A Study of External Galaxies Detected by the COBE Diffuse Infrared Background Experiment

Abstract

A comparison of the COBE Diffuse Infrared Background Experiment (DIRBE) all-sky survey with the locations of known galaxies in the IRAS Catalog of Extragalactic Objects and the Center for Astrophysics Catalog of Galaxies led to the detection of as many as 57 galaxies. In this paper, we present the photometric data for these galaxies and an analysis of the seven galaxies that were detected at λ > 100 μm. Estimates of the ratio of the mass of the cold dust (CD) component detected at Td = 20-30 K to a very cold dust (VCD) component with Td ≈ 10-15 K suggest that between 2%-100% of the cirrus-like CD mass can also exist in many of these galaxies as VCD. In one galaxy, M33, the DIRBE photometry at 240 μm suggests as much as 26 times as much VCD may be present as compared to the cirrus-like component. Further submillimeter measurements of this galaxy are required to verify such a large population of VCD. We also present 10 galaxies that were detected in the sky region not previously surveyed by IRAS and that can be used to construct a flux-limited all-sky catalog of galaxies brighter than 1000 Jy with a modest completeness limit of about 65%.

Full text

THE ASTROPHYSICAL JOURNAL, 500:554È568, 1998 June 20
1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.(
A STUDY OF EXTERNAL GALAXIES DETECTED BY THE COBE DIFFUSE INFRARED
BACKGROUND EXPERIMENT
STEN ODENWALD
Raytheon STX, Code 630.0, Goddard SFC, Greenbelt, MD 20771
JEFFREY NEWMARK
Applied Research Corporation, Landover, MD 20785
AND
GEORGE SMOOT
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720
Received 1995 October 6; accepted 1998 January 29
ABSTRACT
A comparison of the Di†use Infrared Background Experiment (DIRBE) all-sky survey withCOBE1
the locations of known galaxies in the IRAS Catalog of Extragalactic Objects and the Center for Astro-
physics Catalog of Galaxies led to the detection of as many as 57 galaxies. In this paper, we present the
photometric data for these galaxies and an analysis of the seven galaxies that were detected at j[100
km. Estimates of the ratio of the mass of the cold dust (CD) component detected at K to aTd\20È30
very cold dust (VCD) component with K suggest that between 2%È100% of the cirrus-likeTdB10È15
CD mass can also exist in many of these galaxies as VCD. In one galaxy, M33, the DIRBE photometry
at 240 km suggests as much as 26 times as much VCD may be present as compared to the cirrus-like
component. Further submillimeter measurements of this galaxy are required to verify such a large popu-
lation of VCD. We also present 10 galaxies that were detected in the sky region not previously surveyed
by IRAS and that can be used to construct a Ñux-limited all-sky catalog of galaxies brighter than 1000
Jy with a modest completeness limit of about 65%.
Subject headings: di†use radiation È dust, extinction È galaxies: ISM È galaxies : photometry È
infrared: galaxies È surveys
1.INTRODUCTION
The Infrared Astronomical Satellite (IRAS) has shown
that many galaxies are strong sources of far-IR radiation,
particularly at 60 and 100 km Houck, & Neuge-(Soifer,
bauer The nature of galactic far-IR emission among1987).
late-type spirals is widely acknowledged to be radiation
reprocessed by dust grains present in a variety of sizes and
temperatures in the interstellar medium & Mezger(Cox
Boulanger, & Puget This radiation may1989; Desert, 1990).
have as its origin the UV-rich light from new generations of
OB stars in spiral arms, the nonthermal continuum produc-
ed by an active nuclear source & Patten or(Knapp 1991),
the di†use interstellar radiation Ðeld itself Mezger,(Mathis,
& Panagia 1983).
An open question regarding the infrared properties of
““ normal ÏÏ galaxies is the number of dust components con-
tributing to their far-IR luminosities. We deÐne very cold
dust (VCD) as dust having K, cold dust (CD) asTd\15
K, and warm dust (WD) with K. The15 ¹Td\30 Tdº30
continua of galaxies often appear to be dominated by one
or more of these thermal components at far-IR and sub-
millimeter wavelengths.
A preliminary study of our Milky Way using the COBE,
far-IR absolute spectrophotometer (FIRAS; et al.Wright
implies a two-component continuum model with a1991)
dust emission index la,ofa\2.0, and andTCD \20.4
1The National Aeronautics and Space Administration/Goddard Space
Flight Center (NASA/GSFC) is responsible for the design, development,
and operation of the COBE. ScientiÐc guidance is provided by the COBE
Science Working Group. GSFC is also responsible for the development of
the analysis software and for the production of the mission data sets.
K. The VCD component, however, contributedTVCD \4.77
only 0.11% of the total dust emission. et al.Sodroski (1994)
used COBE DIRBE all-sky data to show that the CD com-
ponent is closely associated with the di†use H Iclouds and
cold molecular clouds that make up the far-IR ““ cirrusÏÏ
discovered by IRAS at 100 km et al.(Soifer 1984).
et al. and & Krugel investigatedChini (1986) Chini (1993)
a sample of SbÈSc spiral galaxies. They also found two
thermal components in the far-IR; one WD component
typically has K, while a cooler CD component hasTdB53
K. Wynn-Williams, & DuncanTdB16 Eales, (1989)
included in their study several of the galaxies observed by
et al. but were unable to reach the same conclu-Chini (1986)
sions, Ðnding only conclusive evidence for a single CD com-
ponent. & Sauvage reviewed the results fromThuan (1992)
a variety of surveys and through a series of cross-
correlations between the CfA Galaxy Catalog et al.(Huchra
hereafter the CfA Catalog) and the Hasurvey of gal-1992 ;
axies by and showed that the expectedKennicutt (1983)
correlation between star-forming activity and total far-IR
luminosity does not occur in the case of ““ normal ÏÏ spiral
galaxies. Star-forming regions are apparently less important
in determining the integrated far-IR luminosity of a normal
(nonÈactive galactic nucleus, nonstarburst) spiral galaxy
than some other parameter of the galaxian interstellar
medium. They suggest that CD grains associated with
cirrus-like dust at K may be the principle source ofTdB20
far-IR luminosity at j[60 km. This is consistent with the
Ðndings of & Schwering and et al.Walterbos (1987) Rice
who showed that 85% of the far-IR emission from(1990)
M31 (Sb-type) and 40% of the far-IR emission from M33
(Scd) are associated with cirrus-type dust. et al.Sodroski
554

EXTERNAL GALAXIES DETECTED BY COBE 555
show a similar trend for the Milky Way (SbÈSc) with(1994)
the Galactic plane cirrus component contributing as much
as 90% of the far-IR luminosity.
Newmark, & Smoot (1995, hereafter Paper I)Odenwald,
reported the detection of 56 galaxies between 12 and 240
km based on a comparison of the IRAS Catalog of Extra-
galactic Objects & Lonsdale hereafter the(Fullmer 1989;
IRAS Catalog) and the CfA Catalog, with the all-sky
DIRBE survey. DIRBEÏs large beam size of makes it an0¡.7
ideal instrument for determining the integrated far-IR Ñux
from an entire galaxy, not just its bright nuclear regions.
With the exception of M31 and the Magellanic Clouds, all
galaxies bright enough to be detected by DIRBE will be
observed as unresolvable point sources so that all photo-
metric measurements of external galaxies are measurements
of the integrated total emission. DIRBEÏs photometric
overlap with the IRAS 12È100 km bands and additional
observations at 140 and 240 km provide important con-
straints to the strength of any dust components with tem-
peratures in the range from (K) \25. Not all of the15 \Td
galaxies listed in Paper I were detected at 140 or 240 km,
but a signiÐcant subset of seven galaxies was bright enough
to have measurable emission at either or both of these
wavelengths.
In this paper, we continue an analysis of the DIRBE
galaxy sample by obtaining photometry for each galaxy in
the DIRBE 12È240 km bands. We will then use the photo-
metric data to constrain the quantities of dust in the
detected galaxies, which in the far-IR produce signiÐcant
emission between 100 and 240 km.
2.OBSERVATIONS
Between 1989 December and 1990 September the DIRBE
instrument on board NASAÏs COBE satellite surveyed the
entire sky in 10 photometric bands covering the wavelength
region from 1.25 to 240 km. A detailed description of the
DIRBE instrument and the COBE mission is given by
et al. An extensive discussion of the absoluteBoggess (1992).
calibration of the DIRBE photometry may be found in
COBEÏs DIRBE Explanatory Supplement et al.(Hauser
but for convenience we will brieÑy review some of the1995),
salient issues that a†ect galaxy photometry.
The two-dimensional beam proÐles in the 12È100 km
bands were determined by measuring the intensity changes
of numerous bright stars during the course of the entire
mission as they transited the beam. For the 140 and 240 km
bands, transits of the planets Jupiter and Saturn were used.
The instantaneous DIRBE beam proÐle in each band is
constrained by an internal Ðeld stop to The beam0¡.7]0¡.7.
solid angles for the 12È240 km bands are (1.42, 1.48, 1.51,
1.44, 1.36, and 1.33) ]10~4 sr with uncertainties of (4, 5, 15,
12, 26, and 37) ]10~7 sr, respectively.
The COBE attitude control system provides information
on the instantaneous pointing direction of the satellite
throughout the mission. This information is provided by a
combination of Sun sensors, Earth horizon sensors, rate-
integrating gyros, and magnetometers and is used to estab-
lish an attitude solution for each detector as a function of
observing time. The uncertainty in the attitude solution is
based on sightings of speciÐc calibration objects in the0@.93
near-IR 1.25È4.9 km bands.
The DIRBE photometry has been corrected for detector
gain variations due to environmental inÑuences and instru-
ment instabilities and then calibrated to physical units.
Observations of approximately 144 celestial calibrators
(e.g., stars, planetary nebulae, H II regions) are then used to
establish a relative photometric system that is stable over
the duration of the mission. This relative photometric scale
is then converted to an absolute scale by observing a small
sample of well-known discrete sources.
As described in the DIRBE Explanatory Supplement
(°4.5.3.3) the bright star Sirius was used to calibrate the 12
km band, the planetary nebula NGC 7027 was used for the
25 km band, the 60 and 100 km bands were calibrated by
using the planet Uranus, and Jupiter was used for the 140
and 240 km bands. The quadrature sums of the contribu-
tions to the absolute Ñux calibration in the 12È240 km
bands are 16, 9, 14, 10, and 10 percent, respec-CF\12,
tively. The primary source of the uncertainty is in the
adopted brightnesses of Sirius, NGC 7027, and the planets
Jupiter and Uranus.
The initial detection search described by et al.Odenwald
was conducted using the cold mission, averaged(1995)
sky-map data. The positions searched for galaxy detections
were obtained from the IRAS Catalog. We also searched
the CfA Catalog for detections in the 4% of the sky not
covered by the IRAS survey that according to the CfA
Catalog included 1251 optically identiÐed galaxies brighter
than ]14.5 mag. Initial estimates of the point-source Ñux in
the sky-map data were obtained by removing a simple two-
dimensional background model from the position of each
galaxy in the sky map.
In order to be considered a detection, the candidate had
to be visible at a signal-to-noise ratio (S/N) of at least 5.0
after background removal in a single band, or at a S/N
exceeding four in at least two bands. In the latter case, the
e†ective S/N [4(2)1@2\ 5.6 matched or exceeded the
single-band threshold. The search was then considered to be
complete to a uniform S/N \5.
As the IRAS survey discovered (see et al.Low 1989),
much of the sky at j[60 km is covered by interstellar,
infrared cirrus clouds that can have considerable internal
structure from a few arcminutes to many degrees in scale.
Large-beam observations of such a background can lead to
spurious detections of unresolved cirrus cloud features, as
well as bona Ðde noncirrus background sources. If a candi-
date satisÐed the S/N criterion, the surrounding 7¡ ]7¡
Ðeld was then inspected visually to ensure that the candi-
date could not plausibly be assigned to an unrelated cirrus
feature. We also consulted the Catalog of Infrared Obser-
vations et al. to eliminate prominent, Galactic(Gezari 1993)
IR sources that may coincide with the 3 ]3 pixel DIRBE
beam patch centered on the galaxy position. The initial
search for extragalactic detections produced 53 candidates
in the IRAS Catalog and three candidates in the CfA
Catalog that were detected by DIRBE at j[60 km. We
also include M31 that was detected and resolved by
DIRBE.
3.GALAXY DETECTIONS AND PHOTOMETRY
3.1. T he Final Catalog
In the results of the initial search in areFigure 1 Paper I
presented. Each image represents a 7¡ ]7¡ Ðeld centered on
the pixel coinciding with the cataloged galaxy position. A
second-order background model has been removed from
each Ðeld, and the pixel intensities have been rescaled so
that the intensity of the pixel nearest the galaxy position is
normalized to 1.0. This particular image transformation,

FIG.1a
FIG. 1.È(aÈf ) Sky regions surrounding each of the DIRBE galaxies identiÐed in Tables and The Ðeld of view in each band is 21 ]21 pixels at a12.
resolution of 21@pixel. The images in each band have been logarithmically stretched to facilitate the characterization of the degree of background clutter.
556

FIG.1b
557

FIG.1c
558

FIG.1d
559

FIG.1e

EXTERNAL GALAXIES DETECTED BY COBE 561
FIG.1f
used only in the inspection for foreground clutter, optimizes
the contrast of the foreground clutter in the Ðeld.
It is evident that many of the Ðelds contain signiÐcant
structure at 60 and 100 km. The degree of cirrus contami-
nation for each galaxy Ðeld is determined in our automated
search of the several thousand galaxies in the IRAS and
CfA galaxy catalogs by a set of quality indices based on
the Point Source Catalog (Version 2 hereafterIRAS 1988,
PSC) Ñags: SES1, SES2, CIRR1, CIRR2, and CIRR3.
According to the Explanatory SupplementIRAS (1988,
°VII-36.H.1.b), these indices warn of the presence of
extended, Galactic foreground structure, the total number
of sources detected at 100 km in a 1 square degree box
centered on the IRAS point source, and the degree of sig-
niÐcant cirrus contamination. From a consideration of
these indices and the S/N of the DIRBE emission at the
nominal positions of the candidate galaxy, we identiÐed 57
IRAS galaxies that were also detected by DIRBE and
present the corresponding DIRBE Ðelds in Figure 1.
3.2. Point-Source Photometry
The DIRBE sky maps have been optimized for preser-
ving the photometric accuracy of extended emission at
scales larger than the DIRBE beam. The time-ordered data
is the preferred data to use to obtain accurate point-source
photometry; however, this is a time-consuming procedure
beyond the scope of the present study. Point-source photo-
metry used in this study is derived from the cold, mission-
averaged DIRBE sky-map data. A correction factor is
applied to the peak emission detected toward a galaxy to
reÑect the true beam response.
Because of the pixelization procedure, the peak IR emis-
sion from a galaxy or other unresolved source identiÐed in
the sky-map data may be up to pixel from its nominal^1
2
optical position. Moreover, since the DIRBE beam-
response function is not Ñat over its full Ðeld of view, this
can cause the galaxy emission peak detected by DIRBE to
be as much as 1 pixel di†erent from the true galaxy position.
We have compensated for these e†ects by calculating the
response of the DIRBE beam in each band at the location
of the galaxy rather than at the sky-map position by using
an azimuthally averaged template of the beam response
computed from many transits of calibrator sources across
the beam (N. Odegard 1995, private communication). From
the beam template at the position of the optical galaxy, we
determined the responsivity of the detector, and thenRb,
used the sky-map Ñux density at this pixel, to obtain aSp,
corrected peak Ñux density, Typically, 0.9 \Sc\Sp/Rb.
so the correction was, generally, less than 10%.Rb\1.0
The results of this photometric analysis are presented in
for the galaxies detected at 140 and 240 km and inTable 1 for the remaining galaxies with upper limits at 140Table 2
and 240 km. Column (1) is the galaxy name; column (2) is
the galaxy type; column (3) is the maximum B-band angular
size in arcminutes of the optical galaxy; columns (4)È(9) give
the background-subtracted and beam-response corrected
DIRBE peak Ñux densities, in janskys or their 3 pupperSc,
limits.
A color correction, F\C]F(DIRBE), must be applied
to the photometric values in Tables and if the source12
e†ective temperatures are di†erent from that of the DIRBE
absolute calibrators. Tables of these correction factors may
be found in the DIRBE Explanatory Supplement (e.g.,
Appendix B). For a dust grain emission that follows lawith
a\1.5 and K, the correction factors for theTd\25
DIRBE 60È240 km bands are C\[0.81, 0.96, 0.92, 1.05]. A
variation in the estimated temperature of ^5¡ leads to an
uncertainty in the correction factors by ^3%.
The overall uncertainty in each band consists of the
quadrature sum of the DIRBE absolute calibration of

562 ODENWALD, NEWMARK, & SMOOT Vol. 500
TABLE 1
PHOTOMETRY OF DETECTED IRAS GALAXIES
Size S(12) S(25) S(60) S(100) S(140) S(240)
Galaxy Type (arcmin) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy)
(1) (2) (3) (4) (5) (6) (7) (8) (9)
M31 ............. Sb IÈII D180 \100 \100 700 3706 7545 6242
M33 ............. Sc IIÈIII 70.8 \240 \331 422 1272 1176 1131
M63 ............. Sbc 12.6 \46 \39 42 183 909 452
M82 ............. Amorp. 11.2 58 240 1790 2345 1387 \1158
M83 ............. SBc II 12.9 \185 \350 319 751 949 414
NGC 253 ....... Sc(s) 25.1 \164 \282 1417 2863 2270 925
NGC 4945 ...... Sc 20.0 \81 \97 722 1604 2207 778
NOTE.ÈDIRBE-integrated photometry for M31 at 1.25, 2.2, 3.5, and 4.9 km is as follows: 534, 461, 245, and
128 Jy, respectively.
B10% described in and the uncertainty in the°2
background-subtraction procedure that is B15%, which
gives a total uncertainty for the Ñuxes in Tables and of12
B18%. Including the color-correction uncertainty in quad-
rature increases the over all absolute photometric uncer-
tainty of the DIRBE photometry to B20%.
As we can see in column (3), the maximum angular sizes
of the galaxies are, with few exceptions, smaller than the 42@
TABLE 2
IRAS GALAXIES NOT DETECTED AT 140 OR 240 MICRONS
Size S(12) S(25) S(60) S(100) S(140) S(240)
Galaxy Type (arcmin) (Jy) (Jy) (Jy) (Jy) (Jy) (Jy)
(1) (2) (3) (4) (5) (6) (7) (8) (9)
M51 ............... Sbc IÈII 11.2 \26.4 \28.7 130.9 411.3 \916.7 \569.3
M61 ............... Sc(s) I.2 6.0 \400.6 \495.9 \168.0 99.6 \780.2 \601.3
M66 ............... Sb II.2 9.1 \250.9 \411.7 \188.2 209.4 \1017.3 \600.4
M94 ............... RSab 11.2 \48.3 \42.4 71.0 166.8 \1173.6 \547.9
M101 ............. Sc(s) I 28.8 \16.9 12.2 82.8 259.0 \530.3 \497.5
NGC 55 .......... Sc 32.4 \112.6 \129.6 89.4 212.0 \1149.2 \442.8
NGC 134......... Sbc IIÈIII 8.1 \137.4 \156.7 17.1 78.0 \781.2 \278.5
NGC 300......... Sc 20.0 \55.2 \56.1 23.0 100.5 \892.5 \467.8
NGC 613......... SBb II 5.8 \76.6 \84.4 41.5 80.8 \1065.4 \605.9
NGC 891......... Sb 13.5 \65.7 \148.6 60.3 251.1 \1049.3 \707.6
NGC 838......... Irr(pec) 1.7 \215.2 \230.7 \85.4 61.2 \1102.8 \300.9
NGC 986......... SBab 3.7 \25.3 \29.4 31.9 65.0 \456.0 \193.3
NGC 1072........ SB(pec) 1.7 \238.5 \429.9 240.9 401.1 \899.7 \228.8
NGC 1097A ...... RSBbc IÈII 9.3 \44.0 \43.0 59.4 138.4 \809.5 \311.7
NGC 1313........ SBc IIIÈIV 9.1 \9.0 \13.5 65.7 294.3 \1053.3 \762.1
NGC 1365........ SBb I 9.8 \28.0 \39.4 95.2 223.0 \979.9 \312.9
NGC 1385........ Sc III 3.0 \84.0 \110.2 23.4 55.2 \922.4 \347.2
NGC 1448........ Sc(II) 8.1 \57.9 \38.1 \15.5 35.8 \389.6 \375.6
NGC 1808........ Sbcpec 7.2 \67.3 \79.0 116.5 207.2 \799.0 \390.2
NGC 2403........ Sc(s) 17.8 \94.2 \95.5 71.3 142.6 \591.1 \470.6
NGC 2798........ Sapec 2.8 \335.9 \404.4 48.3 42.1 \1372.1 \144.0
NGC 3079........ SBm 7.6 \67.1 \71.3 51.8 127.7 \1266.5 \236.4
NGC 3256........ LIRG 3.5 \56.4 \68.5 110.8 \281.2 \732.8 \368.8
NGC 3310........ Sbcpec 3.6 \45.3 \51.0 46.8 86.3 \1104.3 \487.1
NGC 3556........ Sc III 8.3 \33.8 \48.1 38.4 103.9 \580.9 \632.8
NGC 3621........ Sc(s) II.8 10.0 \126.0 \175.2 55.2 153.2 \1020.5 \490.9
NGC 4030........ SAbc 4.3 \441.3 \553.4 163.3 109.7 \947.2 \549.4
NGC 4038........ Scpec 2.6 \257.2 \377.8 \100.5 88.8 \919.7 \485.5
NGC 4102........ Sb II 3.2 \14.3 \17.3 62.2 89.6 \260.7 \291.6
NGC 4485........ S(tidal) 2.4 \28.2 \39.0 57.5 115.9 \532.8 \480.3
NGC 4559........ Sc(s) II 10.5 \78.8 \87.6 \33.0 42.7 \646.3 \293.3
NGC 4631........ Sc 15.1 \67.6 \73.2 102.8 257.9 \795.6 \408.1
NGC 5005........ Sb(s) II 5.4 \36.5 \53.7 19.5 68.3 \1125.5 \328.1
NGC 5128........ S0]Spec 18.2 \85.7 \176.4 267.0 641.2 \1316.2 \939.2
NGC 7552........ SBbc IÈII 3.5 \37.7 \27.8 60.8 147.7 \1092.3 \338.8
NGC 7582........ SBab 4.6 \184.4 \216.1 63.1 133.8 \595.1 \422.2
NGC 7793........ Sd(s) IV 9.1 \102.0 \94.0 22.8 69.5 \1257.8 \634.1
Arp 220........... LIRG 1.5 \64.6 \91.5 112.5 143.0 \198.9 \483.0
IC 342 ............ Scd 17.8 \58.7 \76.9 253.2 555.9 \1517.3 \869.6
IC 694 ............ Sc]Sc 1.2 \26.9 \42.6 133.4 152.5 \665.3 \257.9
IC 5179 ........... Sc(s) II 2.3 \319.2 \407.5 60.5 71.0 \1167.1 \285.3
Mrk 201 .......... ... 1.8 \26.8 \28.8 26.7 \61.0 \555.2 \162.7
Mrk 231 .......... LIRG 1.3 \35.7 16.0 29.9 24.5 \449.7 \275.1
Mrk 273 .......... ... 1.1 \23.6 \7.8 20.2 \37.4 \339.3 \209.4
M106 ............. Sb(s) 18.6 \98.0 \133.6 20.4 46.6 \721.3 \623.0

No. 2, 1998 EXTERNAL GALAXIES DETECTED BY COBE 563
TABLE 3
DIRBE GALAXY CANDIDATES IN IRAS UNSURVEYED REGION
S(60) S(100) S(140) S(240)
Galaxy Type lb(Jy) (Jy) (Jy) (Jy)
(1) (2) (3) (4) (5) (6) (7) (8)
Strong Candidates
NGC 92........... Sa 315.9 [67.9 38.2 49.8 \334.4 \85.2
NGC 3588 ........ ... 224.6 ]66.8 55.1 49.2 \537.4 \221.7
IC 749 ............. Sc 153.5 ]71.0 15.3 50.9 \131.4 \95.1
IC 751 ............. Sc/Irr 154.5 ]71.2 21.6 48.5 \236.5 \368.1
C0943[3008 ...... Irr 262.1 ]17.3 87.9 189.5 \449.7 \260.2
C1148]3909 ...... Irr 165.6 ]72.7 18.9 53.6 \255.0 \103.0
C1204]3930 ...... Sbc 157.9 ]74.7 25.3 57.1 \370.1 \270.3
C2139]3137 ...... ... 82.4 [15.7 \44.9 \95.6 \818.4 415.9
C2205]1812 ...... ... 77.5 [29.7 38.2 42.8 \576.1 \297.9
C2230]0750 ...... Irr 74.4 [41.4 \12.7 113.6 592.7 376.9
Less ConÐdent Candidates
NGC 3433 ........ Sc 238.2 ]57.0 147.8 103.0 \458.5 \32.9
NGC 3476 ........ ... 241.2 ]57.6 161.7 86.8 \52.8 \164.6
C0004[4144 ...... ... 332.0 [72.9 18.5 41.6 310.1 \296.7
C0004[4138 ...... ... 333.2 [73.1 \19.9 41.2 263.8 \109.9
C0007[4641 ...... Scd 323.0 [69.2 19.3 34.6 691.1 \344.2
C1033]0021 ...... Scd 247.3 ]47.7 \75.8 144.1 1090.5 \415.4
C1100]0752 ...... ... 245.3 ]57.8 122.4 104.0 \271.8 \320.2
C1208]3957 ...... ... 154.1 ]74.8 \29.1 70.1 \429.5 386.0
C2136]2936 ...... Sc 80.5 [16.8 62.6 197.9 684.1 \183.9
DIRBE beam. For the purposes of galaxy photometry, this
large beam size implies that all but the nearest galaxies (e.g.,
M31) will remain unresolved, and we do not make any
allowances for Ñux outside the DIRBE beam. All of the
Ñuxes listed in Tables and with the exception of M31,12,
which was resolved, are ““ peak ÏÏ Ñuxes of the brightest pixel
coincident with the nominal galaxy position.
3.3. Galaxies Not Included in IRAS Survey
There are two gaps in the IRAS extragalactic survey at
ecliptic latitudes from b\^45¡ and ecliptic longitudes
centered at j\330¡ and j\150¡. From the CfA Catalog,
there are 1251 galaxies in these IRAS-unsurveyed regions.
DIRBE has detected signiÐcant emission (total S/N in all
detected bands [5.0) from 19 of these galaxies that we list
in along with their appropriate upper limits orTable 3
background-corrected peak Ñuxes in each band in columns
(5)È(8). No independent conÐrmation of their far-IR emis-
sion currently exists in the literature. We expect that many
of the weak candidates may ultimately be found to be spu-
rious. We have also included in column (2) the morphologi-
cal type of each galaxy listed in the CfA Catalog, as well as
the galaxyÏs galactic longitude and latitude in columns (3)
and (4).
We see from the available galaxy classiÐcations that the
preponderance are spirals. As for the galaxies identiÐed in
Tables and all of the candidates were detected in more12,
than one band. The galaxies C0004[4144, C0004[4138,
C0007[4641, C1033]0021, C1208]3957, and
C2136]2936 had emission at 140 or 240 km, which was
considerably higher than at 60È100 km, which seems unrea-
sonable based on the measured spectra of the seven DIRBE
galaxies in that have the same morphological types.Table 1
Also, the three galaxies NGC 3433, NGC 3476, and
C1100[0752 are considerably brighter at 60 km than at
100 km, which also does not agree with typical detected
galaxies in We consider this ensemble of nine gal-Table 1.
axies to be photometrically suspect in the bands for which
the highest Ñuxes were determined. This leaves 10 new
galaxies detected by DIRBE in the region not surveyed
by IRAS, with measured continua that appear to be physi-
cally reasonable compared to the presumed archetypes in
Table 1.
We conclude that DIRBE has detected at least 10 new
far-IR emitting galaxies in a region of the sky not previously
surveyed by IRAS. The remaining nine galaxies may also
include among them additional, weak far-IR galaxies on the
basis of the large number of late-type spirals that appear to
dominate the list of DIRBE candidate galaxy detections.
To gauge the completeness of the DIRBE search, in
the galaxies listed in Tables and were comparedTable 4 1 2
to the listing of IRAS galaxies with S(100) [50 Jy. There
are 14 IRAS galaxies brighter than 150 Jy, of which only
Ðve were found by DIRBE, all were in DIRBE Ðelds
showing signiÐcant foreground confusion at the DIRBE
beam resolution. We conclude that for S(100) [150 Jy, the
DIRBE survey is only 35% complete. For S(100) [1000 Jy,
the completeness is about twice as high, but su†ers from
small number statistics. Given the extent to which DIRBE
is a†ected by foreground clutter, any combined IRAS/
DIRBE galaxy catalog would be signiÐcantly incomplete at
virtually all Ñux levels in any Ñux-limited catalog.
TABLE 4
COMPLETENESS ESTIMATES
Range Detections
(Jy) IRAS Number DIRBE Number (%)
(1) (2) (3) (4)
0È49.......... 209 16 8
50È99 ........ 45 13 29
100È149...... 20 13 65
150È249...... 5 2 50
250È499...... 4 1 25
500È999...... 2 0 0
[1000 ....... 3 2 66

564 ODENWALD, NEWMARK, & SMOOT Vol. 500
FIG. 2.ÈColor-corrected IRAS and DIRBE photometry at 60 and 100
km compared for IRAS galaxies detected by DIRBE. The points are
plotted with error bars representing the estimated photometric uncer-
tainties of 10% for the IRAS photometry and 20% for the(Rice 1988)
DIRBE photometry (see °2).
3.4. Comparison to IRAS
In terms of the photometry of known IRAS galaxies, we
have compared the published IRAS-integrated Ñuxes at 60
and 100 km to the corresponding color-corrected (Td\25
K, a\2.0) DIRBE-integrated Ñuxes for the 24 brightest,
unresolved galaxies detected by DIRBE and show the
results in The IRAS Ñuxes were obtained from theFigure 2.
survey of large optical galaxies detected by Rice (1993),
et al. and & Malkan TheSoifer (1989), Spinoglio (1989).
most recent published integrated IRAS Ñuxes have been
used based on co-added, area-integrated studies with the
Ñuxes reported in the photometric scale. For thePSC
DIRBE-detected galaxies that the surveys have in common,
the ratio, R, of the DIRBE to IRAS-integrated Ñuxes at 60
km was 0.94 ^0.12. This suggests that for these galaxies,
the DIRBE and IRAS photometry are consistent with no
statistically signiÐcant di†erence. The dispersion in the rela-
tive absolute photometry between the two surveys at 60
km in is about 15%, which is similar to what wasFigure 2a
estimated in We Ðnd in however, that the°3.2. Figure 2b,
100 km photometry can be Ðtted by a linear regression, with
the IRAS photometry about 10% lower than the corre-
sponding DIRBE photometry, and that following a com-
pensation for this e†ect, R\1.13 ^0.16 at 60 km.
3.5. T he Andromeda Galaxy: M31
The Andromeda galaxy was the only galaxy (excluding
the Large and Small Magellanic Clouds) large enough rela-
tive to the DIRBE beam size to be resolved. showsFigure 3
the Ðelds spanning 5¡ ]5¡ in each of the DIRBE bands.
The resolution is too poor to see any details; however,
the apparent ellipticity of the image is somewhat higher at
the shorter wavelengths. It is evident from that atFigure 3
short wavelengths the nuclear bulge component appears to
play a larger role in shaping the overall image that is seen,
while at longer wavelengths the disk component predomi-
nates to a greater degree with increasing wavelength, giving
the image a Ñatter shape. At 3.5È4.9 km the Rayleigh-Jeans
component of the stellar emission is minimal, and at longer
wavelengths emission produced by star-forming regions
and interstellar dust in the disk of the galaxy predominate,
which accounts for the signiÐcant decrease in image bright-
ness near 12È25 km. The integrated photometry of M31
from DIRBE is presented in and was obtained byTable 1
subtracting from each image pixel a smooth two-
dimensional background Ðt constrained by the o†-galaxy
pixels. The image intensities in each pixel were then
summed over all pixels and multiplied by the calibrated
DIRBE beam solid angles in each band to obtain integrated
Ñux densities in janskys.
4.DUST EMISSION
The galaxies not detected at 140 and 240 km provide little
information for dust components cooler than K,Td\40
which peaks at 100 km. We will, therefore, restrict our dis-
cussions to only those seven galaxies detected beyond 100
km. The spectra of these galaxies are presented in Figure 4
based on their integrated Ñux densities in each DIRBE
band.
We assume that the dust emissivity follows the canonical
ladust emissivity law. We also assume, following et al.Eales
and et al. that a\2.0. For a galaxy(1989) Wright (1991),
for which photometry exists in two or more bands, the
photometry can be Ðtted to obtain estimators for the dust
temperature, and the optical depth, at 100 kmbyT
d
,q
100,
using a dust emission function of the form
I(l)\q100(l/l100)2.0B(l,Td) , (1)
where B(l, is the Planck function evaluated at a tem-Td)
perature and a frequency l. The color correctionsTd
described in have been applied to the DIRBE photo-°3.2
metric data. The dust-model Ðts were computed for the
color-corrected DIRBE Ñuxes to obtain the Ðnal Ðtted dust
temperatures and optical depths for each galaxy. Since the
galaxies in have detected Ñuxes at j[100 km, it isTable 1
feasible to compute two-component Ðts to their spectra to

No. 2, 1998 EXTERNAL GALAXIES DETECTED BY COBE 565
FIG. 3.ÈCOBE/DIRBE image of M31. Each Ðeld represents 5¡ ]5¡ of the DIRBE, mission-averaged data sets in each band. The DIRBE images have
been normalized to 1.0 for the brightest pixel and logarithmically stretched so that the residual background emission after background subtraction can be
seen. The peak intensities after background subtraction are 2.2, 1.9, 1.1, 0.55, 1.8, 1.9, 9.1, 23.6, and 17.2 MJy sr~1 for the 1.25È240 km DIRBE bands. The
stretching factors in these logarithmic gray-scaled images are 7:1 for 1.25È3.5 km ; 5 :1 for 4.9 km; 1:1 for 12, 25, 140, 240 km; 3:1 for 60 km; and 4:1 for
100 km.
determine the actual contribution by VCD with near 15Td
K. shows the results of such a Ðtting operation.Figure 4
Solid lines indicate the best Ðt for a two-component model,
while the dashed line indicates the best one-component
model. The result of this Ðtting procedure is also presented
in Columns (1)È(3) are the galaxy name, morpho-Table 5.
logical class, and distance in megaparsecs; column (4) gives
the single component Ðtted temperature; and columns (5)È
(8) give the two-component Ðtted temperatures and Ñuxes.
The Ñux density in column (6) is the predicted Ñux of the
presumably cirrus-like CD component evaluated at 100 km,
and column (8) is the predicted Ñux of only the presumed
VCD-like component evaluated at 240 km. The Ðtting
uncertainties in the temperatures and Ñuxes are ^3 K and
^10%, respectively, which deÐnes the range of models that
lead to similar Ðts within the range of the 1 perror bars in
each band.
5.DUST MASSES
Using the two-component thermal Ðts as an initial model,
we can investigate whether signiÐcant VCD may be present
in these galaxies based on the detections at 140 and 240 km.
Blackbody dust grains with K have their peak emis-Td\15
sion at 245 km and contribute only 25% of their peak emis-
sion at 100 km, which means that the DIRBE 240 km band
is optimally placed to measure the strength of this thermal
component. By subtracting the Ðtted thermal model from
the measured continuum we can estimate, from the residual
240 km emission or its upper limit, a mass limit for this
VCD that would be consistent with the galaxy continua and
upper limits as measured by DIRBE.
For dust grains with an emissivity coefficient, theQlPl1,
dust mass based on the galaxyÏs measured infrared Ñux
density, the estimated dust temperature, and its distance is
given by & Patten and Andreani,Knapp (1991) Clements,
& Chase as(1993)
Md\4.8S100D2(e144@T[1) , (2)
where is expressed in units of solar mass; is theMdS100
residual Ñux density, or its upper limit, at 100 kmin
janskys; Dis the distance to the galaxy in megaparsecs; and
is the Ðtted dust temperature of the CD component.TCD,
This can be rescaled for dust grains with followingQlPl2
& Patten asKnapp (1991)
M100 \3.4S100D2(e144@TCD [1) . (3)
For purposes of estimating the mass of the emitting CD in
TABLE 5
DERIVED PROPERTIES
Distance TdTCD S(100) TVCD S(240) M100 M240
Galaxy Type (Mpc) (K) (K) (Jy) (K) (Jy) log (M_) log (M_) Ratio
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
M31 ........ Sb IÈII 0.6 21 13 3734 11 1050 8.6 7.4 0.07
M33 ........ Sc IIÈIII 0.7 22 18 1165 10 480 6.7 8.1 26.6
M63 ........ Sbc 0.7 23 13 300 10 14 7.5 5.8 0.02
M83 ........ SBc II 6.8 23 17 804 10 136 6.80 6.81 1.0
N4945 ...... Sc 7.5 23 17 1820 11 240 8.3 7.9 0.46
M82 ........ Starburst 6.4 25 22 1885 10 72 9.3 9.5 1.6
N253 ....... Starburst 7.1 23 20 2688 12 350 9.1 9.1 1.0

566 ODENWALD, NEWMARK, & SMOOT Vol. 500
FIG. 4.ÈColor-corrected spectra of the galaxies at 140 and 240 km in the DIRBE photometric system. Solid curves show the continua produced from
two-component dust models. Similar dashed lines indicate a pure, one-component dust model. The error bars are based on the total DIRBE absolute
photometric uncertainty for the color-corrected Ñuxes described in Also included with diamond symbols are published IRAS photometry by et al.°2. Rice
and The triangular symbols represent the photometry of et al. et al. and & Harvey as(1990) Rice (1993). Ja†e (1984), Hildebrand (1977), Rickard (1984)
described in the text. The axes in each panel are labeled in microns and in the log (janskys).
each galaxy, which we deÐne in column (9) asTable 5,
M100, we use the for the far-IR emission obtained forTCD
the CD component of the spectral Ðts.
Limits to the mass of a potential VCD component in
each galaxy with an adopted temperature equal to the Ðtted
component with a temperature of are obtained byTVCD
computing the di†erence between the DIRBE photometry
at 240 km and the level of the Ðtted component. The rele-
vant relation between the far-IR emission limit at 240 km
and the e†ective dust mass, M240, is given by
M240 \270S240D2(e59@TVCD [1) . (4)
presents the derived dust properties obtained in thisTable 5
way. Columns (9) and (10) are the estimated masses, in solar
units, of the Ðtted dust components, together with the
distance-independent ratio of M240/M100 in column (11).
6.DISCUSSION
M31.ÈA comparison of the IRAS et al. and(Rice 1990)
DIRBE photometry at 60 and 100 km yields S(IRAS,
and 2507 and S(DIRBE) \700 and 3706 Jy,PSC) \496
respectively. The photometric uncertainties for the IRAS-
integrated Ñuxes are described by et al. asRice (1990)
^7%È15%. & Schwering also used IRASWalterbos (1987)

No. 2, 1998 EXTERNAL GALAXIES DETECTED BY COBE 567
data to obtain 610 ^20 and 2850 ^100 for the 60 and 100
km bands, respectively, and from their 12È100 km photo-
metry determined a spectrum consistent with two (a\2.0)
components at and 40 K, with the CD componentTd\21
contributing a luminosity of 1.4 ]109et al.L_. Habing
obtained IRAS-integrated Ñuxes at 60 and 100 kmof(1984)
690 ^140 and 3800 ^800 Jy. Their a\1.0 dust-model Ðt
yielded K and a total mass of 3000Td\34 M_.
The DIRBE data at 60 and 100 km are also reproduced
by a dust component similar to their estimate as shown by
the dashed lines in but DIRBE photometry atFigure 4a,
140 and 240 km yielded a very di†erent overall Ðt to
account for the rising emission that peaks near 140 km and
yields K.TdB13
M33.ÈThe far-IR spectrum of this galaxy was sum-
marized by et al. based on the original photo-Rice (1990)
metry of without color corrections added. TheyRice (1988)
obtained a best Ðt of T\30.6 K for an a\1.0 spectrum by
combining the IRAS photometry with synthetic 160 and
360 km data obtained for four Virgo spirals by combining
the photometry of these galaxies. Our Ðts show a better
match to K ; however, the DIRBE photometry atTd\18
240 km requires a substantial VCD-like component with
K, which would lead to predicted submillimeterTd\10
integrated Ñux densities exceeding those found in, for
example, active galaxies such as M82, M83, and NGC 253.
This component does not appear in the photometry of the
Virgo galaxies that were used as templates to model the
M33 continuum beyond 100 km. The DIRBE Ðeld of the
galaxy shown in does not indicate any obviousFigure 1
presence of background emission that might have led to an
increase in the Ñux assigned for this galaxy at 240 km, so the
unusually high emission detected by DIRBE at this wave-
length remains a mystery unless it is genuinely intrinsic to
the galaxy itself. The 12 and 25 kmIRAS photometry by
et al. shows evidence for a dust componentRice (1990)
warmer than the CD since S(12 km) \S(25 km), but at 240
km the Rayleigh-Jeans portion of this component would
not be adequate to account for the emission detected by
DIRBE or signiÐcantly a†ect the Ðts we obtained for the
CD component. & Malkan obtained 60 and 100M63.ÈSpinoglio (1989)
kmIRAS photometry for these galaxies of 42.9 and 148.1
Jy. & Harvey observed M63 at 160 km withRickard (1984)
a40Aaperture obtaining a peak Ñux of 45.9 Jy and an
estimated dust model for a\1.0 with K. DIRBETd\31
photometry integrated over the full disk of the galaxy at 140
and 240 km shows substantially more emission than
detected by & Harvey in the 40Acore of theRickard (1984)
galaxy and leads to a Ðt with K.TdB13
M83.ÈM83 was observed with an 83Abeam by
et al. at 540 km with a Ñux density ofHildebrand (1977)
14 ^6 Jy. Based on an assumed temperature of 35 K for
a\2.0 near 540 km, they derive a total dust mass of
3]109for a distance of 7.9 Mpc. The DIRBE photo-M_
metry and Ðts require two dust components to match the
et al. data, but require a signiÐcantlyHildebrand (1977)
cooler VCD emission, near 10 K, to match all of the photo-
metry.
M82.ÈA variety of far-IR observations are available for
this well-studied starburst galaxy. et al. detectedElias (1978)
it at 1 mm at 2.7 ^0.7 Jy with a 55Abeam and concluded its
continuum was consistent from 50 kmto1mmwitha25K
dust component (a\3.0). More recently, Becklin, &Ja†e,
Hildebrand used a 42Abeam to detect it at 400 km,(1984)
obtaining 30 Jy with ^10% photometry. This photometry
matched the extrapolated behavior of a 45 K a\1.5 spec-
trum based on the earlier 40È140 km results by &Telesco
Harper et al. conclude that no large dust(1980). Ja†e (1984)
component with would be consistent with20\Td(K) \45
the combined far-IR and submillimeter data to within the
photometric errors. Our Ðts are consistent with a cirrus-
like, CD population with K as the primary com-TdB22
ponent, and a weaker 10 K VCD component to properly
match the submillimeter data.
NGC 253.ÈThe edge-on, starburst galaxy NGC 253 was
observed, but not detected above 8 Jy, at 1.67 mm by Ade,
Rowan-Robinson, & Clegg but observations by(1976),
et al. succeeded in detecting it at 540 kmHildebrand (1977)
at 25 ^6 Jy with an 83Abeam. Their summary of the extant
far-IR observations suggested a single component with
K, a\3.7 ^0.9. The estimated bolometric lumi-Td\50
nosity was 1.5 ]1010 and the estimated dust mass wasL_,
8]106The combined Ðts from 140È540 km provide aM_.
good match to a 20 K cirrus component and a 12 K VCD-
like component assuming dust with a\2.0 throughout.
NGC 4945.ÈThis is a late-type spiral with a high level of
nuclear activity. According to it has beenKoornneef (1993)
described as a poststarburst galaxy with the highest nuclear
Ñux density at 100 km of any known external galaxy. The
spectrum is well Ðtted by a CD-type 20È23 K primary com-
ponent.
We see that the DIRBE observations provide modest
constraints on the magnitude of a VCD component in M31,
M33, M63, M83, NGC 253, and NGC 4945 on the basis of
direct detection of the far-IR continuum level for these gal-
axies at 140 and/or 240 km. The VCD components vary
considerably in their estimated sizes given the crude method
used to represent them via continuum Ðtting and range
from about 2% to over 100% of the cirrus-like component.
M33 has an anomalously high estimated VCD component
because of the large DIRBE measurement for its 240 km
emission. We can Ðnd no obvious explanation in the
DIRBE data for this photometric measurement as a result
of, for example, contamination from foreground Galactic
cirrus, and suggest that a submillimeter measurement of this
galaxy would help verify whether this galaxy does indeed
have a substantial VCD component as the DIRBE obser-
vations seem to indicate.
7.SUMMARY
From an initial catalog of 57 candidates detected by
COBE/DIRBE at j[60 km, we describe seven galaxies
detected for the Ðrst time between 140 and 240 km. The
sample consists of Ðve spirals and two starburst galaxies,
with the spirals having dust temperatures similar to Galac-
tic cirrus emission. These galaxies can be represented by a
single CD population having K, which domi-TdB20È25
nates the far-IR emission from these galaxies between
60È240 km, and with a possible weak VCD component with
K that contributes a peak emission near 240TdB10È15
km. This shows that CD similar to Galactic cirrus is at least
thermally a common ingredient of late-type galaxies such as
the Milky Way.
A comparison of the mass ratios of the CD and VCD
indicates that VCD can exist in many of these galaxies in
quite large amounts compared to the detectable cirrus-like
CD components seen in our own Milky Way by DIRBE

568 ODENWALD, NEWMARK, & SMOOT
and IRAS. Our limits indicate that nearly as much dust
could be involved in VCD components as in the dust
detected at 100 km, although for most of the ““ normal ÏÏ
spirals in our survey, the VCD component is often less than
15% of the total dust mass detected. The DIRBE data are
largely compatible with a single component in Ðve of the
galaxies: M31, M63, M83, NGC 253, and NGC 4945. The
remaining galaxies, M33 and M82, require a more compli-
cated decomposition than a†orded by a single dust com-
ponent and require at least one additional dust temperature
component of the VCD-type to account for the DIRBE 240
km photometry. Further large-beam, submillimeter obser-
vations in the spectral region from 200 km to 1 mm are
required to advance the discussion of VCD in these gal-
axies, especially in the case of M33 with its unusually high
VCD mass estimate.
The authors gratefully acknowledge the e†orts of the
DIRBE data-processing and validation teams in producing
the high-quality data sets used in this investigation. COBE
is supported by NASAÏs Astrophysics Division. Goddard
Space Flight Center, under the scientiÐc guidance of the
COBE Science Working Group, is responsible for the devel-
opment and operation of COBE. We also thank the referee
for a careful reading of the manuscript and for providing
several very helpful suggestions as to how the discussion
could be made clearer. These comments also led us to
include additional unpublished results about M31 and the
region of the sky not previously surveyed by the IRAS satel-
lite.
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