![]() | Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved |
3.4.5. Warm ULIRGs and QSOs
"Warm" ULIRGs (S25 / S60
0.2) probably represent
transition objects between ULIRGs and QSOs
(Sanders et al 1988b).
The ISO spectrophotometry discussed above strongly supports the conclusion
of the earlier IRAS work that warm ULIRGs indeed contain powerful AGNs. For
instance, the warm ULIRG Mrk 231 (the most luminous ULIRG within z
0.1) is similar to the infrared-loud but optically selected QSOs I Zw1 and
Mrk 1014. Most of the warm ULIRGs have Seyfert spectra
(Veilleux et al 1997,
1999).
High-resolution HST and ground-based observations show that warm ULIRGs
typically
have a compact hot dust source at the nucleus, surrounded by a luminous host
galaxy with embedded compact star clusters and large-scale, tidal tails
(Surace et al 1998,
Surace & Sanders
1999,
Lai et al 1998,
Scoville et al 2000).
Although the AGN is probably dominating, circumnuclear
star formation may nevertheless significantly contribute to the total
luminosity of warm ULIRGs
(Condon et al 1991b,
Downes & Solomon
1998).
In the case of Mrk 231
Taylor et al (1999)
found a smooth, approximately circular radio continuum source of
diameter ~ 1"
(1 kpc), centered on the core-jet structure of the bright AGN. The extended
radio source is associated with a massive rotating molecular disk seen at
low inclination
(Downes & Solomon
1998) and with a luminous
(2L* for no extinction correction) near-IR stellar
disk
(Lai et al 1998).
If the far-IR-radio correlation for star-forming galaxies
(Condon et al 1991a,
Condon 1992)
is applied to the parameters of Mrk 231's disk, the implied far-IR
luminosity is 1.3 × 1012
L, or ~ 70% of
the 60 + 100 µm IRAS luminosity. If the red color of the
disk's near-IR emission results from extinction (AV ~ 12;
Lai et al 1998),
the corrected K-band luminosity of the disk is
LK(0) 9
~ 2 × 1010
L
. If the
K-band emission comes from an old stellar population with
M/LK ~ 100, the mass of the disk within the central kpc has
to be 2 × 1012
M
(an 8
M
disk!).
This mass is two orders of magnitude greater than the dynamical mass within
the central kpc derived from the CO rotation curve
(Downes & Solomon
1998).
It is thus more likely that the K-band light comes from a population of
(young) red supergiant or AGB stars. The required mass then is 20 to 100
times smaller, and the ratio of bolometric to K-band luminosity is
102
(Thatte et al 1997).
The red disk may then be responsible for a significant fraction of the
far-IR luminosity of Mrk 231, in accordance with the radio data. The
relative weakness
of the mid-infrared line emission observed by ISO-SWS would then require
very high extinction or, perhaps more likely, an aged starburst. The
situations
for the IR-loud QSOs I Zw 1 and Mrk 1014 are similar, but even more extreme
stellar masses would be required there to account for the large K-band
stellar
luminosities in both galaxies (~ 1.4 × 1011
L
) by
an old stellar population. In both cases the dynamical masses implied by
the molecular disk rotation excludes this possibility
(Schinnerer et al
1998,
Downes and Solomon 1998).
In summary, ISO has made substantial contributions to
our understanding of ULIRGs. The resulting picture is fairly complex; there
is no simple story to tell. ULIRGs do not seem to undergo an obvious
metamorphosis
from a starburst-powered system of colliding galaxies to a buried and then
finally a naked QSO at the end of the merger phase. Average luminosity and
molecular gas content do not change with merger phase. AGN activity and
starburst
activity often occur at the same time. On average, starburst activity does
seem to dominate the bolometric luminosity in most objects. However, the
most luminous ULIRGs appear to be AGN dominated. "Warm" ULIRGs are AGN
dominated,
and "cold" ULIRGs are star-formation dominated. The relative contributions
of starburst and AGN activity are in part controlled by local effects. Warm
ULIRGs/infrared-loud QSOs are advanced transition objects with powerful
central
AGNs. In a few well-studied cases, these AGNs are surrounded by luminous
(aged?) starburst disks. Locally, (U)LIRGs are a spectacular curiosity. They
contribute only ~ 1% to the local infrared radiation field, and ~ 0.3%
to the total bolometric emissivity
(Sanders & Mirabel
1996,
Heckman 2000).
We will show in the next chapter that this fraction increases
dramatically (factor
102) at higher
redshift.
9 LK is here defined as the luminosity emitted in the K-band (in units of solar luminosities) Back.