of the starlight, with only 41%-46% of the total in the extragalactic optical background and the remaining 59%-54% reprocessed by dust into the infrared background. The solutions presented here provide an intriguing picture of the cosmic mean history of galaxies over much of the Hubble time. In particular, the process of galaxy formation appears to have undergone an early period of substantial inflow to assemble interstellar gas at z>~3, a subsequent period of intense star formation and chemical enrichment at 1

The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4

Article

Sep 1997

We report new observations of Zn II and Cr II absorption lines in 10 damped Lyα systems (DLAs), mostly at redshift zabs 2.5. By combining these results with those from our earlier survey and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one-third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped Lyα systems are mostly metal poor, at all redshifts sampled; the column density-weighted mean for the whole data set is [Zn/H] = -1.13 ± 0.38 (on a logarithmic scale), or approximately 1/13 of solar. (2) There is a large spread, by up to 2 orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Lyα systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts z 2, the typical metallicity of the damped Lyα systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of ΩDLA by Storrie-Lombardi et al., and with the metal ejection rates in the universe at these epochs deduced by P. Madau from the ultraviolet luminosities of high-redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped Lyα systems from z > 3 to ≈ 2, consistent with the rise in the comoving star formation rate indicated by the relative numbers of U and B drop-outs in the Hubble Deep Field. Although such comparisons are still tentative, it appears that these different avenues for exploring the early evolution of galaxies give a broadly consistent picture. At redshifts z < 1.5, DLAs evidently do not exhibit the higher abundances expected from a simple, closed-box model of global chemical evolution, although the number of measurements is still very small. We speculate that this may be due to an increasing contribution of low surface brightness galaxies to the cross section for damped Lyα absorption and to the increasing dust bias with decreasing redshift proposed by S. M. Fall and collaborators. However, more DLAs at intermediate redshifts need to be identified before the importance of these effects can be assessed quantitatively. The present sample is sufficiently large for a first attempt at constructing the metallicity distribution of damped Lyα systems and comparing it with those of different stellar populations of the Milky Way. The DLA abundance histogram is both broader and peaks at lower metallicities than those of either thin or thick disk stars. At the time when our Galaxy's metal enrichment was at levels typical of DLAs, its kinematics were closer to those of the halo and bulge than a rotationally supported disk. This finding is at odds with the proposal that most DLAs are large disks with rotation velocities in excess of 200 km s-1, based on the asymmetric profiles of absorption lines recorded at high spectral resolution. Observations of the familiar optical emission lines from H II regions, which are within reach of near-infrared spectrographs on 8-10 m telescopes, may help resolve this discrepancy.

Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

Abstract and Figures

Recommended publications

The Faint Sub-mm Galaxy Population: HST Morphologies and Colors

Connecting galaxy and supermassive black hole growth during the last 8 billion years

Probing the Most Extreme Luminous Infrared Galaxies in the Universe

Interacting galaxies in the IllustrisTNG simulations - IV: Enhanced Supermassive Black Hole Accretio...