HST NICMOS observations of a sample of 24 luminous (LIGs: L_(IR) [8-1000
microns] = 10^(11.0-11.99) L_sun) and ultraluminous (ULIGs: L_(IR) > 10^(12.0)
L_sun) infrared galaxies are presented. The observations provide, for the first
time, high resolution HST imaging of the imbedded 1.1 - 2.2 micron nuclear
regions of these mergers. All but one of the ULIGs are observed to have at
least one compact (50-200 pc) nucleus, and more than half contain what appear
to be blue star clusters. The warm infrared galaxies (i.e., the transition
sources) are observed to have bright nuclei which account for most of the light
of the galaxy. This, combined with the tendency for the light of ULIGs to
become more centrally concentrated as a function of increasing wavelength,
implies that most of their energy is generated within a region 50-200 pc
across.Comment: LaTex, 6 pages with 1 postscript and 1 jpg figure, and 1 postscript
  table, To appear in the proc. of the Ringberg workshop on "Ultraluminous
  Galaxies: Monsters or Babies" (Ringberg castle, Sept. 1998), Ap&SS, in pres

Evans, A. S.

English

arXiv

HST NICMOS observations of a sample of 24 luminous (LIGs: L_(IR)[8-1000 μm] = 10^(11.0-11.99) L_⊙) and ultraluminous (ULIGs:L_(IR) ≳ 10^(12.0) L_⊙) infrared galaxies are presented.The observations provide, for the first time, high resolution HST imaging of the imbedded 1.1 - 2.2 μm nuclear regions of these mergers. All but one of the ULIGs are observed to have at least one compact (50-200 pc) nucleus, and more than half contain what appear to be blue star clusters. The warm infrared galaxies (i.e., the transition sources) are observed to have bright nuclei which account for most of the light of the galaxy.This, combined with the tendency for the light of ULIGs to become more centrally concentrated as a function of increasing wavelength, implies that most of their energy is generated within a region 50-200 pc across

Evans, Aaron S.

Caltech Authors - Main

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NICMOS OBSERVATIONS OF LUMINOUS &
ULTRALUMINOUS INFRARED GALAXIES
AARON S. EVANS
California Institute of Technology 105-24
Pasadena, CA 91125
Abstract.
HST NICMOS observations of a sample of 24 luminous (LIGs: LIR[8−
1000µm] = 1011.0−11.99L⊙) and ultraluminous (ULIGs: LIR >∼10
12.0L⊙) in-
frared galaxies are presented. The observations provide, for the first time,
high resolution HST imaging of the imbedded 1.1 - 2.2 µm nuclear regions
of these mergers. All but one of the ULIGs are observed to have at least one
compact (50-200 pc) nucleus, and more than half contain what appear to be
blue star clusters. The warm infrared galaxies (i.e., the transition sources)
are observed to have bright nuclei which account for most of the light of the
galaxy. This, combined with the tendency for the light of ULIGs to become
more centrally concentrated as a function of increasing wavelength, implies
that most of their energy is generated within a region 50-200 pc across.
1. Introduction
There exists strong morphological and spectroscopic evidence that ultra-
luminous infrared galaxies (ULIGs) are the by-products of the merger/
interaction of molecular gas-rich galaxies, and that their extreme luminosi-
ties result from circumnuclear starbursts and active galactic nuclei (AGN).
During the ultraluminous phase, the radiation from the starburst/AGN,
most of which is emitted at UV-to-optical wavelengths, is absorbed by cir-
cumnuclear dust and re-emitted at infrared/submillimeter wavelengths.
Of direct relevance to this conference is whether stars or AGN account
for the majority of the energy output of ULIGs. Based on the similarities in
the luminosity functions of ULIGs and QSOs, and the evidence for morpho-
logical pecularities and large molecular gas masses in some QSOs, Sanders
et al. (1988a,b) proposed an evolutionary connection between ULIGs and
QSOs. A population of “warm” ULIGs (i.e., f25/f60 > 0.2, similiar to
the colors of Seyfert galaxies) were singled out as an advanced stage of
2 AARON S. EVANS
TABLE 1
Ultraluminous Infrared Galaxies
Name Class
a
z log L
ir
Nulear NICMOS
log L

Separation Charateristis
NGC 4418    0.0073 11.00 24 kp Linear nulear struture with bionial dark lanes
Zw049.057 H II 0.0131 11.22    Inlined spiral with linear dust lane
NGC 6090 H II 0.0294 11.51 3.6 kp Galaxies separate - early in merger phase
NGC 2623    0.0185 11.54    Clusters surrounding an extended nuleus
IC 883 LINER 0.0231 11.60    Disk with lusters and a tidal tail
NGC 7469 Seyfert 1 0.0166 11.60 28 kp Point-like nuleus in spiral
VV 114 H II 0.0201 11.62 5.2 kp Colliding galaxies
NGC 6240 LINER 0.0243 11.82 0.8 kp Double nulei with tails
VIIZw031 H II 0.0542 11.94    Compat spiral with asymmetri outer struture
IR 15250+36 LINER 0.0534 12.00    Nuleus surrounded by lusters
IR 09320+61 LINER 0.0400 12.01    Point-like nuleus with twisting spiral isophotes
IR 10565+24 H II 0.0430 12.02 6.2 kp Spiral galaxy interating with ompanion
IR 08572+39 LINER 0.0582 12.09 5.6 kp point-like nulear galaxy with ompanion
IR 22491-18 H II 0.0773 12.10 2.2 kp Double nulei + tidal tails
IR 05189-25 Seyfert 2 0.0427 12.10    Elliptial-like galaxy with Point-like nuleus
Mrk 273 LINER 0.0378 12.11 0.7 kp Diuse and point-like nulei
Arp 220 LINER 0.0185 12.19 0.3 kp Embedded double nulei in lose proximity
PKS 1345+12 Seyfert 2 0.1224 12.22 4.0 kp Radio galaxy with double nulei
IR 12112+03 LINER 0.0727 12.26 3.8 kp Point-like + extended nulei with sikle-shaped tail
IR 14348-1447 LINER 0.0825 12.27 4.9 kp Colliding spirals
IR 17208-00 H II 0.0429 12.40    Complex, Arp220-like nuleus surrounded by lusters
IR 07598+65 Seyfert 1 0.149 12.45    Point-like soure with extended struture
Mrk 1014 QSO 0.163 12.49    QSO nuleus + spiral tails
3C 48 QSR 0.398 12.50    QSR with extended struture
a
Based on optial emission-line ratios (Kim 1995; Veilleux et al. 1995).
the ULIG phenomenon, occuring between the “cool” ultraluminous phase,
when enhanced star formation is occuring and the AGN is being built, and
the optical QSO phase, when most of the gas and dust have been con-
sumed/blown out by star formation and the AGN.
The high column densities intrinsic to ULIGs have made attempts to
image their nuclear regions at optical wavelengths difficult. Observations
with ground-based arrays sensitive to radiation emitted at near-infrared
wavelengths (where the extinction is ten times lower than at optical wave-
lengths) have improved our ability to observe the morphology and colors
of imbedded regions, but have been limited by atmospheric seeing (FWHM
≈ 0.7′′: Murphy et al. 1996; Kim 1996). The availability of the Near-Infrared
Camera and Multiobject Spectrometer (NICMOS), recently installed on the
Hubble Space Telescope (HST), thus offers a unique opportunity to study
ULIGs by combining near-infrared technology with the high resolution and
photometric accuracy possible with a space-based instrument.
As part of NICMOS GTO program, a sample of 15 ULIGs (9 cool, 4
warm, 2 QSOs) were observed with Camera 2 of NICMOS at 1.1, 1.6, and
2.2 µm. For comparison, images of a sample of 9 LIGs were also obtained.
Camera 2 provides a 0.1′′ − 0.2′′ resolution (FWHM) at 1.1–2.2 µm and
HST NICMOS OBSERVATIONS 3
a 19′′ field of view, which correspond to a resolution of 25-200pc and pro-
jected field of view of 4-40 kpc for the redshift range of 0.01–0.15. The
specific issues to be addressed are: 1) Is there evidence of imbedded star
clusters or multiple/unresolved nuclei that may be responsible for the far
infrared emission? 2) Is there evidence for interactions/mergers in infrared
galaxies that were previously observed to show none? 3) Is there evidence
that a large fraction of the light from ULIGs emanates from increasingly
more compact regions as a function of increasingly wavelength, as might be
expected for an AGN? 4) How do the imbedded cluster colors compare with
the colors of the underlying galaxy? Below, each of these issues is addressed.
An H0 = 75 km s
−1 Mpc−1 and q0 = 0.0 are assumed throughout.
2. Morphologies and Nuclear Luminosities
Table 1 contains a general summary of the properties of the 24 galaxies
observed, and Figure 1 shows several examples of their near-infrared mor-
phologies. The statistics of the data are as follows: 22/24 galaxies show
morphological disturbances to varying degrees (note that the two excep-
tions, IR 05189-25 and 07598+65, have large-scale, low surface-brightness
optical debris outside of the nuclear region: Surace et al. 1998); 11/24 of the
galaxies show evidence of spiral structure/arms; 11/24 have double nuclei
with projected separations ranging from 0.3–7.0 kpc (two additional galax-
ies, NGC 4418 and 7469, have companion galaxies ≈26 kpc away, i.e., out
of the NICMOS field of view); 16/24 have one or more blue star clusters.
In ground-based images of VIIZw031 (Figure 1) and IR 10565+24, both
have elliptical galaxy-like nuclear appearances (Djorgovski et al. 1990; Mur-
phy et al. 1996). In the NICMOS images, the central 0.5–1.0′′ of the galaxies
have spiral structure, supporting the idea that the formation of a LIG or
ULIG involves at least one spiral galaxy. Similar nuclear spiral structure is
also revealed in HST images of NGC 7252 (Whitmore et al. 1993).
Putative nuclei with L2.2µm ≈ 2 × 10
7
− 1011 L⊙ have been identified
in all galaxies. Several of these “nuclei” have been detected in CO, and all
but a few have strong Paα and radio emission. With the exception of IR
17208-00, all of the ULIGs contain at least one compact (i.e., FWHM <∼70-
200 pc) nucleus. The more compact cores tend to be the more luminous.
In particular, all of the warm ULIGs, whose nuclei at first glance look like
stars, have very luminous nuclei.
3. Nuclear Concentrations
Using 1.1′′ and 11′′-diameter apertures, the percentage of the total galac-
tic light contained within the nucleus of the galaxies were measured. The
smaller aperture size was selected to contain the first airy disk of the un-
4 AARON S. EVANS
Figure 1. NICMOS images of six of the LIGs/ULIGs. Blue=1.1 µm, Green=1.6 µm,
and Red=2.2 µm. The galaxy name and log(LIR) are provided in the bottom corners of
each panel. The field of view is ≈ 19′′ × 19′′/panel. North is up, east is to the left.
HST NICMOS OBSERVATIONS 5
resolved 2.2 µm nuclei, thus minimizing photometric errors due to the dif-
ferent resolutions of the 1.1, 1.6, and 2.2 µm images. The ULIGs with the
lowest f60/f100 ratio (e.g. the cool ULIGs Arp 220, IR 12112+03) had the
lowest value of f(1.1′′)/f(11′′) [≈ 0.15-0.3 at 2.2 µm], whereas the warm
ULIGs, which have high f60/f100 ratios, have high values of f(1.1
′′)/f(11′′)
[≈ 0.70]. The galaxy UGC 5101 (IR 09320+61: Figure 1), has an interme-
diate value of f(1.1′′)/f(11′′) [=0.42], indicating that its 2.2 µm emission
emanates from the nucleus and underlying disk in nearly equal amounts.
Finally, f(1.1′′)/f(11′′) increases as a function of increasing wavelength
in nearly all of the galaxies, possibly indicating that their emission becomes
more centrally concentrated at longer wavelengths. This is due in part to
the waning contribution of light from the extended underlying stellar pop-
ulation at wavelengths longward of 1.6 µm.
4. Infrared Colors
Figure 2 is a plot of m1.1−1.6 vs. m1.6−2.2 for all of the galaxies in the sample,
extracted using a 1.1′′-diameter aperture. Most of the LIGs and cool ULIGs
are in a linear scatter with a slope consistent with starlight reddened by a
screen of cold dust, however, a QSO embedded in warm (> 1000K) dust
cannot be ruled out for galaxies at the high end of the linear scatter of
points (e.g., Arp 220, IR 17208-00, IC 883, NGC 2623). Whether starlight
or AGN, the nuclear energy sources of these galaxies are heavily embedded.
While all of the galaxies have m1.1−1.6 colors between 0.7 and 2.0, the
warm ULIGs have high values of m1.6−2.2 and lie on or to the right of
the QSO–1000K dust line. Thus, in addition to being extremely compact
and bright, the nuclei of the warm galaxies are also abnormally red. The
similarities in the colors and nuclear compactness of warm ULIGs and QSOs
lends strong support to the idea that warm ULIGs possess QSO nuclei.
Two LIGs and one cool ULIG also lie in the region of Figure 2 occupied
by the warm ULIGs. One of the LIGs, NGC 7469, is a warm infrared galaxy
(f25/f60 = 0.22) with a bright Seyfert 1 nucleus embedded in a spiral galaxy.
The cool ULIG, UGC 5101, contains a bright nucleus embedded in a spiral
galaxy. In an 11′′-diameter aperture, both galaxies are indistinguishable
from any of the other cool ULIGs. Thus, UGC 5101 and NGC 7469 may be
intermediate cases, possessing warm, QSO-like infrared nuclei embedded in
cool (and very luminous) infrared galaxies.
5. Star Clusters
A similar color analysis has been done of the clusters (L2.2µm ≈ 10
4
−107L⊙)
in a few of the galaxies. While no attempt has been made to constrain the
ages of the clusters, the cluster colors appear to be bluer than the underlying
6 AARON S. EVANS
Figure 2. Infrared Magnitude Color Diagram for LIGs/ULIGs measured in 1.1′′-diameter
aperture. The dash-dotted line extending from UGC 5101 (triangle), VV 114 (*), and
NGC 7469 (*) indicate how far the galaxies move on the diagram when their colors are
measured in 11′′-diameter apertures. Adapted from Scoville et al. (in prep).
galaxies, which are bluer than the nuclei. If the assumption is made that the
effects of extinction are moderate or negligible outside of the nuclear region,
the star clusters are much younger than the underlying stellar population
and are probably by-products of the merger event.
Acknowledgements
It is a pleasure to thank my collaborators N. Scoville, N. Dinshaw, M.
Rieke, D. Hines, R. Thompson, J. Surace and the NICMOS GTO Team.
This research was supported NASA grant NAG 5-3042.
References
Djorgovski, S., de Carvalno, R. R., & Thompson, D. J. 1990, AJ, 99, 1414
Kim, D.-C. 1995, Ph.D. Thesis, University of Hawaii
Murphy, T. et al. 1996, AJ, 111, 1025
Sanders, D. B., Soifer, B. T., Elias, J. H., et al. 1988a, ApJ, 325, 74
Sanders, D. B., Soifer, B. T., Elias, J. H., et al. 1988b, ApJ, 328, L35
Surace, J.A., Sanders, D.B. Vacca, W.D., et al. 1998, ApJ, 492, 116
Veilleux, S., Kim, D. -C., Sanders, D. B., et al. 1995, ApJS, 98, 171
Whitmore. B. C., Schweizer, F., Leitherer, C., et al. 1993, AJ, 106, 1354
This figure "evans_fig1.jpg" is available in "jpg"
 format from:
http://arxiv.org/ps/astro-ph/9903272v1


NICMOS observations of luminous & ultraluminous infrared galaxies

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NICMOS OBSERVATIONS OF LUMINOUS &
ULTRALUMINOUS INFRARED GALAXIES
AARON S. EVANS
California Institute of Technology 105-24
Pasadena, CA 91125
Abstract.
HST NICMOS observations of a sample of 24 luminous (LIGs: LIR[8−
1000µm] = 1011.0−11.99L⊙) and ultraluminous (ULIGs: LIR>∼10
12.0L⊙) in-
frared galaxies are presented. The observations provide, for the first time,
high resolution HST imaging of the imbedded 1.1 - 2.2 µm nuclear regions
of these mergers. All but one of the ULIGs are observed to have at least one
compact (50-200 pc) nucleus, and more than half contain what appear to be
blue star clusters. The warm infrared galaxies (i.e., the transition sources)
are observed to have bright nuclei which account for most of the light of the
galaxy. This, combined with the tendency for the light of ULIGs to become
more centrally concentrated as a function of increasing wavelength, implies
that most of their energy is generated within a region 50-200 pc across.
1. Introduction
There exists strong morphological and spectroscopic evidence that ultra-
luminous infrared galaxies (ULIGs) are the by-products of the merger/
interaction of molecular gas-rich galaxies, and that their extreme luminosi-
ties result from circumnuclear starbursts and active galactic nuclei (AGN).
During the ultraluminous phase, the radiation from the starburst/AGN,
most of which is emitted at UV-to-optical wavelengths, is absorbed by cir-
cumnuclear dust and re-emitted at infrared/submillimeter wavelengths.
Of direct relevance to this conference is whether stars or AGN account
for the majority of the energy output of ULIGs. Based on the similarities in
the luminosity functions of ULIGs and QSOs, and the evidence for morpho-
logical pecularities and large molecular gas masses in some QSOs, Sanders
et al. (1988a,b) proposed an evolutionary connection between ULIGs and
QSOs. A population of “warm” ULIGs (i.e., f25/f60 > 0.2, similiar to
the colors of Seyfert galaxies) were singled out as an advanced stage of
2 AARON S. EVANS
TABLE 1
Ultraluminous Infrared Galaxies
Name Class
a
z log L
ir
Nulear NICMOS
log L

Separation Charateristis
NGC 4418    0.0073 11.00 24 kp Linear nulear struture with bionial dark lanes
Zw049.057 H II 0.0131 11.22    Inlined spiral with linear dust lane
NGC 6090 H II 0.0294 11.51 3.6 kp Galaxies separate - early in merger phase
NGC 2623    0.0185 11.54    Clusters surrounding an extended nuleus
IC 883 LINER 0.0231 11.60    Disk with lusters and a tidal tail
NGC 7469 Seyfert 1 0.0166 11.60 28 kp Point-like nuleus in spiral
VV 114 H II 0.0201 11.62 5.2 kp Colliding galaxies
NGC 6240 LINER 0.0243 11.82 0.8 kp Double nulei with tails
VIIZw031 H II 0.0542 11.94    Compat spiral with asymmetri outer struture
IR 15250+36 LINER 0.0534 12.00    Nuleus surrounded by lusters
IR 09320+61 LINER 0.0400 12.01    Point-like nuleus with twisting spiral isophotes
IR 10565+24 H II 0.0430 12.02 6.2 kp Spiral galaxy interating with ompanion
IR 08572+39 LINER 0.0582 12.09 5.6 kp point-like nulear galaxy with ompanion
IR 22491-18 H II 0.0773 12.10 2.2 kp Double nulei + tidal tails
IR 05189-25 Seyfert 2 0.0427 12.10    Elliptial-like galaxy with Point-like nuleus
Mrk 273 LINER 0.0378 12.11 0.7 kp Diuse and point-like nulei
Arp 220 LINER 0.0185 12.19 0.3 kp Embedded double nulei in lose proximity
PKS 1345+12 Seyfert 2 0.1224 12.22 4.0 kp Radio galaxy with double nulei
IR 12112+03 LINER 0.0727 12.26 3.8 kp Point-like + extended nulei with sikle-shaped tail
IR 14348-1447 LINER 0.0825 12.27 4.9 kp Colliding spirals
IR 17208-00 H II 0.0429 12.40    Complex, Arp220-like nuleus surrounded by lusters
IR 07598+65 Seyfert 1 0.149 12.45    Point-like soure with extended struture
Mrk 1014 QSO 0.163 12.49    QSO nuleus + spiral tails
3C 48 QSR 0.398 12.50    QSR with extended struture
a
Based on optial emission-line ratios (Kim 1995; Veilleux et al. 1995).
the ULIG phenomenon, occuring between the “cool” ultraluminous phase,
when enhanced star formation is occuring and the AGN is being built, and
the optical QSO phase, when most of the gas and dust have been con-
sumed/blown out by star formation and the AGN.
The high column densities intrinsic to ULIGs have made attempts to
image their nuclear regions at optical wavelengths difficult. Observations
with ground-based arrays sensitive to radiation emitted at near-infrared
wavelengths (where the extinction is ten times lower than at optical wave-
lengths) have improved our ability to observe the morphology and colors
of imbedded regions, but have been limited by atmospheric seeing (FWHM
≈ 0.7′′: Murphy et al. 1996; Kim 1996). The availability of the Near-Infrared
Camera and Multiobject Spectrometer (NICMOS), recently installed on the
Hubble Space Telescope (HST), thus offers a unique opportunity to study
ULIGs by combining near-infrared technology with the high resolution and
photometric accuracy possible with a space-based instrument.
As part of NICMOS GTO program, a sample of 15 ULIGs (9 cool, 4
warm, 2 QSOs) were observed with Camera 2 of NICMOS at 1.1, 1.6, and
2.2 µm. For comparison, images of a sample of 9 LIGs were also obtained.
Camera 2 provides a 0.1′′ − 0.2′′ resolution (FWHM) at 1.1–2.2 µm and
HST NICMOS OBSERVATIONS 3
a 19′′ field of view, which correspond to a resolution of 25-200pc and pro-
jected field of view of 4-40 kpc for the redshift range of 0.01–0.15. The
specific issues to be addressed are: 1) Is there evidence of imbedded star
clusters or multiple/unresolved nuclei that may be responsible for the far
infrared emission? 2) Is there evidence for interactions/mergers in infrared
galaxies that were previously observed to show none? 3) Is there evidence
that a large fraction of the light from ULIGs emanates from increasingly
more compact regions as a function of increasingly wavelength, as might be
expected for an AGN? 4) How do the imbedded cluster colors compare with
the colors of the underlying galaxy? Below, each of these issues is addressed.
An H0 = 75 km s
−1 Mpc−1 and q0 = 0.0 are assumed throughout.
2. Morphologies and Nuclear Luminosities
Table 1 contains a general summary of the properties of the 24 galaxies
observed, and Figure 1 shows several examples of their near-infrared mor-
phologies. The statistics of the data are as follows: 22/24 galaxies show
morphological disturbances to varying degrees (note that the two excep-
tions, IR 05189-25 and 07598+65, have large-scale, low surface-brightness
optical debris outside of the nuclear region: Surace et al. 1998); 11/24 of the
galaxies show evidence of spiral structure/arms; 11/24 have double nuclei
with projected separations ranging from 0.3–7.0 kpc (two additional galax-
ies, NGC 4418 and 7469, have companion galaxies ≈26 kpc away, i.e., out
of the NICMOS field of view); 16/24 have one or more blue star clusters.
In ground-based images of VIIZw031 (Figure 1) and IR 10565+24, both
have elliptical galaxy-like nuclear appearances (Djorgovski et al. 1990; Mur-
phy et al. 1996). In the NICMOS images, the central 0.5–1.0′′ of the galaxies
have spiral structure, supporting the idea that the formation of a LIG or
ULIG involves at least one spiral galaxy. Similar nuclear spiral structure is
also revealed in HST images of NGC 7252 (Whitmore et al. 1993).
Putative nuclei with L2.2µm ≈ 2 × 10
7
− 1011 L⊙ have been identified
in all galaxies. Several of these “nuclei” have been detected in CO, and all
but a few have strong Paα and radio emission. With the exception of IR
17208-00, all of the ULIGs contain at least one compact (i.e., FWHM <∼70-
200 pc) nucleus. The more compact cores tend to be the more luminous.
In particular, all of the warm ULIGs, whose nuclei at first glance look like
stars, have very luminous nuclei.
3. Nuclear Concentrations
Using 1.1′′ and 11′′-diameter apertures, the percentage of the total galac-
tic light contained within the nucleus of the galaxies were measured. The
smaller aperture size was selected to contain the first airy disk of the un-
4 AARON S. EVANS
Figure 1. NICMOS images of six of the LIGs/ULIGs. Blue=1.1 µm, Green=1.6 µm,
and Red=2.2 µm. The galaxy name and log(LIR) are provided in the bottom corners of
each panel. The field of view is ≈ 19′′ × 19′′/panel. North is up, east is to the left.
HST NICMOS OBSERVATIONS 5
resolved 2.2 µm nuclei, thus minimizing photometric errors due to the dif-
ferent resolutions of the 1.1, 1.6, and 2.2 µm images. The ULIGs with the
lowest f60/f100 ratio (e.g. the cool ULIGs Arp 220, IR 12112+03) had the
lowest value of f(1.1′′)/f(11′′) [≈ 0.15-0.3 at 2.2 µm], whereas the warm
ULIGs, which have high f60/f100 ratios, have high values of f(1.1
′′)/f(11′′)
[≈ 0.70]. The galaxy UGC 5101 (IR 09320+61: Figure 1), has an interme-
diate value of f(1.1′′)/f(11′′) [=0.42], indicating that its 2.2 µm emission
emanates from the nucleus and underlying disk in nearly equal amounts.
Finally, f(1.1′′)/f(11′′) increases as a function of increasing wavelength
in nearly all of the galaxies, possibly indicating that their emission becomes
more centrally concentrated at longer wavelengths. This is due in part to
the waning contribution of light from the extended underlying stellar pop-
ulation at wavelengths longward of 1.6 µm.
4. Infrared Colors
Figure 2 is a plot ofm1.1−1.6 vs.m1.6−2.2 for all of the galaxies in the sample,
extracted using a 1.1′′-diameter aperture. Most of the LIGs and cool ULIGs
are in a linear scatter with a slope consistent with starlight reddened by a
screen of cold dust, however, a QSO embedded in warm (> 1000K) dust
cannot be ruled out for galaxies at the high end of the linear scatter of
points (e.g., Arp 220, IR 17208-00, IC 883, NGC 2623). Whether starlight
or AGN, the nuclear energy sources of these galaxies are heavily embedded.
While all of the galaxies have m1.1−1.6 colors between 0.7 and 2.0, the
warm ULIGs have high values of m1.6−2.2 and lie on or to the right of
the QSO–1000K dust line. Thus, in addition to being extremely compact
and bright, the nuclei of the warm galaxies are also abnormally red. The
similarities in the colors and nuclear compactness of warm ULIGs and QSOs
lends strong support to the idea that warm ULIGs possess QSO nuclei.
Two LIGs and one cool ULIG also lie in the region of Figure 2 occupied
by the warm ULIGs. One of the LIGs, NGC 7469, is a warm infrared galaxy
(f25/f60 = 0.22) with a bright Seyfert 1 nucleus embedded in a spiral galaxy.
The cool ULIG, UGC 5101, contains a bright nucleus embedded in a spiral
galaxy. In an 11′′-diameter aperture, both galaxies are indistinguishable
from any of the other cool ULIGs. Thus, UGC 5101 and NGC 7469 may be
intermediate cases, possessing warm, QSO-like infrared nuclei embedded in
cool (and very luminous) infrared galaxies.
5. Star Clusters
A similar color analysis has been done of the clusters (L2.2µm ≈ 10
4
−107L⊙)
in a few of the galaxies. While no attempt has been made to constrain the
ages of the clusters, the cluster colors appear to be bluer than the underlying
6 AARON S. EVANS
Figure 2. Infrared Magnitude Color Diagram for LIGs/ULIGs measured in 1.1′′-diameter
aperture. The dash-dotted line extending from UGC 5101 (triangle), VV 114 (*), and
NGC 7469 (*) indicate how far the galaxies move on the diagram when their colors are
measured in 11′′-diameter apertures. Adapted from Scoville et al. (in prep).
galaxies, which are bluer than the nuclei. If the assumption is made that the
effects of extinction are moderate or negligible outside of the nuclear region,
the star clusters are much younger than the underlying stellar population
and are probably by-products of the merger event.
Acknowledgements
It is a pleasure to thank my collaborators N. Scoville, N. Dinshaw, M.
Rieke, D. Hines, R. Thompson, J. Surace and the NICMOS GTO Team.
This research was supported NASA grant NAG 5-3042.
References
Djorgovski, S., de Carvalno, R. R., & Thompson, D. J. 1990, AJ, 99, 1414
Kim, D.-C. 1995, Ph.D. Thesis, University of Hawaii
Murphy, T. et al. 1996, AJ, 111, 1025
Sanders, D. B., Soifer, B. T., Elias, J. H., et al. 1988a, ApJ, 325, 74
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Surace, J.A., Sanders, D.B. Vacca, W.D., et al. 1998, ApJ, 492, 116
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NICMOS Observations of Luminous and Ultraluminous Infrared Galaxies

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