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arXiv:astro-ph/0608603v1 28 Aug 2006
Submitted to the Astrophysical Journal
Mid-Infrared Selection of Brown Dwarfs and High-Redshift Quasars
Daniel Stern1, J. Davy Kirkpatrick2, Lori E. Allen3, Chao Bian4, Andrew Blain4, Kate Brand5,
Mark Brodwin1, Michael J. I. Brown6, Richard Cool7, Vandana Desai4, Arjun Dey8, Peter
Eisenhardt1, Anthony Gonzalez9, Buell T. Jannuzi8, Karin Menendez-Delmestre4, Howard A.
Smith3, B. T. Soifer4,10, Glenn P. Tiede11 & E. Wright12
ABSTRACT
We discuss color selection of rare objects in a wide-field, multiband survey span-
ning from the optical to the mid-infrared. Simple color criteria simultaneously identify
and distinguish two of the most sought after astrophysical sources: the coolest brown
dwarfs and the most distant quasars. We present spectroscopically-confirmed examples
of each class identified in the IRAC Shallow Survey of the Bo¨otes field of the NOAO
Deep Wide-Field Survey. ISS J142950.9+333012 is a T4.5 brown dwarf at a distance
of approximately 42 pc, and ISS J142738.5+331242 is a radio-loud quasar at redshift
z= 6.12. Our selection criteria identify a total of four candidates over 8 square degrees
of the Bo¨otes field. The other two candidates are both confirmed 5.5< z < 6 quasars,
previously reported by Cool et al. (2006). We discuss the implications of these dis-
coveries and conclude that there are excellent prospects for extending such searches to
cooler brown dwarfs and higher redshift quasars.
1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Mail Stop 169-506, Pasadena,
CA 91109 [e-mail: stern@zwolfkinder.jpl.nasa.gov]
2Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125
3Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138
4Division of Physics, Math, and Astronomy, California Institute of Technology, Pasadena, CA 91125
5Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
6Princeton University Observatory, Peyton Hall, Princeton University, Princeton, NJ 08544
7Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721
8National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719
9Department of Astronomy, University of Florida, Gainesville, FL 32611
10Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125
11Department of Physics and Astronomy, Bowling Green State University, Bowling Green, OH 43403
12Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, CA 90095
– 2 –
Subject headings: surveys — stars: brown dwarfs — quasars: high-redshift — stars:
individual (ISS J142950.9+333012) — quasars: individual (ISS J142738.5+331242)
1. Introduction
Wide-area surveys are one of the most powerful tools for observational astronomy, and have
led to discoveries ranging from Earth-crossing asteroids to the most distant quasars. Historically,
when technology allows a wavelength regime to be newly probed, either in terms of sensitivity or
area, one of the first tasks is a large, shallow survey to see what astrophysical phenomena lurk
in the uncovered territory. In recent years, major advances from this line of research include the
discovery of the coolest Galactic stars by the Two Micron All Sky Survey (2MASS), ultraluminous
infrared galaxies by the Infrared Astronomical Satellite, the most distant quasars by the Sloan
Digital Sky Survey (SDSS), and the power spectrum of the cosmic microwave background, first
by the Cosmic Background Explorer and later refined by the Wilkinson Microwave Anisotropy
Probe. Such fundamental scientific discoveries have been a major incentive and reward for NASA’s
Explorer program and other large projects. The pace of scientific discovery relies on such programs
continuing.
The mid-infrared regime has been made newly accessible by the launch of the Spitzer Space
Telescope (Werner et al. 2004). At its least competitive, shortest waveband, 3.6µm, Spitzer is still
more than five orders of magnitude more efficient than the largest ground-based observatories for
areal surveys. For the longest wavebands, ground-based observations are simply not possible. Even
compared to previous space-based missions, Spitzer offers several orders of magnitude increase in
mapping efficiency.
Thus inspired, we have undertaken a shallow, wide-area 3.6 to 8.0µm survey with Spitzer,
summarized in §2. We discuss two of the rare, interesting astronomical sources which are ideally
suited to selection by combining deep optical data with shallow mid-infrared data: the coolest
Galactic brown dwarfs and the most distant quasars. The former, of course, are not actually rare
in the cosmos; their faint optical magnitudes merely delayed their discovery until recent years and
continue to make them “rare” in terms of known, spectroscopically-confirmed examples. Section 3
discusses the selection criteria used to identify such sources and §4 describes our spectroscopic
observations which confirmed both a cool brown dwarf (spectral class T4.5) and a high-redshift
(z= 6.12) quasar. The implications for these discoveries are described in §4, and §5 summarizes
the results and discusses future prospects. Throughout we adopt a (Ωm,ΩΛ) = (0.3,0.7) flat
cosmology and H0= 70 km s−1Mpc−1. Unless otherwise stated, all magnitudes are quoted in the
Vega system.
– 3 –
2. Multiwavelength Surveys of Bo¨otes
The Bo¨otes field is a 9 deg2field which has been the target of deep observations across the
electromagnetic spectrum. Bo¨otes was initially selected as the North Galactic field of the NOAO
Deep Wide-Field Survey (NDWFS; Jannuzi & Dey 1999), which obtained deep optical (BWRI)
and moderately-deep near-infrared (Ks) images across the entire field.13 These images reach ap-
proximate 5σpoint source depths of BW= 27.1, R= 26.1, I= 25.4 (B. Jannuzi et al., in prep.)
and Ks= 19.0 (A. Dey et al., in prep.). Subsequently, the field has been observed at X-ray energies
with the Chandra X-Ray Observatory (Murray et al. 2005), with a z′filter using the Bok 2.3m
telescope at Kitt Peak (R. Cool in prep.), more deeply in the near-infrared (JKs) as part of the
FLAMINGOS Extragalactic Survey (FLAMEX; Elston et al. 2006), in the infrared with the Spitzer
Space Telescope (Eisenhardt et al. 2004; Papovich et al. 2004), and at radio frequencies using the
Westerbork Synthesis Radio Telescope (1.4 GHz; de Vries et al. 2002) and the Very Large Array
(325 MHz; S. Croft et al., in prep.). Approximately 20,000 spectroscopic redshifts in the Bo¨otes
field have been obtained by the AGN and Galaxy Evolution Survey (AGES; C. Kochanek et al., in
prep.) and Brodwin et al. (2006) reports on nearly 200,000 photometric redshifts in this field.
The mid-infrared imaging of Bo¨otes is central to this paper. As part of a guaranteed-time
observation program, 8 deg2of the field was imaged with the Infrared Array Camera (IRAC; Fazio
et al. 2004) at 3.6 to 8 µm. Eisenhardt et al. (2004) presents the survey design, reduction,
calibration, and initial results. The survey, called the IRAC Shallow Survey, identifies ≈270,000,
200,000, 27,000, and 26,000 sources brighter than 5σVega magnitude limits of 18.4, 17.7, 15.5, and
14.8 at 3.6, 4.5, 5.8, and 8.0 µm, respectively, where IRAC magnitudes are measured in 6′′ diameter
apertures and corrected to total magnitudes assuming sources are unresolved at the 1.
′′66 −1.
′′98
resolution of IRAC.
3. Mid-Infrared Selection of Rare Sources
Two core science goals of the IRAC Shallow Survey are the identification of the coolest stars and
the identification of the most distant quasars. Both are optically-faint sources that are difficult to
find, but are highly sought after for their astrophysical significance. Brown dwarfs probe the stellar-
to-planetary link (e.g., Kirkpatrick 2005), while the highest redshift quasars probe the conditions
of the early universe and the onset of cosmic reionization (e.g., Fan et al. 2006). Currently,
there are less than 100 T-type brown dwarfs known and only ten quasars at z > 6. Both brown
dwarfs and the most distant quasars are substantially brighter at mid-infrared wavelengths than at
optical wavelengths. Therefore, wide-area, shallow, infrared surveys are ideally suited to identifying
13The third data release is publicly available at http://www.archive.noao.edu/ndwfs. The optical filter specifi-
cations are BW(λc= 4111 ˚
A, FWHM = 1275 ˚
A), R(λc= 6514 ˚
A, FWHM = 1511 ˚
A), and I(λc= 8205 ˚
A, FWHM
= 1915 ˚
A). The Rand Ifilters are part of the Harris filter set and the photometry was calibrated to match the
Cousins system. The near-infrared filter conforms to the standard filter set.
– 4 –
Fig. 1.— Color-color diagrams for unresolved sources in the Bo¨otes field. As indicated, the asterisk
refers to ISS J142950.9+333012 (T4.5 brown dwarf), the filled circle refers to ISS J142738.5+331242
(z= 6.12 quasar), and the open circles refer to Bo¨otes field 5 < z < 6 quasars from Cool et al.
(2006). The dashed line in the right panel illustrates the empirically determined wedge largely
populated by luminous, unobscured AGN (Stern et al. 2005). The dotted line illustrates the
selection criteria employed in Cool et al. (2006) to identify luminous AGN which are too faint to
be detected in all IRAC channels, [3.6] −[4.5] >0.4. Dots illustrate typical colors of sources which
are unresolved in the Iband (stellarity index ≥0.8). In the left panel, sources with 18 < I < 20
and ≥5σdetections in [3.6] and [4.5] are plotted. In the right panel, sources with 10 < I < 20
and ≥5σdetections in all four IRAC passbands are plotted. Photometry of M, L, and T dwarfs
from Dahn et al. (2002; optical) and Patten et al. (2006; IRAC) are plotted in red, as indicated.
SDSS quasars at z≈6 are plotted as inverted-Y’s (Jiang et al. 2006). Quasars and typical stars
are clearly separated on the basis of their mid-infrared colors. The blue line illustrates the colors of
the SDSS quasar template from Richards et al. (2006) for 3 ≤z≤7, subject to the Madau (1995)
formulation for the opacity of the intergalactic medium as a function of redshift. The line becomes
thicker for z≥5.5.
samples of both types of sources.
Brown dwarfs have red colors due to their cool temperatures. At mid-infrared wavelengths the
spectra of most stars generally follow a Rayleigh-Jeans tail, giving them mid-infrared Vega colors
near zero. For cooler stars and brown dwarfs, however, the presence of deep molecular absorptions
results in very different emergent spectra (Kirkpatrick 2005). Specifically, the fundamental bands of
CH4and CO between 3 and 5 µm (Oppenheimer et al. 1998; Rayner & Vacca 2005) and additional
bands of H2O, CH4, and NH3between 5 and 12 µm (Roellig et al. 2004) dramatically recarve the
spectral energy distributions (SEDs) of these objects and give them unique IRAC colors (Patten
– 5 –
ISS J142950.9+333012
60 40 20 0 −20 −40 −60
∆ R.A. (arcsec)
−60
−40
−20
0
20
40
60
∆ Dec. (arcsec)
ISS J142738.5+331242
60 40 20 0 −20 −40 −60
∆ R.A. (arcsec)
−60
−40
−20
0
20
40
60
∆ Dec. (arcsec)
Fig. 2.— Finding charts for ISS J142950.9+333012 (T4.5 brown dwarf) and ISS J142738.5+331242
(z= 6.12 quasar) from the NDWFS I-band imaging. The fields are 2′×2′, centered on the targets.
North is at the top, and east is to the left.
et al. 2006). Shortward of 3 µm, H2O bands in L and T dwarfs (and CH4bands in T dwarfs
only) cause deep depressions in the near-infrared spectra (e.g., McLean et al. 2003), and pressure-
broadened Na Iand K Iresonance doublets suppress much of the flux below 1 µm (Kirkpatrick et al.
1999; Burgasser et al. 2003a), making brown dwarfs extremely faint in the optical. Specifically,
the colors of known brown dwarfs later than type mid-T are R−I > 3.5 (Kirkpatrick et al. 1999;
Dahn et al. 2002), 0.7<[3.6] −[4.5] <2, and 0 <[5.8] −[8.0] <0.8 (Patten et al. 2006).
High-redshift quasars have red colors primarily due to absorption by foreground neutral hy-
drogen in the intergalactic medium which strongly suppresses the intrinsic UV emission of these
AGN. At the highest redshifts, z∼
>6, very little flux is detectable below Lyα, providing the highest
redshift quasars with similar optical colors to cool stars. Longward of Lyα, luminous quasars are
well-approximated by a power law and are easily identified in mid-infrared color-color diagrams
(e.g., Stern et al. 2005).
Therefore, both the coolest brown dwarfs and the highest redshift quasars should easily be
identifiable by selecting unresolved sources with very red optical colors and relatively flat (in fν)
mid-infrared SEDs. Fig. 1 illustrates these selection criteria for the Bo¨otes field. Note that at the
highest redshifts, z > 7, quasars drop out of the optical completely. Typical colors of optical (I-
band) point sources are presented. These color-color plots clearly separate stars and quasars, at least
for typical, hot stars and typical, moderate-redshift quasars: most stars have mid-infrared colors
near zero, while quasars are distinguished by their redder mid-infrared colors. Three confirmed
quasars in this field at 5.39 ≤z≤5.85 identified by Cool et al. (2006) are indicated, as are
twelve z≈6 quasars from the SDSS (Jiang et al. 2006). The IRAC color-color criteria empirically
– 6 –
determined by Stern et al. (2005) and Cool et al. (2006) to select luminous AGN are indicated.
Additionally, the colors of M, L, and T dwarfs from Dahn et al. (2002) and Patten et al. (2006)
are plotted. The thick solid line shows the expected colors of 3 ≤z≤7 quasars, calculated using
the Richards et al. (2006) SDSS quasar template subject to the Madau (1995) formulation for the
opacity of the intergalactic medium as a function of redshift. As can be seen, the Lyαforest causes
high-redshift quasars to become very red in R−Iat z∼
>5, while the mid-infrared colors vary
only slightly over this large redshift range. Cool stars and high-redshift quasars are identifiable
from their red R−Iand [3.6] −[4.5] colors. Longer-wavelength, [5.8] −[8.0] colors can provide
additional information, but require deeper data to obtain robust detections in these less-sensitive
passbands.
As seen in Fig. 1, both brown dwarfs and high-redshift quasars should be easily identified
using the simple (Vega-system) selection criteria of (i) R−I≥2.5, (ii) [3.6] −[4.5] ≥0.4, and (iii)
unresolved at I-band. To restrict the number of spurious sources identified in the catalogs, we also
require (iv) BW−I≥2.5. These constraints implicitly require robust detections (or robust non-
detections) in the various bands. In particular, the I-band morphology criterion requires I∼
<23
for the NDWFS survey. According to the work of Patten et al. (2006), the IRAC color criterion
eliminates sources hotter than spectral class T3. The Bo¨otes 4.5µm catalog (ver. 1.3) identifies
30 candidates matching these selection criteria, which are trimmed to four robust candidates after
visual inspection. The most common cause of a false positive is source blending. One source,
ISS J142918.1+343731, appeared modestly robust after visually inspecting the ground-based imag-
ing. However, the source resides near a z > 1 galaxy cluster identified by Eisenhardt et al. (2006).
Hubble Space Telescope imaging of the cluster (GO 10836; P.I. S. Perlmutter) shows that the po-
tential candidate is compact, but clearly resolved, and thus unlikely to be either a brown dwarf
or a high-redshift quasar. Two of the final four candidates have already been spectroscopically
confirmed as 5.5< z < 6 quasars by Cool et al. (2006). The remaining two were targeted spectro-
scopically during Spring 2006, as discussed next. Table 1 presents all four Bo¨otes field candidates,
in order of decreasing 3.6µm flux, and Fig. 2 presents finding charts for the two sources described
in §4.
4. Spectroscopic Observations and Discussion
Initial spectroscopic follow-up of candidates was obtained with the Multi-Aperture Red Spec-
trometer (MARS; Barden et al. 2001) on the Mayall 4m telescope at Kitt Peak. MARS is an
optical spectrograph which uses a high resistivity, p-channel Lawrence Berkeley National Labora-
tory CCD with little fringing and very high throughput at long wavelengths (∼
<10,500 ˚
A). On the
nights of UT 2006 March 24 −26, we obtained spectra of red sources in the Bo¨otes field using
the 1.
′′7 wide long slit, OG550 order-sorting filter, and the VG8050 grism. Across much of the
optical window, the instrument configuration provides resolution R≈1100 spectra, as measured
from sky lines filling the slit. ISS J142950.9+333012 was observed for 1.5 hr on UT 2006 March
– 7 –
24, split into three dithered 1800 s exposures. ISS J142738.5+331242 was observed for 1 hr on
UT 2006 March 25, split into three 1200 s exposures. The data were processed following standard
optical, slit spectroscopy procedures. The nights were not photometric, but relative flux calibra-
tion of the spectra was achieved with observations of the spectrophotometric standards Feige 34
and PG 0823+546 (Massey & Gronwall 1990) obtained during the same observing run. The ex-
tracted, calibrated MARS spectra are presented in Figs. 3 and 5. The bright star 3.
′′3 east of
ISS J142738.5+331242 made extraction of the fainter target challenging, resulting in systematic
fluctuations of the background at the 1 µJy level.
Near-infrared spectroscopy of ISS J142950.9+333012 was obtained with the cryogenic, cross-
dispersed Near-Infrared Echelle Spectrograph (NIRSPEC; McLean et al. 1998) on the Keck II 10m
telescope atop Mauna Kea. We first obtained J- and H-band spectroscopy on UT 2006 April 05.
An AB nod sequence with a total on-source integration time of 200 s per grating setting was used.
For both grating settings, the G2 V star GSPC P300-E from Colina & Bohlin (1997) was used for
both telluric correction and flux calibration. An additional J-band spectrum was acquired on UT
2006 May 11. On this night a 300 s integration was taken both on-source and off-source, and the
F0 star BD+66 1089 was acquired for telluric correction and flux calibration. Fig. 4 presents the
combined near-infrared spectrum.
4.1. ISS J142950.9+333012: Mid-T Brown Dwarf
The spectrum of ISS J142950.9+333012 shows the classic signatures of a T dwarf. The optical
spectrum in Fig. 3 shows a sharp rise to the longest wavelengths, indicative of a cool temperature
and strong absorption by the pressure-broadened wings of K I(and to some extent Na I). Even
more telling are the J- and H-band spectra in Fig. 4 that show strong CH4and H2O absorption,
the former of which is the hallmark of spectral class T.
As this object has both optical and near-infrared spectra, we can classify on both the optical
and near-infrared classification schemes. The optical typing of T dwarfs is somewhat crude because
the ≤1µm spectra show less variation than at longer wavelengths. Nonetheless, Burgasser et al.
(2003a) have established standards for classes T2, T5, T6, and T8. In the 6000 −10000 ˚
A range the
best diagnostic is the 9300 ˚
A band of H2O. Unfortunately our MARS spectrum has not been telluric
corrected so the depth of this water feature will be influenced by both the earth’s atmosphere as
well as the atmosphere of the brown dwarf itself. This feature in ISS J142950.9+333012 is not as
deep as in the spectrum of a T8, so the true spectral type must be earlier than that. Comparisons
with the T2, T5, and T6 standards obtained with Keck (Burgasser et al. 2003a) show that the
overall slope most resembles that of the T5. Given the coarseness of classification in this wavelength
regime, we can assign only a crude optical spectral type of T5±2.
In the near-infrared the situation is much improved. In this wavelength regime there is a full
set of standards for each spectral subtype from T0 to T8 (Burgasser et al. 2006). Using Keck
– 8 –
Fig. 3.— Optical spectrum of ISS J142950.9+333012, optically classified as a T5±2 brown dwarf,
obtained with the MARS spectrograph on KPNO 4m telescope. The relative flux calibration
was determined from observations of standard stars from the same observing runs with the same
instrumental configurations. The spectrophotometric scale was estimated from the imaging. The
dotted spectrum shows 2MASS J055919.14−140448.8, classified as a T5 brown dwarf at optical
wavelengths (Burgasser et al. 2003a).
NIRSPEC spectra from McLean et al. (2003) of the Burgasser et al. (2006) standards, we find
that the individual J-band spectra best match a type intermediate between T4 and T5. A similar
fit to the H-band data alone gives the identical result. These results point to a solid near-infrared
spectral type of T4.5.
Shown in Figs. 3 - 4 are comparisons of the spectra of ISS J142950.9+333012 and 2MASS J0559−1404,
which is the optical T5 standard and typed as T4.5 on the Burgasser et al. (2006) near-infrared
scheme. (That is, 2MASS J0559−1404 has the same type as ISS J142950.9+333012 in both wave-
length regimes.) Note the similarities between the two spectra. 2MASS J0559−1404 has a well
measured trigonometric parallax of 97.7±1.3 mas (Dahn et al. 2002) and an absolute magnitude of
MJ= 13.75±0.04, which allows us to estimate a distance to ISS J142950.9+333012 of 42 pc, assum-
ing both T dwarfs are single. However, 2MASS J0559−1404 is the most overluminous object in the
early-/mid-T “hump” on the Hertzsprung-Russell diagram (see Vrba et al. 2004; Golimoski et al.
2004), leading some researchers to believe that it might be a close, equal-magnitude double despite
all current evidence to the contrary (Burgasser et al. 2003b; Gelino & Kulkarni 2005; Liu et al.
– 9 –
Fig. 4.— Near-infrared spectrum of ISS J142950.9+333012 obtained with the NIRSPEC spectro-
graph on the Keck II telescope, classified as a Galactic T4.5 brown dwarf from these data. The
relative flux calibration was determined from observations of standard stars from the same observ-
ing runs with the same instrumental configurations. The spectrophotometric scale was estimated
from the imaging. The dotted line shows the infrared spectrum of 2MASS J0559−1404, classified
as a T4.5 brown dwarf in the near-infrared (McLean et al. 2003).
2006). Correcting for this possibility, we find that ISS J142950.9+333012 might be as close as 30 pc.
No other optically classified T dwarfs are known of spectral type T5; the only other T dwarf with
a measured trigonometric parallax and near-infrared type of T4.5 is SDSS J020742.48+000056.2.
The parallax measurement of 34.85±9.87 mas for SDSS J0207+0000 (Vrba et al. 2004) implies
MJ= 14.51 ±0.64, which is very uncertain but lends some weak support to the closer distance
estimate for ISS J142950.9+333012.
The first images to detect the brown dwarf were the NDWFS I-band observations obtained
on UT 2000 April 28, 3.7 yr prior to the IRAC imaging. Comparing ten nearby sources detected
in both the I-band and 3.6µm observations, ISS J142950.9+333012 has a detected proper motion
of 0.
′′1±0.
′′03 yr−1, in a southerly direction. This is comparable in amplitude to the expected reflex
solar motion for a source at ≈40 pc. Interestingly, the star 5.
′′7 east of the brown dwarf shows a
higher proper motion, µ= 0.
′′3±0.
′′03 yr−1in the NW direction. The colors of this R= 22.3 star
(ISS J142951.3+333010) are relatively blue, BW−R= 0.6, R −I= 0.6, suggesting a relatively hot
white dwarf at a distance of several hundred pc, moving at several hundred km sec−1.
– 10 –
The 4.5µm flux of the brown dwarf is 2.7 mag brighter than the survey limit (i.e., V/Vmax =
0.024), whereas the Imagnitude is only 0.9 mag above the limit (V /Vmax = 0.3). This suggests that
the I < 23 requirement imposed to provide robust morphological selection of unresolved sources
is a significant limiting factor. We estimate that our selection criteria restrict our sensitivity to
brown dwarfs of spectral type T3 to T6. The former limit comes from the IRAC color criterion
(Patten et al. 2006). The latter limit comes from available data (J.D. Kirkpatrick et al., in prep.)
suggesting that I-band flux drops dramatically for spectral types cooler than T6. From Vrba et al.
(2004) and Golimoski et al. (2004), the range T3 to T6 corresponds very roughly to Teff = 1500
to 1100 K. Using a model which forms brown dwarfs at a constant rate over 10 Gyr with power
law mass functions of index 0.4 to 1.3 (Reid, Gizis, & Hawley 2002) and the theoretical models of
Burrows, Sudarsky, & Lunine (2003) which give luminosities and Teff as a function of brown dwarf
mass and age, we expect 3 −5 brown dwarfs in the IRAC shallow survey to meet our selection
criteria. Intriguingly, there should be a similar number of dwarfs with Teff <750 K above the [4.5]
flux limit, although our I < 23 requirement would exclude them from the present sample.
Given our desire to understand more fully the physical nature of the L/T transition, the newly
discovered T4.5 brown dwarf can serve as another probe of the overluminosity of the early-/mid-T
hump. Its magnitudes of J= 16.88 and Ks= 16.99 make it a difficult but not impossible target for
a dedicated near-infrared parallax program such as the on-going one at the US Naval Observatory
in Flagstaff (Vrba et al. 2004). More importantly, ISS J142950.9+333012 is the first example of
a field T dwarf selected by mid-infrared photometry supplemented by other ground-based optical
and near-infrared data. This implies that a very similar selection technique to be employed by
the Wide-Field Infrared Survey Explorer (WISE; Eisenhardt & Wright 2003), planned for launch
in 2009, is sound and will be capable of discovering other T dwarfs, and hopefully cooler Y dwarfs
(Kirkpatrick 2003). WISE will sample hundreds of times more volume than the IRAC Shallow
Survey in bands similar to [3.6] and [4.5], and should reveal whether there are brown dwarfs closer
to the Sun than Proxima Centauri.
4.2. ISS J142738.5+331242: z= 6.12 Quasar
The spectrum of ISS J142738.5+331242 (Fig. 5) clearly shows the strong Lyαemission and
strong Lyαdecrement of a z≥6 quasar. At a redshift of z= 6.12, ISS J142738.5+331242 is
emitting when the universe was 0.89 Gyr old, or only 7% of its current age. This is the tenth z≥6
quasar identified to date, with the prior nine identified by the Sloan Digital Sky Survey (SDSS; Fan
et al. 2006). ISS J142738.5+331242 was identified independently by McGreer et al. (2006) using
different selection criteria. Two characteristics separate ISS J142738.5+331242 from the other nine
z≥6 quasars known. First, while the other nine were identified from 6550 deg2of the wide-area,
shallow SDSS optical survey with J-band follow-up, ISS J142738.5+331242 was identified in a more
sensitive, multi-wavelength survey of only 8 deg2. Consequently, this is the least luminous quasar
known at z≈6. Secondly, the Faint Images of the Radio Sky at Twenty-cm survey (FIRST; Becker,
– 11 –
Fig. 5.— Spectrum of ISS J142738.5+331242, a quasar at z= 6.12, obtained with the MARS
spectrograph on the Kitt Peak 4m Mayall telescope. The relative flux calibration was determined
from observations of standard stars from the same observing run with the same instrumental
configuration. As the nights were not photometric, the spectrophotometric scale has been estimated
from the imaging.
White, & Helfand 1995) identifies a source with an integrated flux of 1.03 mJy within 1 arcsec of
the quasar coordinates. ISS J142738.5+331242 is thus the only z≥6 mJy-radio source currently
known.
The evolution of the fraction of quasars which are radio loud, and, in fact, the definition and
very existence of such a dichotomy, has been the subject of substantial literature. Some researchers
prefer a definition based on the radio-optical ratio Rro of the specific fluxes at rest-frame 6 cm
(5 GHz) and 4400 ˚
A (Kellerman et al. 1989). The other common definition divides the populations
at some rest-frame radio luminosity; e.g., Gregg et al. (1996) uses a cutoff value for the 1.4
GHz specific luminosity, L1.4 GHz = 1032.5h−2
50 ergs s−1Hz−1to separate radio loud and radio quiet
sources.14 The latter definition is immune to obscuration from dust, and, as argued by Peacock,
Miller, & Longair (1986) and Miller, Peacock, & Mead (1990), is the more physically meaningful
definition. Based on radio observations of all z > 4 quasars known as of mid-1999 and using the
radio luminosity definition, Stern et al. (2000) found that approximately 12% of quasars are radio
14An Einstein-de Sitter cosmology is assumed.
– 12 –
loud, with no evidence of this fraction depending on either redshift (for 2 ∼
<z∼
<5) or optical
luminosity (for −25 ∼
>MB∼
>−28). For a typical radio spectral index α=−0.5 and an Einstein-
de Sitter cosmology for comparison with previous literature, ISS J142738.5+331242 has a radio
luminosity of L1.4 GHz = 1.33 ×1033 h−2
50 ergs s−1Hz−1, classifying it as radio loud. McGreer et al.
(2006) show that this source is still classified as radio loud based on a radio-optical ratio definition.
ISS J142738.5+331242 is thus the most distant radio-loud quasar known.
ISS J142738.5+331242 is only slightly fainter in luminosity than the z≥6 SDSS quasars, so
we consider all ten z > 6 quasars as a single sample, deferring issues of the likelihood of our having
found such a source in our drastically smaller survey (discussed next). The implication is that the
radio loud fraction remains near 10% out to z≈6.5. Conventional wisdom and morphological
studies suggest that luminous, radio loud AGN are preferentially identified with early-type galaxies
(e.g., McLure et al. 1999). Theory can explain the trend, since early-type galaxies are likely
the products of major mergers and two coalescing supermassive black holes appear necessary to
create black holes of sufficient spin to generate highly collimated jets and powerful radio sources
(e.g., Wilson & Colbert 1995). Assuming the radio loud – luminous host galaxy relation remains
robust at high redshift, the apparent discovery that ≈10% of quasars are radio loud out to the
highest redshifts probed has interesting implications for the formation epoch of massive galaxies.
In hierarchical models of galaxy formation, late-type (less massive) systems form first and mergers
are required to form the early-type (more massive) systems. Eventually, therefore, one expects the
radio-loud fraction of AGNs to fall precipitously with redshift. Our results show this epoch lies
beyond z≈6, providing further evidence for an early formation epoch for massive galaxies. The
stellar masses of i-dropout galaxies in the Great Observatories Origins Deep Survey (Giavalisco
et al. 2004) leads to a similar conclusion from a very different data set and line of argument (Yan
et al. 2005, 2006; Eyles et al. 2006).
How likely was the discovery of this distant quasar in an 8 deg2field? Interpolating the
Ksand 3.6µm photometry for ISS J142738.5+331242 implies mAB [(1 + z)4400˚
A] ≈19.6, or
MAB(4400) = −27.2. For a typical quasar optical spectral index, the conversion between AB-
system MAB(4400) and Vega-system MBis MB=MAB (4400) + 0.12 (e.g., Stern et al. 2000),
implying MB=−27.1 for ISS J142738.5+331242. Our J-band photometry implies a continuum
flux density of ≈18 µJy at 1 µm, or a rest-frame UV luminosity of M(1450) = −26.0, making this
source fainter than any of the z≈6 quasars identified by the SDSS (Fan et al. 2006). McGreer
et al. (2006) found a slightly brighter absolute magnitude, M(1450) = −26.4, likely due to their
alternate methodology whereby a quasar template fit to the IRAC data was used to derive the
rest-frame UV luminosities.
We estimate the number of high-redshift quasars expected from our selection criteria using the
Fan et al. (2004) high-redshift quasar luminosity function, derived from the SDSS. We approximate
high-redshift quasar spectra as step functions, with zero flux below redshifted Lyαand a flat SED
(in fν) redward of Lyα, and we approximate the NDWFS I-band filter as a tophat function. Our
selection criteria restrict our sensitivity to quasars at 5.5∼
<z∼
<6.5. The lower redshift limit comes
– 13 –
from the R−Icolor requirement, determined from the Richards et al. (2006) model discussed in
§3; indeed, the z= 5.39 quasar identifed by Cool et al. (2006) is too blue in R−Ito meet our
selection criteria (Fig. 1). The upper redshift limit corresponds to Lyαshifting out of the I-band
filter. The Fan et al. (2004) luminosity function predicts 3.3 quasars at 5.5< z < 6.5 with I < 23
in our 8 deg2survey. This prediction exactly matches the current results, though, notably, the
faintest of the high-redshift Bo¨otes quasars has I= 22.0, suggesting that more quasars remain to
be discovered with 22 < I < 23 and that the faint end slope of the high-redshift quasar luminosity
function is steeper than currently assumed. Of the 3.3 quasars predicted at 5.5< z < 6.5, only 0.3
are expected to be at z > 6, or, for 12% of quasars being radio-loud (Stern et al. 2000), we only
had a 4% chance of identifying a z > 6 radio-loud quasar in this survey. While it is premature to
make strong claims from this small sample, our results imply possible rapid evolution in the faint
end of the quasar luminosity function and in the radio loud fraction at high redshift.
5. Summary and Future Prospects
We report the discovery of the first mid-infrared selected field brown dwarf and the discovery of
the most distant radio loud source known. Fig. 6 plots the observed SEDs of these two sources, with
a model brown dwarf spectrum from Burrows, Sudarsky, & Hubeny (2006) and a model high-redshift
quasar from Richards et al. (2006). Interestingly, despite nine orders of magnitude difference in
luminosity distance, or nearly 20 orders of magnitude difference in luminosity, the broad-band
optical colors, the broad-band mid-infrared colors, and the multi-wavelength brightnesses of these
two extremely disparate sources are nearly identical. With only BWRI and IRAC photometry, there
is no possibility to separate mid-T brown dwarfs and high-redshift quasars. As shown in Fig. 7,
however, near-infrared photometry offers the possibility to separate the two source types. While
quasars have red J−Kscolors, the latest T dwarfs have blue near-infrared colors, J−Ks∼
<0.5.
We have presented simple color criteria which very efficiently identify astrophysically inter-
esting sources. Using the Bo¨otes 4.5µm-selected catalog, the criteria presented in §3 identify 30
potential candidates which were trimmed down to four robust candidates after visual inspection.
The primary weakness of the criteria is that we have only found objects at the edge of current
observations, not beyond them – e.g., ISS J142950.9+333012 is the among the 50 coldest brown
dwarfs known and ISS J142738.5+331242 is the 6th most distant quasar known. To push to new
territory such as Y dwarfs and z > 7 quasars will require modifying the selection criteria in §3,
and, most likely, surveying more of the celestial sphere. In our current search, the I≤23 criterion
imposed to ensure robust morphological selection of unresolved sources is the most restrictive re-
quirement. Probing deeper would allow the detection of fainter sources, but to identify Y dwarfs
and z > 7 quasars will likely require robust morphological information at longer wavelengths.
We consider the effects of relaxing the selection criteria identified in §3. If we retain the
requirement I≤23 but drop the morphological requirement for a point source, we obtain 434
candidates. Going a magnitude more deeply in I-band, where NDWFS photometry is still robust
– 14 –
Fig. 6.— Spectral energy distributions of the two sources discussed in this paper,
ISS J142950.9+333012, a T4.5 brown dwarf, and ISS J142738.5+331242, a z= 6.12 quasar. Based
on broad-band photometry, the optical and IRAC properties of these two very different sources are
nearly identical; only at near-infrared wavelengths do they differ significantly. The dotted line illus-
trates a model brown dwarf spectrum from Burrows et al. (2006) for Teff = 1000 K, g= 105cm s−2,
solar metallicity, and a modal cloud particle size of 100µm. The solid line illustrates a model quasar
at z= 6.12 from Richards et al. (2006), assuming no flux is detected below redshifted Lyα.
but star-galaxy morphological separation fails, nearly quadruples the number of candidates to 1598
sources. Galaxies at z∼
>1 have red R−Icolors and IRAC [3.6] −[4.5] ≥0.4 (e.g., see model
tracks in Stern et al. 2005), thereby causing significant contamination. At z∼
>7, quasars will fall
out of the NDWFS I-band, so one might think that selecting optical dropouts with IRAC [3.6] −
[4.5] ≥0.4 would provide efficient criteria to identify the most distant quasars. Unfortunately, red
galaxies are again a contaminant: there are nearly 10,000 sources in the Bo¨otes field with [3.6] −
[4.5] ≥0.4 and no detection in the optical passbands. We are currently experimenting with various
schemes to trim these large samples that arise when morphological criteria aren’t available. One
possibility, amenable to searching for cool dwarfs, is to search for sources with blue near-infrared
colors (e.g., J−Ks<0.5), but red colors in [3.6] −[4.5]. The former criterion should identify both
hot and cold Galactic stars, while the latter criterion eliminates the hot stars. Since extragalactic
sources are typically redder in J−Ks(e.g., Fig. 7 in Elston et al. 2006), these criteria should
identify T dwarfs (and colder) irrespective of morphology. Another solution would be to obtain
better morphological measurements, as are available in the (smaller area) Extended Groth Strip
– 15 –
Fig. 7.— Color-color diagrams for cool stars and high-redshift quasars. Black symbols are for
sources in the Bo¨otes field: the asterisk refers to ISS J142950.9+333012 (T4.5 brown dwarf), the
filled circle refers to ISS J142738.5+331242 (z= 6.12 quasar), and the open circle refers to the
z= 5.39 quasar in Cool et al. (2006). The vertical dotted line illustrates the selection criteria
employed in Cool et al. (2006) to identify luminous AGN, [3.6] −[4.5] >0.4. Photometry of M,
L, and T dwarfs from Patten et al. (2006) are plotted in red, as indicated. Photometry of z≈6
SDSS quasars from Jiang et al. (2006) are plotted as inverted-Y’s. The solid blue line illustrates
the colors of the SDSS quasar template from Richards et al. (2006) for 3 ≤z≤7, subject to the
Madau (1995) formulation for the opacity of the intergalactic medium as a function of redshift (the
line becomes thicker for z≥5.5). The vertical line separates high-redshift quasars and dwarfs later
than T3 from the hotter dwarfs. The near-infrared J−Kscolor looks like a promising diagnostic
to separate the coolest brown dwarfs from the high-redshift quasars.
and the COSMOS surveys, or should also be obtainable with the new generation of wide-field,
near-infrared cameras.
Our observations illustrate some of the interesting sources identifiable from wide-area mid-
infrared surveys. After Spitzer has depleted its cryogen, expected to occur in early- to mid-2009,
wide-area 3.6 and 4.5 µm surveys are likely to be an emphasis for the observatory. Shortly thereafter,
the launch of the WISE will provide full-sky, mid-infrared images. Such surveys, combined with
the deep, complementary optical data expected from the Panoramic Survey Telescope and Rapid
Response System (Pan-STARRS; Kaiser et al. 2005) and the Large Synoptic Survey Telescope
(LSST; Tyson et al. 2005), should prove very valuable for studying both the nearest, coldest stars
– 16 –
and for identifying the most distant, luminous quasars. The former will enhance our knowledge of
star formation and Galactic structure. The latter will probe the first cosmic structures, the history
of the intergalactic medium, and literally expand the limits of human knowledge.
We thank Chris Kochanek and Steve Willner for useful comments on the manuscript. This
work is based on observations made with the Spitzer Space Telescope, which is operated by the
Jet Propulsion Laboratory, California Institute of Technology. Support was provided by NASA
through an award issued by JPL/Caltech. This work also made use of images and/or data products
provided by the NDWFS, which is supported by the National Optical Astronomy Observatory
(NOAO). NOAO is operated by AURA, Inc., under a cooperative agreement with the National
Science Foundation. AD and BJ are supported by NOAO. We thank the staff of KPNO and Keck
for their expert assistance with our observations. Research has benefited from the M, L, and T
dwarf compendium housed at http://DwarfArchives.org. The authors also wish to recognize
and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea
has always had within the indigenous Hawaiian community; we are most fortunate to have the
opportunity to conduct observations from this mountain.
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This preprint was prepared with the AAS L
A
T
EX macros v4.0.
– 19 –
Table 1. Photometry of Bo¨otes Field Candidates.
Target BWR I J Ks[3.6] [4.5] [5.8] [8.0] Notes
ISS J142950.9+333012 >27.1>26.1 22.12 16.88 16.99 15.76 15.04 15.05 14.56 T4.5 dwarf
ISS J142738.5+331242 >27.1>26.1 22.03 19.83 18.17 16.57 15.92 15.28 14.81 z= 6.12
ISS J142729.6+352209 >27.1 23.99 21.38 · · · 18.44 17.33 16.70 >15.5>14.8z= 5.53
ISS J142516.3+325409 >27.1 23.76 21.15 · · · · · · 17.41 16.77 >15.5>14.8z= 5.85
Note. — Photometry is all Vega-based, total magnitudes. Optical photometry is from NDWFS. Near-infrared
photometry is from FLAMEX (Elston et al. 2006). Mid-infrared photometry is from the IRAC Shallow Survey
(Eisenhardt et al. 2004). Non-detection limits are the average 5σlimits for the relevant bands across the entire field.
Catalogued FLAMEX near-infrared photometry for the z= 6.12 quasar was corrupted by the bright, neighboring
star. Photometry above comes instead from DAOPHOT analysis of the images, using stars in the field to model the
PSF.
