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OCR for page 344
Ultraluminous Infrared Garages
B.T. SOIFER
California Institute of Technology
ABSTRACT
The IRAS all-sk r survey has provided astronomers with a first deep
New of the sky in the "thermal infrared", i.e., from 10 microns to 100
microns. One of the major discoveries of this survey has been a population
of sources having the bolometric luminosities of quasars, but where more
than 90% of the luminosity emerges in the infrared. These objects, more
numerous than quasars, are found exclusively in interact~ng/merging galaxies
that are extremely rich in interstellar gas. We have accumulated evidence
that suggests that these systems are indeed quasars obscured by many tens
of magnitudes of extinction.
We have suggested that these Ultraluminous Infrared Galaxies are
the formation stage of quasars, and that colliding galaxies, ultraluminous
infrared galaxies, and quasars might all be linked through an evolutionary
sequence where the infrared bright phase is one in which the quasar Is
formed in the nucleus of a merger system, and is enshrouded in gas and
dust, while the UV excess quasars are the end state of quasar evolution
where most of the enveloping dust cloud has been dissipated, and the
quasar is visible directly.
INTRODUCTION
Viewing We universe through a new portion of the electromagnetic
spectrum has always lead astronomers to major discovenes. Quasars and
neutron stars are just two examples of discoveries made as a result of the
Radio and X-ray sky surveys of the 1950's, 1960's, and 1970's. In 1983 the
344
OCR for page 345
HIGH-ENER~ ~TROP~ICS
345
Infrared Astronomical Satellite (IRAS) performed the latest of these all
sky surveys that has lead to major discoveries.
Before IRAS, our view of the infrared universe was limited to sly
surveys at 2.2 microns (Neugebauer and Leighton 1969) and 4-30 microns
(Pnce and Walker 1976) as well as studying objects found by other means.
The infrared sky surveys vastly increased our understanding of the Galaxy
and its constituents, but lacked the sensitivity to go significantly beyond the
Galaxy. The Caltech 2 micron sky survey contained one external galaxy,
M31, while the AFGL sly survey contained a handful of extragalactic
objects.
The IRAN sky survey, with its 3 orders of magnitude improvement
in sensitivity over previous surveys, and extension to 60 microns and 100
microns, drastically increased the numbers of galaxies detected purely by
their infrared emission, finding ~ 0.5 galaxies/square degree, and altered
our understanding of the extragalactic sly, permitting an unbiased view of
the local universe in the infrared.
TF~ WAS SKY SURVEY
The IRAS all sly survey was performed at 12 microns, 25 microns, 60
microns, and 100 microns using a 57 cm telescope and focal plane entirely
cooled to 2.7K with superfluid helium. The telescope was contained in
the toroidal shaped, 700 liter capacity helium cryostat. The satellite was
launched on January 25, 1983, and collected data for the 300 days that the
superfluid helium lasted. The primary scientific goal of the IRAS mission
was to perform the all sly survey, and approximately 60% of the satellite's
time was devoted to this purpose. 1b produce a highly reliable census of
the inerdally fixed sources in the presence of many "local" contaminants
such as cosmic ray hits on the detectors, dust particles crossing the field
of view of the telescope, asteroids and comets, the telescope scanned the
sky 6 tunes over the 300 day mission. The multiple sightings of particular
sources were used to filter and separate the inertially fixed sources from the
"moving sources" such as asteroids and comets, and the transient events
local to the telescope environment. These data were combined to produce
the first TRAS Point Source Catalog which covered 96% of the sly. At
high galactic latitude the IRAS Point Source Catalog was complete to 0.4,
0.5, 0.6, arid 1.5 Jy at 12 microns, 25 microns, 60 microns, and 100 microns
respectively with reliability > 99.8%. Recently the same data have been
reprocessed to produce a catalog at high galactic latitudes (the IRAS Faint
Source Catalog) that is a factor of ~ 2.5 more sensitive at all wavelengths,
at the cost of reduced reliability (> 98%).
The IRAS survey has a fairly modest sensivity compared to modern
optical surreys. In flux per octave, i.e., uS,~, the IRAS Point Source Catalog
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346
AMERICAN AND SOVIET PERSPECTIVES
reached 3 x 10-14 W/m2 at 60 microns, equal to the flux per octave of a
15.0 mag object at B. In terms of detectable luminosity, extragalactic sources
seen by IRAS are comparatively local. Sources with the infrared luminosity
of the Milly Way are detected to a redshift z ~ 0.03, while sources with
the bolometric luminosity of a quasar, where this energy emerges in the
infrared, are detected to redshifts of z ~ 0.3 (in this paper, we adopt Ho
= 75 ~n/s/Mpc and QO = 0~. Thus the IRAS survey is indeed a survey of
the local universe. The combination of the sensitivity of the IRAS survey,
and the wavelength at which the infrared luminosity emerges from galaxies
results in the vast majored of the ~ 20,000 extragalactic objects discovered
in the IRAS Point Source Catalog being found at 60 microns.
Since the IRAS survey was done at wavelengths substantially longward
of the peak in stellar photospheres, those sources seen in the infrared
radiate via mechanisms substantially different from those found to be
bright in optical suIveys. The predominant mechanism responsible for the
infrared flow from galaxies is thermal emission from dust; the dust absorbs
and reradiates energy originally emitted at shorter wavelengths. Infrared
bright galaxies are quite dusW and therefore have either been missed, or
appeared innocuously in visible surveys.
TlIE LOCAL UNIVERSE IN THE INFRARED
With an unbiased view of the infrared sly available for the first time,
several groups have investigated the local luminosity function of infrared
bright galaxies using IRAS data (Rieke and Lebofsky 1986; Soifer et al.
1986, 1987; Lawrence et al. 1987; Smith et al. 1987~. All these analyses
agree remarkably well, and have demonstrated that the space density of
infrared bright galaxies does not vary significantly over a factor 3 in distance
(from a median redshift of ~ 2,200 Km/s in Soifer, et al. 1987; to
~ 7,000 Km/s in Lawrence e' al. 1987~.
1b understand the importance of such infrared bright galaxies requires
placing this class of objects in the context of other lmown classes of
extragalactic objects. We have done this by comparing the bolometnc
luminosity functions of the major classes of extragalactic objects (Soifer
et al. 1987~. Since most luminosity functions for extragalactic objects are
derived at B (4400 A), it was necessary to apply bolometric corrections
to most such luminositr functions, to place them in the same units. This
comparison is shown in Figure 1.
The luminosity functions of these sources immediately shows that
infrared bright gal~es are an important, but not dominant source of lumi-
nosi~ in the local universe. For Loot < 2 x 10~° Go, the infrared bright
galaxies have a density about 20% of that for galaxies found in the vis~-
ble. For L`'o' > 2 x 10~° L:>, infrared bright galaxies become increasingly
OCR for page 347
HIGH-ENERGY ASTROP~SICS
10°
10
lo-2 _~
10-3 _
10-4 _
. .
-
o
D
~ 10-5 ~
Cal
to -,.\\
· IRAS GA LAX I ES - ALL
° IRAS GALAXIES - NON-VIRGO
-NORMAL GALAXIES
X STARBURST GALAXIES
~ SEYFERT GALAX I ES
° QUASARS
$ x ~ Y~\
10-7
lo-8
lo-l9o! 109 1010 loll
LBOL [Le]
t ~
1
lol2 lol3
347
FIGURE 1 The bolometric luminosity functions of the major class" of extragalactic
emitted (taken from Soifer et al. 1987). Separate bolometric corrections have been applied
for each class of object. The infrared galaxies become the dominant extragalactic population
at Lbol > 3 X 1011 Lain.
OCR for page 348
348
AMERICAN AND SOVIET PERSPECT~ES
important in the local universe, since the space density of normal galaxies
drops exponential with luminosity, while the infrared bright galaxies ap-
pear to decrease as ~ L-2. At Lbol > 3 x 1011 Lo, the infrared bright
galaxies become the dominant population in the local universe, exceeding
in space density the quasars. Overall the infrared luminosity of galaxies
represents ~ 25% of that emerging in stellar photospheres, and 60-80~o of
this infrared luminosity is due to young, massive stars (Soifer et al. 1987~.
ULTRALUMINOUS INFRARED GATES
We have studied the ~ 300 brightest Intragalactic sources at 60 microns
in 14,500 square degrees at high galactic latitude. This IRAS Bright Galaxy
Sample has been most amenable to study at other wavelengths, since it
represents the brightest of the infrared luminous galaxies. Of these IRAS
Bnght Galaxies 10 have bolomeuic luminosities equivalent lo those of
quasars. We have studied these 10 objects in great detail (Sanders et
al. 1988a), and have found them to possess remarkable properties. The
observed properties of these objects are summarized in Able 1.
Four of these Ultraluminous Infrared Galaxies were previously cata-
loged in other surveys. To of them, ~ 231 and Mk 273, were found
in the Markarian survey of UV excess gal~es, and were known to be
peculiar objects. Markan~an 231 has been lmown for nearly two decades as
an extraordinarily luminous infrared galaxy (Rieke and Low 1972; Young
et al. 1972~. Arp 220 (IC4553) has also been identified as a galaxy with
very peculiar properties. As can be seen immediately from Table 1, all of
the Ultraluminous Infrared Galaxies are rather faint optical galaxies. The
ratio of infrared to visible luminosity is quite large for the ultraluminous
galaxies, ranging from 50-150, as compared to 0.3 for "typical" spirals or
1-5 for "apical" infrared selected galaxies (Soifer et al. 1987).
In the infrared these Ultraluminous Infrared Gal~es are unresolved
at the IRAS angular resolution of ~ 1 '. At optical wavelengths, CCD
images show ALL these systems to be galaxies undergoing strong interac-
tions/mergers. Contour plots of CCD images of these galaxies (Sanders et
al. l9~a) are shown in Figure ~ All of these objects show evidence for
recent or ongoing mergers, i.e., multiple nuclei, tidal tails, strongly distorted
disks, eta
Optical spectroscopy shows that all of these galaxies have strong emis-
sion lines. Three of the systems, MK 231, UG{: 5101, and IRAS 051~25,
have Ha line widths > 2,000 Km/s (FWZI). Mk 231 is a well known Seyfen
1 system, while U5101 and IRAS 051~25 would be classified as interme-
diate Seyfert nuclei based on their optical spectra. Of the remaining 7
Ultraluminous systems, 6 show line widths > 1,000 Km/s, and line ratios
(tOIII]/H,B, iSII]/Hc~, [NII]/Hc~, [OIl/Hc') characteristic of AGN spectra:
OCR for page 349
349
~~'
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OCR for page 350
350
my,
~IERICAN AND SOVIET PERSPECTIVES
4c
2C
-4C
. ~
20
,
6C
TIC
2C
o
-2C
Mrk 273
~ 3 . .
UGC OS101
JO . ,. ;:
0. ,4' "
To JO ~0 -= to
FIGURE 2 Contour maps of optical images of the 10 Ultraluminous Infrared Galaxies
found in the IRAS Bnght Galaxy Sample (from Sanders et al. 198Sa). Note the irreg~
shapes, tidal mils and double nuclei; this is clear evidence of mergers in all these systems.
Only IRAS 2249-18 is classified as an HII region spectrum based on the
above line ratio.
PhotometIy of the nuclear regions of these galaxies reveals that the
near infrared colors of these galaxies show a very large spread, spanning
the entire range of colors from slightly reddened spiral gal~es to highly
reddened quasars. As will be discussed later there is enough dust (As >
100 may) obscuring the underlying energy source that it is unclear whether
these observations are a meaningful probe of the nuclear regions.
The overall energy distn~utions of these objects are shown in Figure 3.
All of the objects plotted here have very large ratios of infrared to visible
luminosity, immediately demonstrating why they are found in 60 micron
selected samples. All of these systems have similar ratios of infrared to radio
continuum emission, with log [fir/uS~ (1.49Ghz)] ~ 6.5, so these are indeed
OCR for page 351
HIGH-ENERGY ASTROP~ICS
351
"radio quiet" sources. The energy distributions are ordered by increasing
S~60 microns)/S~100 microns) ratio. Into systematic trends emerge from
this plot. The fraction of energy emerging at shorter infrared wavelengths
increases with increasing S`~60 microns)/S`, (100 microns) ratio, while the
separation between cold dust and near infrared components becomes less
obvious in the 'warmer" objects.
The eno~ous infrared luminosities, coupled with the fact that the
emission is clearly thermal emission by dust, implies that these are gas-nch
systems. The amount of dust required to produce the observed infrared
luminosity is Mauri > lO8M,3, so we would expect > 10~°M~> of gas
lo accompany this dust. Surpnsingly, the vast masons of this gas is in
molecular for, with 9 of the Ultraluminous Galaxies baring been detected
in the 2.6mm J = 1-0 line of CO (table 1~. Even more surprising has been
the finding that a large fraction, as much as 70-80% of this gas is confined
to the central few Kpc of the galaxy (Scoville et al. 1986; Scoville e' al.
1989~.
COLLIDING GALAXIES FORMING QUASARS?
Based on the observations of the Ultraluminous Infrared galaxies found
in the IRAS Bright Galaxy sample described above, we have suggested
(Sanders et al. 1988a) that these systems are heavily dust enshrouded
quasars. The evidence for this comes from the luminosities, emission lines,
near infrared colors, and luminosity to gas mass ratios. We believe that
a quasar is powering the vast luminosity emerging from these objects,
however the quasar is so heavily enshrouded in dust that we new the
quasar primarily through its bolometric luminosity. An alternate view
of these Ultraluminous Infrared Galaxies, as systems undergoing "mega
starbursts", has been presented by several groups (e.g., Rieke e! al. 1985~.
The fact that all of these systems are found ~ merging, gas-rich
systems has lead us to suggest that the merger of two gas-rich galaxies is
fundamental to the process of quasar formation. We believe that when
two gas-rich galaxies interact in a near direct collision, the nuclei merge
rapidly (lbomre and lbomre 1972), so disrupting the angular momentum
and gravitational field of the system that the gas is funneled rapidly into
the nuclear region of the merged system. This rapid accumulation of gas in
the central environment of the galaxy will lead inevitably to an enormous
"starburst." Enough gas accumulates in the central Kiloparsec of the system
that the gas becomes self-gravitating, triggering further collapse of the gas.
If a massive black hole exists In the center of the galaxy, as suggested
by much current observational data (e.g., Filippenko and Sargent 1985;
Dressier and Richstone 1988), this rapid funneling of interstellar gas into
the central environment of the galaxy is exactly the source needed to trigger
OCR for page 352
352
AMERICAN AND SOVIET PERSPECTIVES
Am 220
IR 1211~03
I R 2249 -18
-
~n
V)
a
(z< 0.081)
IR OS18-2S
Ark =1
TR 08S7~39
~Ultroluminous IRAS Goloxies
M,` 273 1 ~ \ ·. · ~
\; '"
ma;
it_ \\
..:----
.----'~
IR 1525~36 / ~ ~· . ·
\ \
\ ·.
UGC Oslo' ~\\ - .. -
\ ! ... is
\ ' ... '''I
\ ..
.l
me,
N O'er
,
.
. %~
1 1 ~
t04 3000 t00 10
A (Jim)
-
.~
xlo
2
1 ~1
1 0.1
FIGIJPE 3 The energy distributions form 0.44 microns to 350 microns for the Ult~lumi-
nous Infrared Galaxies (from Sanders et al. 1988a). I-ne energy distributions are ordered
from lowest 60 micronsf100 microns color temperature (top) to highest 60 micronsllOO
microns color temperature (bottom).
OCR for page 353
HIGH-ENERGY ASTROPHYSICS
353
formation of the quasar that is powered by gravitational energy released as
the gas falls onto the black hole.
We expect that the formation of a quasar embedded in 10~°M~ of gas
and dust in the center of a galaxy is an enormously disruptive event, one
that will ultimately result in a visible quasar. Qualitatively, we expect that as
the energy release of the quasar begins to disrupt the surrounding gas and
dust, the quasar will take on more and more of the properties of a "normal"
quasar, be. revealing a strong nuclear opticallUV/near infrared continuum,
very broad emission lines, etc. We have found evidence (Sanders et al.
1988b) for such an evolutionary sequence in the ``warm" Ultraluminous
galaxies found in the IRAS database as extragalactic objects with luminosi-
ties of quasars, but with infrared properties intermediate between those
of the Ultraluminous Infrared Galaxies and the "normal" quasars. These
systems, again selected by purely infrared flux density and color criteria,
show equal numbers of Seyfert 1 and Seyfert 2 nuclei. Of these Warm
Ultraluminous Infrared Galaxies, 9/12 are in strongly interacting/merging
systems, and 3 are indeed classified as "normal" quasars by the criteria of
Schmidt and Green (1983) and Veron-Cetty and Veron (1985~.
As we propose this evolutionary scenario, ultimately the disruption
of the surrounding medium is complete and the quasar emerges from its
cocoon as a UV excess quasar. The residual dust and gas in the environment
still produces a significant, but not dominant, fraction of infrared luminosity.
Recent~, we have studied in detail the energy distributions of the brightest
UV excess quasars (Sanders et al. 1989), and found that they indeed emit
approximately 20% of their bolometric luminosity in the infrared, consistent
with this picture, while the infrared selected quasars studied by Low et al.
(1989) might represent a slightly earlier phase in this sequence.
The ideas for the formation of quasars that we have suggested as a
result of our study of the Ultraluminous Infrared Galaxies are not new,
but rather bring new data to support rather old ideas. Alar and Juri
lbomre (1972), in a seminal work, outlined most of these ideas for galaxy
interaction triggering active nuclei while modern, high dynamic range
imaging and spectroscopy of quasars and their environments have lead
many investigators to the conclusion that interactions play a significant role
in quasars (Stockton and McKenty 1983; Hutchings e! al. 1984~. What we
believe the IRAS data has provided is evidence for the formative stages
of the quasar, where the gas and dust that converts the luminosity of the
quasar into a bright infrared source effectively acts as a strong neutral
density alter to allow us to study in detail the environment surrounding the
quasar without the blinding effect of the exceedingly bright central source.
Recent theoretical work (Norman 1989) suggests that the scenario we have
described here is indeed a plausible outcome of a direct collision between
two gas-rich gal~es.
OCR for page 354
354
AMERICAN AND SOVIET PERSPECTIVES
The model we envision for quasar formation has the exceedingly
attractive feature that it ties quasars, the most luminous and presumably
violent objects in the universe to the rare occurrence of a direct merger
of two gas-nch spiral galaxies. This model very nicely provides a physical
process to explain the well known strong evolution of quasars, both in
their rapid increase with redshift, and apparent CUtOD at high redstart.
The former is simply related to the higher rate of collisions of galaxies in
the past, as a result of their closer proximity and their higher mean gas
content, while the latter Is a result of the fact that the galaxies must form
and interact before a quasar can be formed. Clearly, for this model to
work the ultraluminous infrared galaxies must follow the evolution found
for the optically selected quasars (see e.g., Schmidt and Green 1983~. This
prediction is directly testable with the next generation of space infrared
obsenatones, ISO and SIRTF.
CONCLUSIONS
The new view of the universe provided by the IRAS survey has lead to
the discovery of a new class of objects, the Ultraluminous Infrared Galaxies.
We believe that these objects are the first formative stages of quasars in the
nuclei of merging gas rich spiral galaxies. Such an explanation naturally
ties the formation of quasars to a violent, but rare event in the evolution
of spiral galaxies.
ACKNOWLEDGEMENTS
Many colleagues have contoured in major ways to the development
Of the ideas presented here Most notably these are Dave Sanders, Gerry
Neugebauer, Jay Elias, and Nick Scoville. In addition, Keith Matthews,
Dave Carnco, and James Graham have made major contributions to obser-
vations that have helped formulate these ideas. Finally, none of this work
would have been done without the tremendously successful IRAS mission,
and it is a great pleasure to acknowledge the many dedicated engineers,
scientists, technicians, and managers who helped make this project the
success it has been.
This research has been supported by NASA through the TRAS B-
tended Mission, and by the NSF.
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Representative terms from entire chapter:
infrared galaxies
