The Infrared Astronomical Satellite (IRAS) survey uncovered a class of Extremely Luminous Far Infrared Sources (ELFS), exhibiting luminosities up to and occasionally exceeding 10 to the 12th power L sub 0. Arguments are presented to show that sources with luminosities L equal to or greater than 3 x 10 to the 10th power L sub 0 may represent gas rich galaxies in collision. The more conventional explanation of these sources as sites of extremely active star formation fails to explain the observed low optical luminosities of ELFS as well as their high infrared excess. In contrast, a collisional model heats gas to a temperature of approx. 10 to the 6th power K where cooling takes place in the extreme ultraviolet. The UV is absorbed by dust and converted into far infrared radiation (FIR) without generation of appreciable optical luminosity. Gas recombination as it cools generates a Lyman alpha photon only once for every two extreme ultraviolet approx. 50eV photons emitted by the 10 to the 6th power gas. That accounts for the high infrared excess. Finally, the model also is able to explain the observed luminosity distribution of ELFS as well as many other traits

Harwit, Martin

Houck, James R.

Palumbo, Giorgio G. C.

Soifer, B. Thomas

English

NASA Technical Reports Server

N87- 24323
EXTREMELY LUMINOUS FAR-INFRARED SObR_CES (ELFS)*
Martin Harwit and James R. Houck
Center for Radiophyslcs and Space Research, Department of Astronomy
Ithaca, New York 14853-6801
B. Thomas Soifer
Division of Physics, California Institute of Technology
Pasadena, California 91125
Giorgio G. C. Palumbo
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT. The Infrared Astronomical Satellite (IRAS) survey uncovered a class
of Extremely Luminous Far-Infrared Sources (ELFS), exhibiting luminosities up
to and occasionally exceeding 1012 L_. We present arguments to show that sour-
ces with luminosities L >__3×1010 L_ may represent gas-rlch galaxies in colli-
sion. The more conventional explanation of these sources as sites of extremely
active star formation fails to explain the observed low optical luminosities of
ELFS as well as their high infrared excess. In contrast, a collislonal model
heats gas to a temperature of -I06K where cooling takes place in the extreme
ultraviolet. The UV is absorbed by dust and converted into far-infrared radia-
tion (FIR) without generation of appreciable optical luminosity. Gas recombln-
ing as it cools generates a Lyman-a photon only once for every two extreme
ultraviolet -50eV photons emitted by the 106K gas. That accounts for the high
infrared excess. Finally, our model also is able to explain the observed
luminosity distribution of ELFS as well as many other traits.
I. INTRODUCTION TO THE DATA
The most luminous sources identified through the IRAS survey exhibit these
properties:
i) Their FIR luminosities approach and sometimes exceed 1012 L_ (Soifer
et al. 1986),
ii) The FIR flux appears uncorrelated with the optical and near infrared
luminosity which typically remains around i0I0 L_ characteristic of
fairly normal spirals (Houck et al. 1985, Allen et al. 1985),
iii) Many ELFS appear to be irregular, disturbed or interacting galaxies
(Houck et al. 1985),
The authors have submitted a more comprehensive article (Harwlt et al.
1987) on the same topic to The Astrophysical Journal. Here we only s_marize
the findings presented in greater detail in that paper.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 387
https://ntrs.nasa.gov/search.jsp?R=19870014890 2020-03-20T11:07:48+00:00Z
M.O.HARWITETAL.
iv)
v)
vi)
vii)
viii)
ix)
x)
xi)
The FIR luminosity is directly proportional to the 21-ca radio
continuum luminosity (Sanders and Mirabel 1985),
CO observations at 2.6mm indicate a high molecular hydrogen content
~i0 I0 M_ (Sanders and Mirabel 1985), and
The CO lines are abnormally broad, indicating high-velocity motions.
The infrared excess -- the ratio of FIR to Lyman-line radiation -- is
higher by an order of magnitude than in HII complexes (Beck et al.
1986, DePoy et al. 1986).
Low-ionization states of atoms are more prevalent than the more highly
ionized species generally found in HII regions (Allen et al. 1985).
The luminosity distribution function is roughly proportional to
L-5/2 for L > 3x10 I0 L_ but declines less steeply below a break in
the curve at---3xl0 I0 L_ (Lawrence et al. 1986, Soifer et al. 1986).
Only one extragalactic source in ~106 has FIR luminosity above ~1012
L_ (Soifer et al. 1986).
Intense vibrational H 2 emission characterizes many ELFS (Joseph
et al. 1984).
2. MODEL FOR ELFS
We assume that ELFS are the byproduct of collisions among gas-rich galax-
ies. We picture most of the gas in each of the colliding partners to be
concentrated in a central disk, roughly 1 kpc in radius, 0.I kpc thick and
containing -_0 I0 M_ of gas. The indicated pre-collision gas density then is
nH ~ i0 cm- • Galaxy-galaxy collisions are expected to occur at free infall
velocities _ 500 km sec -I. The observed CO line widths are consistent with
such high velocities and the line strengths suggest the high gas content we
have assumed.
If the disks collide face on, the dissipated kinetic energy gives rise to
a luminosity -1012 Lm for about 3x105y. More oblique collisions can account
for lower luminosities, often with longer enduring interaction. A rough numer-
ical calculation that takes into account a wide range of initial orientations
and of center-to-center distances for the colliding pairs gives the luminosity
(the dissipation) distribution function shown in Fig. I. The line drawn
through the data points has been arbitrarily anchored to the probability with
which the most luminous galaxies are observed. There are no other assumptions
aside from the above[listed disk dimensions and mass. The shaded extension of
the curve around I0 L_ esimates the contribution of the many glancing (weak)
collisions. Collisional activity should not make a pronounced contribution for
luminosities well below 3xlO L_, and this appears to account for the break in
the luminosity distribution below that luminosity.
Calculation of the luminosity of §tars that could form from i0 I0 M_ of gas
suggest that a total luminosity of I0 Im L_ could only be reached with unusually
efficient conversion of gas into stars and with an exceptionally high luminosi-
388
re')
I
10-3,
10.4
t0-5 _
1Ue-
ld 7 -
I
__+__
o
EXTREMELY LUMINOUS FAR-INFRARED SOURCES
I
• IRAS Galaxies
o Normal Galaxies
Starburst
• Seyfert
o Quasars
!
Lto , [Le]
Figure 1 - Data on the luminosity distribution function among luminous FIR
sources (Soifer et al. 1986) and a calculated fit (solid line and
shaded area). A derivation of this fit is presented in a
full-length paper by the authors (Harwit et al. 1987).
389
M.O.HARWITETAL.
ty to mass ratio for the stars that are formed. Even then, however, we would
have to account for the exceptionally low optical luminosity, since it is not
clear how one could systematically block all but ~I percent of the optical
emission, converting all of that radiation instead into FIR. It would also be
difficult to explain the low observed atomic hydrogen line-emission or the
radiation from predominantly singly ionized, rather than multiply ionized
species. All of these observations break with those traditionally
characteristic of HII complexes around massive, luminous stars.
The collisional model does not face these difficulties. Observed gas
masses and velocities can account for dissipation rates consistent with the
observed luminosities. At collision (Fig. 2) gas temperatures rise to >I06K.
At this temperature the bulk of the ener_ loss is via cooling through extreme
ultraviolet (EUV) radiation. Photons with energies in the 50eV range are
emitted. There is little optical flux. Much of the EUV emission is absorbed
by grains and converted directly into FIR. Detailed considerations show that
only every second EUV photon, on the average, leads to ionization followed by
recombination -- to a a Lyman-e photon. This accounts for the high infrared
excess. The optical line radiation emitted in this layered model (Fig. 3)
comes largely from a dusty, partially ionized layer. The partial ionization of
the hydrogen accounts for the absence of highly ionized species; those would
quickly jump to a lower ionization state by charge exchange with a hydrogen
atom. Molecular hydrogen is vibrationally excited in this same partly ionized
regime, well upstream of the impact front.
(a) (b) (c) (d) (e)
Molecular Cloud
} Cosmic Roy Particles
Ionized Gas
Figure 2 - Schematic Diagram of Colliding Gas Clouds. Two molecular clouds
approach. (a) Upon contact, the gas between them is ionized and
heated to temperatures above 106K. (b) Ionizing radiation escapes
into the molecular gas and produces an ionized layer even before
that gas has a chance to collide with the stationary layer of hot
gas that has formed. As the collision proceeds, (c,d) the central
layer thickens until no more molecular gas remains. Then the hot
outer layers of the ionized cloud expand outward to meet trailing,
more tenuous halo gas from each of the colliding galaxies (e).
390
EXTREMELY LUMINOUS FAR-INFRARED SOURCES
i s Rodio, X-roy ond Gommo°roy Halo )
Trailing Cloud
Edge
C
o
0
0
la.
Impact Front
Cold Neutral Gas
n ,.. t0:3 cm"3 T_.50K
Molecular, Atomic and
Singly lo.ized Species
nH ~ 103 cm-3
tot9 Pre- ionizt_ Gascm"s T~t K
I hi, he.,. .
Hot Ionized Gas
10t6 ne ~ 4 x 103 cm"3 T -,. t06K
015 Cooling Gas
ne~4xtO 4cm "3 T~105K
Largely Ionized Gos
ne~ 4xtOScm -3 T,,.tO4K
Neutral Gas
n > 4 x 105 cm-3 T_< I04K
Stationary Center__..l
of Impact Zone
Figure 3 - Layered structure of colliding clouds. At the bottom is the central
portion of the colliding disks. At the top the outer pre-collision
gas.
More comprehensive arguments also suggest that the cosmic ray acceleration
rate and hence the radio flux should be proportional to the gas content of
colliding regions, leading to the observed proportionality of FIR to radio
continuum emission.
3. PREDICTION
The 106K layer should give rise to highly ionized species such as OIV and
391
M. O. HARWIT ET AL.
NeV, whose fine-structure emission, respectively at 25.87 and 24.28_m, should
penetrate through obscuring dust layers, to be observable from Earth. That
radiation would not be produced in HII regions and, if observed, would
discriminate against starburst models.
ACKNOWLEDGEMENTS
At Cornell, this work was supported by NASA Grant Nb_W-761 and JPL
Contract 954603. B.T.S. is supported by NASA under the IRAS Extended Mission
Program. G.G.C.P. appreciates the hospitality of Drs. E. A. Boldt and S. S.
Holt during his stay at NASA/GSFC as a NRC/NAS Research Associate.
REFERENCES
Allen, D. A., Roche, P. F., Norris, R. P. 1985, M.N.R.A.S., 213, 67P.
Beck, S. C., Turner, J. L. and Ho, P. T. P. 1986, Ap. J., to be published.
DePoy, D. L., Becklin, E. E., and Wynn-Williams, C. G. 1986, Ap. J., 307, 116.
Harwit, M., Houck, J. R., Soifer, B. T., and Palumbo, G. G. C. 1987, Ap. J.,
submi tted.
Houck, J. R., Schneider, D. P., Danielson, G. E., Beichman, C. A., Lonsdale,
C. J., Neugebauer, G., and Soifer, B. T. 1985, Ap. J., 290, L5.
Joseph, R. D., Wright, G. S., and Wade, R. 1984, Nature, 311, 132.
Lawrence, A., Walker, D., Rowan-Robinson, M., Leech, K. J., and Penston, M. V.
1986, M.N.R.A.S., 219, 687.
Sanders, D. B., and Mirabel, I. F. 1985, Ap. J. (Letters), 298, L31.
Soifer, B. T., Sanders, D. B., Neugebauer, G. Danielson, G. E., Lonsdale, C.
J., Madore, B. F., and Persson, S. E. 1986, preprint.
DISCUSSION
BURBIDGE:
First, I would like to remind everyone that more than thirty years ago collisions were thought to be the
explanation of radio galaxies. The idea did not last! Second, I want to say that my own view is that the
brightest IRAS galaxies are young galaxies forming their first generations of massive (,,_ 100 M®) stars which
give both line luminosity and the dust. A few times 105 stars are needed, and about 10 generations of them,
to make enough dust.
HARWIT:
First, observations tell us that galaxy interaction appears involved. Many of the most extremely luminous
sources clearly appear to be interacting partners.
Second, the most luminous phases may last ,,_ 3 x l0 s yr, perhaps an order of magnitude less than the
lifetime of highly luminous stars. Lower luminosity, more oblique collisions will last an order of magnitude
longer. But we are dealing with events of extremely low probability here. Only one galaxy in a million has
a luminosity in excess of 1012 L®. Quantitatively, our model appears capable of explaining that number of
extremely luminous sources.
SCOVILLE:
I don't think the high LIR/LBrc, line ratio is an insurmountable problem for massive star formation models
in interacting galaxies. The HII regions there will have higher dust abundances than standard Galactic HII
regions because the cloud-cloud velocities are >__100 km s-1 and fresh grains will be supplied continually to
392
EXTREMELY LUMINOUS FAR-INFRARED SOURCES
the ionized gas surrounding the OB stars.
HARWIT:
I would really like to see quantitative estimates of such effects. It appears very difficult to provide a
dust absorption shield which will have no holes, and will prevent all but 1% of the optical radiation from
escaping. The luminosities involved are so high that dust and gas constituting a shrouding envelope could be
blown away by radiation pressure if it did not already have the enormous approach velocities involved in our
model.
SHULL:
Your large ratios of infrared to optical flux are predicated on the assumption that the emitting regions are
buried deep inside large homogeneous slabs. If you take into account a cloudy interstellar medium, wouldn't
much of the optical radiation escape through the 'holes'? Note that much of the cooling will take place in
optically thin optical-IR forbidden lines.
HARWIT:
The emitting regions in our model primarily radiate in the far ultraviolet which is readily absorbed by
the interstellar gas even if it escapes through holes between clouds. No direct optical radiation is involved.
Recombination in tenuous ambient interstellar gas ionized in this way will be extremely slow and result in
low luminosity emission from the galaxies' halos.
de JONG:
You have normalized the infrared luminosity function, predicted on the basis of your model, to the ob-
served infrared luminosity function. This implies a certain collision frequency of galaxies at impact parameters
of ,,_1 kpc. Have you verified that this implied collision frequency is consistent with that directly derivable
from observations?
HARWIT:
Schweizer, in a recent article in Science, estimates that one galaxy in every ten appears to have undergone
an interaction in its past. Such figures are still not well established, but they appear sufficiently high,
particularly if we recall that many initial encounters may be quite weak - just strong enough to bind two
galaxies together in rather eccentric orbits. There may then be several relatively close passes before a
substantial collision occurs. In summary, the number of passes two galaxies make past each other could be
few times higher than the number of captures that take place.
393


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