Extremely Luminous far-infrared galaxies (ELFs) are a class of galaxy discovered independently by several groups. The class is characterized by a quasar-like total luminosity (10(exp 11) to 10(exp 13) solar luminosity) which is radiated almost entirely in the far-infrared. It has been suggested that obscured quasar cores may be responsible for generating this luminosity. Here the author demonstrates that ELFs appear in several guises which can be characterized by the number of quasar cores they contain (zero, one or two). The author develops a unified model to account for these differences

Norris, R. P.

English

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2 I 
ARE EXTREMELY LUMINOUS FAR-INFRARED GALAXIES THE RESULT OF 
MERGING QUASAR CORES? 
R.P.Norris, Australia Telescope National Facility 
Extremely Luminous far-infrared galaxies (ELFs) are a class of galaxy discovered 
independently by several groups. The class is characterised by a quasar-like total luminos- 
ity (loll - 1013 Lo) which is radiated almost entirely in the far-infrared, and it has been 
suggested that obscured quasar cores may be responsible for generating this luminosity. 
Here I demonstrate that ELFs appear in several guises which can be characterised by the 
number of quasar cores they contain (zero, one or two), and develop a unified model to 
account for these differences. 
ARP 220: EVIDENCE FOR MERGING QUASAR CORES 
Arp 220 (IC 4553) has become regarded by many as the archetypal ELF. Evidence 
for intense starburst activity in it is provided by the optical spectra and high molecular 
density (e.g. Solomon et al. 1990). In addition, the disturbed morphology, extended tidal 
plumes and shock- excited infrared spectrum (Joseph et al. 1984) all strongly suggest that 
Arp 220 is the result of a recent merger. 
However, the 40 or so magnitudes of extinction to the core of Arp 220 prevent us from 
using optical techniques to probe its innermost secrets. Radio observations, on the other 
hand, are not affected in this way, and have demonstrated the presence of a quasar-like 
core. The evidence for this is principally the existence of a compact radio nucleus with 
a high brightness temperature (Norris, 1985, 1988), but is supported by the broad lines 
seen in Brackett-alpha emission (DePoy et al. 1987) and in HP absorption (Heckman et 
al. 1983). 
Radio observations at sub-arcsec resolution (Norris, 1988) have produced the sur- 
prising result that the nucleus of Arp 220 contains not one but two compact radio cores. 
These probably represent two separate quasar nuclei for the following reasons: (i) the ap- 
pearance, surface brightness, and scale of these differ sufficiently from the classical ‘radio 
doubles’ to indicate that they cannot be easily explained in terms of lobes from a common 
nucleus; (ii) VLBI observations (Diamond et al. 1989) indicate that both have pc-sized 
central components; (iii) high-resolution near-infrared images (Carico et al. 1990; Graham 
et al. 1990) show two condensations, which are presumably galactic nuclei, coincident with 
the radio cores. 
Each of these cores therefore bears the hallmarks of a compact quasar nucleus, indi- 
cating that the pair represents either a gravitationally lensed core or else a pair of merging 
nuclei. Because of the strong evidence for a aerger, and in the absence of any other evi- 
dence for a gravitational lens, I consider here only the hypothesis that Arp 220 contains a 
pair of merging nuclei. 
The two nuclei of Arp 220 have a projected separation of about 350 pc, compared 
to the kpc separations normally observed in interacting active galaxies. Modelling and 
numerical calculations (e.g. Begelman et al. 1980; Roos, 1985) have shown that two 
merging nuclei would spend only a short period.at such a small separation before spiralling 
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in towards their common potential well. Thus such a configuration is so rare that it can 
account for only a small fraction of ELFs. 
I conclude that Arp 220 no longer appears archetypal, but instead appears to be an 
extremely peculiar ELF. It has been noticeable in this conference how many graphs have 
been shown with their axes extended to accommodate Arp 220! It has many superlative 
claims: it has the highest far-IR excess of any UGC galaxy; it was the first OH megamaser 
(and still the brightest) to be discovered; and it refuses to obey the well- established radio- 
FIR correlation. Its optical spectrum appears at first to be that of a starburst, but closer 
examination (and infrared spectra) show broad quasar-like wings. Of the tens of ELFs 
now studied in detail, Arp 220 remains unique. Thus there is no conflict with the inferred 
brief lifetime and rarity of its closely separated cores. 
Arp 220 is in many senses the most extreme ELF, and yet typifies many of the 
properties of other ELFs, and so we must consider its position in the hierarchy of ELFs. 
ARE QUASAR CORES PRESENT IN ALL ELFS? 
The evidence that Arp 220 is powered by an obscured quasar core has led to the 
suggestion (Sanders et  al. 1988) that all such objects are powered by quasar cores, and 
that ELFs represent nascent quasars. However, the competing hypotheses that at least 
some of these objects are powered by ‘super-starburst’ activity or by inter-galactic collisions 
have also gained currency. The plot is thickened further by the common occurrence of tidal 
tails, interwting companions, and other evidence of disruption. 
A long-baseline radio interferometer provides a keen tool for discriminating between 
these mechanisms, since such an observation is sensitive only to fine structure with high 
brightness temperature. The 275-km Parkes-Tidbinbilla Interferometer (PTI) has therefore 
been used to survey a sample of Seyferts, starbursts, and ELFs. The PTI can detect pc- 
sized quasar cores, with brightness temperatures NN lo8 K, but is blind to the extended 
kpc-sized starburst regions which have typical brightness temperatures of lo4 K. 
In our sample of normal starbursts and Seyferts, we found (Norris et  al. 1988, 1990) 
that radio cores are common in optically selected Seyferts, but rare in optically selected 
starburst galaxies, confirming that the Seyferts are intrinsically different from starbursts, 
and verifying the validity of the technique. 
When we turned our attention to a sample of ELFs previously classified by their 
optical spectral line ratios into Seyfert-like and starburst-like, we found that radio cores are 
common in ELFs with Seyfert-like spectra, but rare otherwise. These results demonstrate 
that there are two separate populations of objects masquerading as ELFs. In one of these 
populations, the ELFs do contain quasar cores and betray them by a Seyfert-like spectrum. 
However, those in the other population, which includes some of the most luminous ELFs, 
do not, and therefore require some other mechanism to explain their activity. 
TOWARDS A UNIFIED MODEL FOR ELFS 
We are forced into the unsatisfactory situation of requiring two different types of 
ELFs: those with Seyfert-like spectra which often contain a radio core, and those with 
starburst-like spectra which rarely contain a radio core. Our observations so far place tens 
of members in each class. Arp 220 appears to occupy a third class: a starburst galaxy 
388 
concealing two quasar cores. Obviously, a model to explain ELFs must explain these 
results, and must also embody the strong evidence for interactions in ELFs of all classes. 
Here I speculate on one such model, although clearly variations on this theme are 
also possible. I assume that some, but not all, galaxies contain quiescent massive black 
holes (MBH) at their centres, legacies of their quasar youths, and consider the later stages 
of the merging of two galaxies. The models and timescales used here are derived from 
calculations and simulations such as those of Begelman e t  al. (1980), Roos (1981, 1989), 
and Hernquist (1989). 
(i) Two spiral galaxies, which may or may not contain a quiescent MBH, start the 
merging process by entering a decaying binary orbit with a separation of a few kpc. During 
this process, shock waves in the disks of the galaxies cause bursts of star formation and 
the consequent heating of large masses of dust. In this stage the object appears as a 
starburst-like ELF. If a MBH is present, then the kinematic disruption within the body of 
the galaxy may start refuelling of the MBH, resulting in a mildly Seyfert-like ELF. The 
Seyfert or quasar core may not appear optically at this stage because of the high extinction 
to the galactic centre. This phase lasts for perhaps lo9 years. 
(ii) Dynainical friction eventually causes the two galactic nuclei to approach each 
other to within a few hundred parsecs, with the luminosity of both starburst and Seyfert 
activity increasing. If two MBH are present, both are fuelled, giving the appearance of two 
independent quasar cores, as in Arp 220. Note that if there are no MBH then no quasar 
cores will be present, so presumably the object will appear as a particularly luminous 
starburst-like ELF. This phase lasts for perhaps lo’ - 10’ years. 
(iii) Eventually, the binary orbit decays and the nuclei merge. The starburst activity 
and MBH refuelling (if present) will continue, but gradually the dust will be blown away 
(or converted into stars) exposing the nucleus. The galaxy will then appear as a normal 
Seyfert or starburst galaxy, depending on whether a MBH is present. This activity may 
continue for about lo7 - 10’ years until the starburst cycle ends, the MBH loss-cone orbits 
are depleted, and the galaxy returns once more to its inactive state. 
Other embellishments to this model are also possible. For example, we may consider 
whether we should, instead of simply classifying objects as Seyfert or starburst, place them 
on a starburst-Liner-Seyfert scale. Their position on this scale would then be determined 
by the ratio of MBH to starburst activity, which in turn may depend simply on the masses 
of MBH within the galaxies. We can also speculate whether the eventual product of several 
such mergers might be an elliptical galaxy. 
Finally, I note that the model above is not a unique solution to the observational 
data. It is obvious that there is a diversity of significant relationships between the different 
classes of active galaxy, and consequently the time is ripe for an ‘HR diagram’ or ‘periodic 
table’ of active galaxies in general. Further observations, particularly at radio and infrared 
wavelengths, will be required before we can determine whether the model presented here, 
or some other model, may play a central role in the construction of this diagram. 
ACKNOWLEDGEMENTS 
I thank James Graham and Dave Carico for permission to cite material prior to 
publication, and Dave Allen, Jim Caswell, and Alan Roy for comments on this paper. 
389 
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DePoy, D. L., Becklin, E. E., & Geballe, T. R., 1987, Astrophys. J. Lett., 316, L63. 
Diamond, P. J., Norris, R. P., Baan, W. A., & Booth, R. S., 1989, Astrophys. J., 340, 
Graham, J. R., Carico, D. P., Matthews, K., Neugebauer, G., & Soifer, B. T., 1990. 
Heckman, T. M., van Breugel, W., Miley, G. K., & Butcher, H. R., 1983, Astron. J., 
Hernquist, L., 1989, Nature, 340, 687. 
Joseph, R. D., Wright, G. S., & Wade, R., 1984, Nature, 311, 132. 
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390 
Kembhavi: Could Sey-El fs  be r e l a t i v i s t i c a l l y  beamed Sb-Elfs?  
Norris: T h i s  is a ve ry  i n t e r e s t i n g  q u e s t i o n  t h a t  w e  are a c t i v e l y  
p u r s u i n g .  My q u e s s  is t h a t  a beaming model ( a s  s u g g e s t e d  by 
B a r t h e l  (1989 and o t h e r s  may be a f a c t o r ,  b u t  it canno t  e x p l a i n  
e v e r y t h i n g .  F o r  example,  B a r t h e l ' s  model would s u g g e s t  t h a t  w e  
shou ld  d e t e c t  S y I s  more o f t e n  than  S y I I s ,  whereas ,  i f  a n y t h i n g ,  
w e  d e t e c t  more S y I I s  t han  SyI s .  
Beckman: I t  is ve ry  i n t e r e s t i n g  t h a t  you f i n d  t h a t  many 
F I R G ' s / E L F ' s  ( l i k e  Arp 2 2 0 )  p r o b a b l y  have "bur i ed"  AGN's. 
However, it is no t  clear t h a t  t h e s e  AGN's make i m p o r t a n t  
c o n t r i b u t i o n s  t o  t h e  o v e r a l l  ene rgy  budget .  R i e k e  showed t h a t  
"u l t r a - luminous"  f a r - IR  g a l a x i e s  are s t r o n g l y  d e f i c i e n t  i n  hard  
X-ray e m i s s i o n  compared t o  t h e  h y p o t h e s i s  t h a t  a l l  of t h e  I R  
e m i s s i o n  is powered by a s t a n d a r d  QSO. H e  concluded t h a t  A G N ' s  
c o u l d  c o n t r i b u t e  o n l y  a small f r a c t i o n  of t h e  I R  l u m i n o s i t y ,  
u n l e s s  t h e y  were ve ry  p e c u l i a r  AGN's t h a t  were abnormal ly  weak a t  
10  keV. 
Norris: Y e s .  I t ' s  i n t e r e s t i n g  t h a t ,  a l t h o u g h  we've shown t h a t  
some, l i k e  Arp 220, - do c o n t a i n  a b u r i e d  q u a s a r ,  many ELF 'S  d o n ' t  
c o n t a i n  one ,  and so w e  are now f o r c e d  to  search f o r  a n  
a l t e r n a t i v e  mechanism. Given t h e  e x i s t e n c e  of t h i s  mechanism, w e  
must t hen  a s k  how i m p o r t a n t  it is i n  t h i n g s  l i k e  Arp 220. I 
d o n ' t  have an  answer ,  b u t  my q u e s s  is t h a t ,  as you s u g g e s t ,  t he  
q u a s a r  core may n o t  a t  p r e s e n t  be t h e  dominant ene rgy  source. 
The e x i s t e n c e  of a q u a s a r  c o r e ,  however, may still  be impor t an t  
as w e  t r y  t o  p l a c e  ELF'S i n t o  t h e  " p e r i o d i c  t a b l e "  of g a l a x i e s ,  
and t h e  core may evo lve  w i t h  t i m e  i n t o  a dominant p o s i t i o n .  
van Breugel: What are t h e  s p e c t r a l  i n d i c e s  of t h e  t w o  r a d i o  
components i n  Arp 2203 I f  bo th  components (one  is even r e s o l v e d )  
have a s t e e p  s p e c t r a l  index  and indeed are AGN's, t h e n  what is 
t h e  p h y s i c a l  n a t u r e  of t h e  r a d i o  e m i s s i o n  (AGN's  a t  r a d i o  
wave leng ths  normal ly  have f l a t  r a d i o  s p e c t r a  and are e x t r e m e l y  
compact; i.e., s y n c h r o t r o n  s e l f - a b s o r b e d ,  
Simkin: Comment on d i s c u s s i o n  by Norris and Van Breugel--"Why is 
t h e  core of  Arp 220 n o t  a f l a t  spec t rum r a d i o  source?" (by  Van 
B r e u g e l ) .  S e y f e r t s  are n o t  r a d i o  loud.  
Norris: I a g r e e .  It's dangerous  to  assume t h a t  S e y f e r t s  shou ld  
have f l a t - s p e c t r u m  cores l i k e  t h e  r ad io - loud  q u a s a r s  do. 
O b s e r v a t i o n a l l y ,  m o s t  S e y f e r t s  do n o t  seem t o  have f l a t - s p e c t r u m  
cores, and t h e i r  r a d i o  l u m i n o s i t i e s  would c l a s s i f y  most of them 
as radio-quiet q u a s a r s .  
39 I 
Norris: They both have steep ( a  - - 0.7)  indices, but then 
observationally I would say that most Seyferts have steep- 
spectrum cores, too. As for the physical origin of this spectral 
index, I'm not sure.what it's telling us. Perhaps an appreciable 
amount of the core emission is actually from a nuclear 
starburst. In the case of Arp 220 I presume that, in addition to 
quasar cores, starburst activity contributes a substantial 
fraction of the energy. 
392 


Are extremely luminous far-infrared galaxies the result of merging quasar cores

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