On the Connection between Interaction
between Galaxies and Activity of AGN
S. Bernhart
Active galactic nuclei (AGN) are among the most powerful objects known in the universe
and their principle energy source is thought to be the famous and still mysterious 'Black
Holes'. In the last few years, high resolution Hubble Space Telescope studies have
provided overwhelming evidence for the presence of supermassive black holes in nearby
active galaxies and for an association between galaxy interactions and AGN. It seems
possible that e.g., quasar hosts involved in recent collisions are not only common, but
perhaps ubiquitous. It has been speculated that the supermassive black holes may be fed
or even formed in galaxy-merging events.
Outline
Galaxy collisions in general
Orders of Magnitude
Historical overview (partly from F. Schweizer 1986, Science,
231, 227)
The importance of collisions
Historical Highlights
Major merger dynamics
Minor mergers
Mergers
New disks
AGN in collisional galaxies
Overview: Curtis Struck, Galaxy Collisions, astro-ph/9908269
Galaxy Collisions – Orders of Magnitude
Close encounter that has a significant effect on one of the galaxies involved, also called
'tidal interaction', 'a near miss is as good as a hit'
Two objects with ~10^12 solar masses meet with typical relative velocities of ~300 km/s,
collision energy is of order 10^53 J, equivalent to about 10^8 to 10^9 SN
Typical timescales of order 3 x 10^8 yrs
Most of the matter involved doesn't collide with anything. Most of the mass in a typical
galaxy consists of collisionless dark matter, that passes through with no effects except for
those due to their collective gravitational forces. Similarly, the probability for direct starstar collisions is of order 10^(-15) for a typical star like the Sun (cross section
~10^17/m^2, surface density of stars near the Sun ~10^(-32)/m^2 )
But there must be direct collisions between the various gas components when two gasrich disk galaxies collide – observational evidence: supersonically colliding gas clouds
Galaxy Collisions – Historical Overview
1940 E. Holmberg: galaxies may collide, experience friction and eventually merge (Holmberg,
E. 1940, ApJ, 92, 200):
galaxy encounters are likely because their main separation is small (~10 to 100 times their
diameter)
during an encounter, they induce tides in each other that cost them orbital energy
their orbits must shrink and the involved galaxies may even coalesce
This hypothesis received little attention during the next 30 years for several reasons:
astronomers thought encounters were rare random events
they ignored Holmberg's evidence that most galaxies orbit in double and multiple systems with
recurring opportunities of colliding
even Zwicky's extensive observations of interacting galaxies in the 1950's (F. Zwicky, Ergeb. Exakten
Naturwiss. 29, 344 (1956); Handb. Phys. 53, 373 (1959)) were not accompanied by similar progress in
theory, partly because of the prevailing notion that gravitation could not produce the long
filaments he photographed
1963 discovery of quasars and their enormous energy outputs, astronomers believed that
gigantic explosions were taking place in galaxies that presumably also explained the filaments
Infrared detector technologies were not yet well-developed
Galaxy Collisions – Historical Overview
The serious scientific study of galaxy collisions began in the early efforts to discover a
morphological classification system for galaxies (Hubble tuning fork scheme).
Shapley states: (Shapley, H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge,
Harvard University Press)): There are also plate spirals... and frankly "pathological" types, (as Baade
calls such freaks) like NGC 5128...and the ring-tail system, NGC 4038-9 ...
The theories that sufficiently explain the relatively simple looking Sc spiral, like Messier 33, and the
most common galaxies in Virgo, must have sufficient flexibility to take care of these aberrant types.
The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will
find some justification, because the individual galaxies are not so far separated but that encounters
may have been fairly numerous, if the time scale has been long enough....
Baade about the frequency of galaxy companions: Hubble and I had a long-standing bet of
$20 for the one who could first convince the other that a system which he found was single. We
could never decide the bet; neither of us could pull out some distant fellow - in some cases there
really was a companion and in other cases there could be. So single galaxies may be rare. (Baade,
W. 1963, Evolution of Stars and Galaxies, ed. by C. Payne-Gaposhkin, (Cambridge, Harvard University Press))
Galaxy Collisions – Historical Overview
The serious scientific study of galaxy collisions began in the early efforts to discover a
morphological classification system for galaxies (Hubble tuning fork scheme).
Shapley states: (Shapley, H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge,
Harvard University Press)): There are also plate spirals... and frankly "pathological" types, (as Baade
calls such freaks) like NGC 5128...and the ring-tail system, NGC 4038-9 ...
The theories that sufficiently explain the relatively simple looking Sc spiral, like Messier 33, and the
most common galaxies in Virgo, must have sufficient flexibility to take care of these aberrant types.
The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will
find some justification, because the individual galaxies are not so far separated but that encounters
may have been fairly numerous, if the time scale has been long enough....
Baade about the frequency of galaxy companions: Hubble and I had a long-standing bet of
$20 for the one who could first convince the other that a system which he found was single. We
could never decide the bet; neither of us could pull out some distant fellow - in some cases there
really was a companion and in other cases there could be. So single galaxies may be rare. (Baade,
W. 1963, Evolution of Stars and Galaxies, ed. by C. Payne-Gaposhkin, (Cambridge, Harvard University Press))
Galaxy Collisions – Historical Overview
The serious scientific study of galaxy collisions began in the early efforts to discover a
morphological classification system for galaxies (Hubble tuning fork scheme).
Shapley states: (Shapley, H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge,
Harvard University Press)): There are also plate spirals... and frankly "pathological" types, (as Baade
calls such freaks) like NGC 5128...and the ring-tail system, NGC 4038-9 ...
The theories that sufficiently explain the relatively simple looking Sc spiral, like Messier 33, and the
most common galaxies in Virgo, must have sufficient flexibility to take care of these aberrant types.
The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will
find some justification, because the individual galaxies are not so far separated but that encounters
may have been fairly numerous, if the time scale has been long enough....
Baade about the frequency of galaxy companions: Hubble and I had a long-standing bet of
$20 for the one who could first convince the other that a system which he found was single. We
could never decide the bet; neither of us could pull out some distant fellow - in some cases there
really was a companion and in other cases there could be. So single galaxies may be rare. (Baade,
W. 1963, Evolution of Stars and Galaxies, ed. by C. Payne-Gaposhkin, (Cambridge, Harvard University Press))
Galaxy Collisions – Historical Overview
The serious scientific study of galaxy collisions began in the early efforts to discover a
morphological classification system for galaxies (Hubble tuning fork scheme).
Shapley states: (Shapley, H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge,
Harvard University Press)): There are also plate spirals... and frankly "pathological" types, (as Baade
calls such freaks) like NGC 5128...and the ring-tail system, NGC 4038-9 ...
The theories that sufficiently explain the relatively simple looking Sc spiral, like Messier 33, and the
most5128
common
galaxies
NGC
– Centaurus
A in Virgo, must have sufficient flexibility to take care of these aberrant types.
The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will
find some justification, because the individual galaxies are not so far separated but that encounters
may have been fairly numerous, if the time scale has been long enough....
Baade about the frequency of galaxy companions: Hubble and I had a long-standing bet of
$20 for the one who could first convince the other that a system which he found was single. We
could never decide the bet; neither of us could pull out some distant fellow - in some cases there
really was a companion and in other cases there could be. So single galaxies may be rare. (Baade,
W. 1963, Evolution of Stars and Galaxies, ed. by C. Payne-Gaposhkin, (Cambridge, Harvard University Press))
NGC 4038-9 - Antennae
Credit: Brad Whitmore (STScI) and NASA
Galaxy Collisions – Historical Overview
The serious scientific study of galaxy collisions began in the early efforts to discover a
morphological classification system for galaxies (Hubble tuning fork scheme).
Shapley states: (Shapley, H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge,
Harvard University Press)): There are also plate spirals... and frankly "pathological" types, (as Baade
calls such freaks) like NGC 5128...and the ring-tail system, NGC 4038-9 ...
The theories that sufficiently explain the relatively simple looking Sc spiral, like Messier 33, and the
most common galaxies in Virgo, must have sufficient flexibility to take care of these aberrant types.
The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will
find some justification, because the individual galaxies are not so far separated but that encounters
may have been fairly numerous, if the time scale has been long enough....
Baade about the frequency of galaxy companions: Hubble and I had a long-standing bet of
$20 for the one who could first convince the other that a system which he found was single. We
could never decide the bet; neither of us could pull out some distant fellow - in some cases there
really was a companion and in other cases there could be. So single galaxies may be rare. (Baade,
W. 1963, Evolution of Stars and Galaxies, ed. by C. Payne-Gaposhkin, (Cambridge, Harvard University Press))
Galaxy Collisions – Historical Overview
1954 pioneering observational paper by Baade & Minkowski: "the radio source Cygnus A
is an extragalactic object, two galaxies in actual collision." (Baade, W. and Minkowski, R. 1954, ApJ,
119, 206)
In the following years other colliding systems were discovered:
Many galaxies came from Dreyer's New General Catalog (NGC) and his later Index Catalog
additions to it (Sulentic, J. W., & Tifft, W. G. 1973, The Revised New General Catalogue of Nonstellar
Astronomical Objects, (Tucson, University of Arizona Press))
Shapley-Ames photographic survey of all galaxies above a certain limiting brightness (Shapley,
H. Arp undertook the search for peculiar galaxies in the Palomar all-sky Schmidt camera
survey (Arp, H. C. 1966, Atlas of Peculiar Galaxies, (Pasadena, California Institute of Technology))
Theoretical work:
1972 Alar and Juri Toomre: landmark paper 'Galactic Bridges and Tails' (A. Toomre and J. Toomre,
Astrophys. J. 178, 623 (1972))
H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge, Harvard University Press))
Paper led to further theoretical studies and to identification of an increasing number of
merging galaxies by observers in the late 1970's
Galaxy Collisions – Historical Overview
1954 pioneering observational paper by Baade & Minkowski: "the radio source Cygnus A
is an extragalactic object, two galaxies in actual collision." (Baade, W. and Minkowski, R. 1954, ApJ,
119, 206)
In the following years other colliding systems were discovered:
Many galaxies came from Dreyer's New General Catalog (NGC) and his later Index Catalog
additions to it (Sulentic, J. W., & Tifft, W. G. 1973, The Revised New General Catalogue of Nonstellar
H. Arp undertook the search for peculiar galaxies in the Palomar all-sky Schmidt camera
survey (Arp, H. C. 1966, Atlas of Peculiar Galaxies, (Pasadena, California Institute of Technology))
Theoretical work:
1972 Alar and Juri Toomre: landmark paper 'Galactic Bridges and Tails' (A. Toomre and J. Toomre,
Astrophys. J. 178, 623 (1972))
Astronomical Objects, (Tucson, University of Arizona Press))
Messier 51, NGC 5194 and companion
Shapley-Ames
photographic
survey of all galaxies above a certain limiting brightness (Shapley,
NGC 5195,
Arp 85, Whirlpool
Galaxy
H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge, Harvard University Press))
Paper led to further theoretical studies and to identification of an increasing number of
merging galaxies by observers in the late 1970's
Galaxy Collisions – Historical Overview
1954 pioneering observational paper by Baade & Minkowski: "the radio source Cygnus A
is an extragalactic object, two galaxies in actual collision." (Baade, W. and Minkowski, R. 1954, ApJ,
119, 206)
In the following years other colliding systems were discovered:
Many galaxies came from Dreyer's New General Catalog (NGC) and his later Index Catalog
additions to it (Sulentic, J. W., & Tifft, W. G. 1973, The Revised New General Catalogue of Nonstellar
H. Arp undertook the search for peculiar galaxies in the Palomar all-sky Schmidt camera
survey (Arp, H. C. 1966, Atlas of Peculiar Galaxies, (Pasadena, California Institute of Technology))
Theoretical work:
1972 Alar and Juri Toomre: landmark paper 'Galactic Bridges and Tails' (A. Toomre and J. Toomre,
Astrophys. J. 178, 623 (1972))
Astronomical Objects, (Tucson, University of Arizona Press))
Messier 51, NGC 5194 and companion
Shapley-Ames
photographic
survey of all galaxies above a certain limiting brightness (Shapley,
NGC 5195,
Arp 85, Whirlpool
Galaxy
H. 1943, Galaxies, 1st ed. (Philedelphia, Blakeston); 1961, Galaxies, 2st ed. (Cambridge, Harvard University Press))
Paper led to further theoretical studies and to identification of an increasing number of
merging galaxies by observers in the late 1970's
Galaxy Collisions – The Importance of Collisions
Toomre & Toomre (1972) suggested that collisions were more likely to occur between
galaxies on eccentric, bound orbits
Large-scale tidal distortions formed at the expense of orbital energy – therefore inevitably
merging of the galaxies (Toomre, A. 1974, in The Formation and Dynamics of Galaxies, (I.A.U. Symp. 58) ed. J. R.
Shakeshaft (Dordrecht: Reidel) p. 347)
Toomre & Toomre (1972): ...Would not the violent mechanical agitation of a close tidal
encounter - let alone an actual merger - already tend to bring deep into a galaxy a fairly
sudden supply of fresh fuel in the form of interstellar material, either from its own outlying
disk or by accretion from its partner? And in a previously gas-poor system or nucleus, would
not the relatively mundane process of prolific star formation thereupon mimic much of the
"activity" that is observed? - Enhanced star formation
Observational evidence in 1978 by Larson and Tinsley (Larson, R. B., & Tinsley, B. M. 1978, ApJ , 219, 46.)
who produced color evolution models for aging stellar populations – large color variations
were predicted in galaxies with significant bursts of SF which they, indeed, found in their
observations
Connection between galaxy collisions and nuclear activity in galaxies
Mergers – Historical Highlights
Toomre (1977): most elliptical might be formed from spirals in mergers; N-body simulations,
rapid merging (in Evolution of Galaxies and Stellar Populations, eds. B. M. Tinsley and R. B. Larson (New Haven, Yale
University Observatory), p 401)
First epoch of numerical simulations assumed collision partners of (near-) equal mass,
modeled by a single (spheroidal or halo) component
confirmation that galaxy collisions from initially nearly bound orbits generally result in rapid
merger
also showed that the merger remnant relaxed rapidly to a quasi-steady state, which was at least
qualitatively similar to galactic bulges or elliptical galaxies – 'violent relaxation'
Second epoch of simulations can be defined by two component (disk-halo) N-body models
and the first attempts to simulate gaseous dissipation in collisions and mergers (e.g. Barnes, J.
E., and Hernquist, L. 1991, ApJ, 370, L65)
revealed more complexity in both the merging process and in merger remnants, e.g. orbital
deceleration: massive halos merge first, and the merging of the denser disks and bulges is
faciliated by the binding of the large halo. As expected, the dynamical heating in this process
forms remnants of much earlier Hubble type than the progenitors, i.e., elliptical-like. However,
the memory of the initial structure and orbit are not immediately erased by the heating and
relaxation processes.
Mergers – Historical Highlights
The gas dynamical models showed that a large fraction of the interstellar gas can be funneled
deep into the core of the merger remnant.
"Third epoch" models (bulge-disk-halo), gas-plus-star models
1996, ApJ, 471, 115), not many major new results, but:
(e.g., Barnes, J. E., and Hernquist, L.
more extensive studies of collisional bars and their interaction with other components of the
merging galaxies
more sophisticated attempts to model the feedback effects of starbursts
large particle numbers simply allowed to produce better models for specific mergers
Great progress in observations pioneered by F. Schweizer (1983, in
Internal Kinematics and Dynamics
of Galaxies, ed. E. Athanassoula, (Dordrecht, Reidel), p. 319; 1998, in Galaxies: Interactions and Induced Star Formation,
Saas-Fee Advanced Course 26, eds. Friedli, D., Martinet, D., & Pfenniger, D.(Berlin, Springer), pg. 105):
examples of galaxies with multiple cores and of galaxies with gas disks that do not rotate at the
same rate as the coextensive stars (indicating a relative tilt), and of galaxies apparently
containing two gas disks; since that time many more multiple core, and "counter-rotating" disk
systems have been found
presence of large scale "tidal" features: ripples, shells, tails, etc., around many merger
candidates; in many of the systems Schweizer reviewed there is no obvious alternative to a
galaxy collision and merger origin.
Meanwhile statistical studies were beginning to provide evidence for enhanced "activity" in colliding
and merging galaxies, in the form of star formation and nuclear activity, especially in optical colors
and radio continuum emission
IRAS (Infrared Astronomical Satellite) mission in 1984, whole sky survey in the far-infrared (with
passbands centered at 60 and 100 microns)
Revealed numerous ultra-luminous infrared galaxies (ULIRGs or ULIGs), many of which were
soon determined to be merger remnants or other types of collisional galaxy (e.g. Soifer, B. T., et al.
1984a, ApJ, 278, L71 ; Soifer, B. T., et al. 1984b, ApJ, 283, L1)
Mergers – Historical Highlights
Big surprise: some colliding and merging galaxies emit up to 99% of their total luminosity in IR,
whereas normal galaxies emit typically less than half their luminosity there – supersonic
collisions of gas clouds seem to have triggered gigantic starbursts
Luminosity rises during the merger process and seems to peak relatively early sometimes
exceeding 10^12 solar luminosities, firmly established: almost all of the most infrared luminous
objects are active merger remnants (e.g. Sanders, D. B. & Mirabel, I. F. 1996, ARAA, 34, 749): "advanced
mergers powered by a mixture of circumnuclear starburst(s) and active galactic (nuclei)...fueled
by an enormous concentration of molecular gas that has been funneled into the merger
nucleus."
Meanwhile statistical studies were beginning to provide evidence for enhanced "activity" in colliding
and merging galaxies, in the form of star formation and nuclear activity, especially in optical colors
and radio continuum emission
IRAS (Infrared Astronomical Satellite) mission in 1984, whole sky survey in the far-infrared (with
passbands centered at 60 and 100 microns)
Revealed numerous ultra-luminous infrared galaxies (ULIRGs or ULIGs), many of which were
soon determined to be merger remnants or other types of collisional galaxy (e.g. Soifer, B. T., et al.
1984a, ApJ, 278, L71 ; Soifer, B. T., et al. 1984b, ApJ, 283, L1)
Mergers – Historical Highlights
Big surprise: some colliding and merging galaxies emit up to 99% of their total luminosity in IR,
whereas normal galaxies emit typically less than half their luminosity there – supersonic
collisions of gas clouds seem to have
triggered
Arp 220
(WISE) gigantic starbursts
Luminosity rises during the merger process and seems to peak relatively early sometimes
exceeding 10^12 solar luminosities, firmly established: almost all of the most infrared luminous
objects are active merger remnants (e.g. Sanders, D. B. & Mirabel, I. F. 1996, ARAA, 34, 749): "advanced
mergers powered by a mixture of circumnuclear starburst(s) and active galactic (nuclei)...fueled
by an enormous concentration of molecular gas that has been funneled into the merger
nucleus."
Mergers – Major Merger Dynamics
Major mergers involve nearly equal progenitors and have
a major effect on both galaxies
Tidal friction: makes colliding galaxies merge in the end;
generalization of 'dynamical friction' of stars, first
described by Chandrasekhar (1943, ApJ, 97, 255): '(a star)
must suffer from a systematic tendency to be
decelerated in the direction of its motion.'
(by Michael Bertschik, MPIA)
Orbital decay: final decay and merging occur rapidly, typically during one orbital revolution
(~10^8 years)
Violent relaxation: in their final stages of merging, two galaxies produce a rapidly changing
gravitational field that scatters and redistributes the stars into a characteristic equilibrium
configuration; Lynden-Bell 1967 calculated the form of the equilibrium configuration – it was
similar to the observed light distribution of ellipticals (Lynden-Bell, D. 1967 MNRAS, 136, 101)
Merging occurs much faster when galaxies rotate in the same sense as they orbit
Mergers – Minor Mergers
Minor mergers involve a significantly smaller companion, and so the primary galaxy is not
highly disrupted
Minor mergers are likely to be much more common than major ones. Hernquist and Mihos
(1995) summarize the evidence for this conclusion (Hernquist, L., & Mihos, J. C. 1995, ApJ, 448, 41 and
ref. therein):
Galaxies have a wide range of masses (over 4-5 orders of magnitude), so it seems
unlikely that collision partners have nearly the same mass. However, the companion to
primary mass ratio probably has to be about 0.1 or less to make the merger "minor",
and more than about 0.01 to have substantial consequences for the primary. Collision
partners are generally members of bound groups or of groups interacting with other
groups or clusters. Thus, hierarchical clustering biases the statistics of collision
partners.
The radial gas flows can be delayed or inhibited by the presence of a compact bulge in
the primary
Simulations show strong gas funneling; up to almost half of the primary gas mass can
be deposited in a dense core, plausibly inducing starbursts and other nuclear activity comparable to the funneling in major mergers
Mergers – New Disks
Major mergers can destroy galaxy disks, converting the progenitors into ellipticals, and
minor mergers can heat and age disks; besides, minor mergers with companion disruption
can lead to the formation of new, or revived disks
Examples:
Disks in ellipticals: until about the mid-1970s there was little evidence for a cool gas, disk
component in ellipticals, but with more sensitive instruments this situation has changed greatly
(see e.g. Bregman, J. N., Hogg, D. E., & Roberts, M. S. 1992, ApJ, 387, 484), moreover, kinematically
decoupled cores have been found now in a high fraction of ellipticals
Counter-rotating disks in S0 and Sa Galaxies: most remarkable is that, in contrast to counterrotating cores in luminous ellipticals, they are extensive, sometimes as large as the parent disk;
can also contain significant mass, up to 20-50% of the disk; this makes it difficult to account for
them in any way except as the result of a merger. NGC 4550 in Virgo is the most famous S0
example (Rubin, V. C. 1994b, A.J., 108, 456)
Polar ring galaxies: generally oriented perpendicular to the main disk, probably usually annular
disks, with empty middles, though in many cases this is hard to determine observationally;
probably formed out of the total or partial disruption of a gas-rich companion (see e.g., Richter, O.G., Sackett, P. D., Sparke, L. S. 1994, AJ, 107 , 99)
Mergers – New Disks
Major mergers can destroy galaxy disks, converting the progenitors into ellipticals, and
minor mergers can heat and age disks; besides, minor mergers with companion disruption
can lead to the formation of new, or revived disks
Examples:
HST WFPC2 images of the central region of NGC4550
Disks in ellipticals: until about the mid-1970s there was little evidence for a cool gas, disk
component in ellipticals, but with more sensitive instruments this situation has changed greatly
(see e.g. Bregman, J. N., Hogg, D. E., & Roberts, M. S. 1992, ApJ, 387, 484), moreover, kinematically
decoupled cores have been found now in a high fraction of ellipticals
Counter-rotating disks in S0 and Sa Galaxies: most remarkable is that, in contrast to counterrotating cores in luminous ellipticals, they are extensive, sometimes as large as the parent disk;
can also contain significant mass, up to 20-50% of the disk; this makes it difficult to account for
them in any way except as the result of a merger. NGC 4550 in Virgo is the most famous S0
example (Rubin, V. C. 1994b, A.J., 108, 456)
Polar ring galaxies: generally oriented perpendicular to the main disk, probably usually annular
disks, with empty middles, though in many cases this is hard to determine observationally;
probably formed out of the total or partial disruption of a gas-rich companion (see e.g., Richter, O.G., Sackett, P. D., Sparke, L. S. 1994, AJ, 107 , 99)
NGC 4650A
Mergers – New Disks
Major mergers can destroy galaxy disks, converting the progenitors into ellipticals, and
minor mergers can heat and age disks; besides, minor mergers with companion disruption
can lead to the formation of new, or revived disks
Examples:
HST WFPC2 images of the central region of NGC4550
Disks in ellipticals: until about the mid-1970s there was little evidence for a cool gas, disk
component in ellipticals, but with more sensitive instruments this situation has changed greatly
(see e.g. Bregman, J. N., Hogg, D. E., & Roberts, M. S. 1992, ApJ, 387, 484), moreover, kinematically
Image courtesy European Southern Observatory
decoupled cores have been found now in a high fraction of ellipticals
Counter-rotating disks in S0 and Sa Galaxies: most remarkable is that, in contrast to counterrotating cores in luminous ellipticals, they are extensive, sometimes as large as the parent disk;
can also contain significant mass, up to 20-50% of the disk; this makes it difficult to account for
them in any way except as the result of a merger. NGC 4550 in Virgo is the most famous S0
example (Rubin, V. C. 1994b, A.J., 108, 456)
Polar ring galaxies: generally oriented perpendicular to the main disk, probably usually annular
disks, with empty middles, though in many cases this is hard to determine observationally;
probably formed out of the total or partial disruption of a gas-rich companion (see e.g., Richter, O.G., Sackett, P. D., Sparke, L. S. 1994, AJ, 107 , 99)
AGN in Collisional Galaxies
Toomre's suggestion (1977) that collisional disruption together with gas dissipation could
feed all kinds of nuclear activity, has inspired many searches for a connection between
galaxy interactions and active galactic nuclei (AGN). These have turned up generally
positive results, however, the correlations are not simple and direct, and many uncertainties
and ambiguities remain.
Searches for an excess of AGNs in interacting galaxies:
Radio (e.g., Hummel 1981 A&A, 96, 111), far-infrared (e.g., Soifer et al. 1984a ApJ, 278, L71), and nearinfrared (e.g., Joseph, R. D., Meikle, W. P. S., Robertson, N. A., & Wright, G. S. 1984, MNRAS, 209, 111) surveys
have found excess emission in the nuclei of interacting systems relative to normal systems;
however, generally the resolution in these studies was not sufficient to determine the source of
the emission.
How often are AGN galaxies involved in collisions?
Problem: most AGNs are at great distances from us (quasar density peaks at z~2), and difficult
to study
(see e.g. the reviews of Heckman, T. M. 1990, in Paired and Interacting Galaxies: I.A.U. Colloq. 124, eds. J.W. Sulentic, W. C.
Keel, and C. M. Telesco (Washington, NASA) p. 359; Stockton, A. 1990, in Dynamics and Interactions of Galaxies, ed. R.
Wielen, (Springer-Verlag, Berlin) p. 440)
AGN in Collisional Galaxies
Early studies of nearby Seyfert galaxies, which contain low luminosity AGNs (Simkin, S. M., Su, H.
J., & Schwarz, M. P. 1980, ApJ, 237 , 404), found asymmetries or morphological disturbances in many
of the host galaxies; suggestion: these asymmetries probably resulted either from internal
causes, like bars, or from tidal interactions: subsequent studies have confirmed the latter
HST observations of quasar hosts have provided the strongest evidence to date of an
association between interactions and AGN, e.g Bahcall et al. (1997) studied 20 nearby,
luminous quasars (Bahcall, J. N., Kirhakos, S., Saxe, D. H., & Schneider, D. P. 1997, ApJ, 479, 642); three
objects appear to be in merging systems with extreme tidal distortions, and 13 have close
companions; since one of the merging systems does not have a close companion, at least 14
out of 20 systems may be involved in tidal interactions. Thus, it seems that quasar hosts
involved in recent collisions are not only common, but perhaps ubiquitous.
Difficulties in understanding the AGN-interactions connection:
The likelihood that the connection is only one part of a triangular relationship, in which
starbursts are the third player; the nature of the remaining part, the connection between
starbursts and AGN, is very unclear (despite a huge literature of studies of individual systems).
At one extreme there is the possibility that the only connection is accidental or indirect, that both
phenomena are triggered by similar conditions.
AGN in Collisional Galaxies
Early studies of nearby Seyfert galaxies, which contain low luminosity AGNs (Simkin, S. M., Su, H.
J., & Schwarz, M. P. 1980, ApJ, 237 , 404), found asymmetries or morphological disturbances in many
of the host galaxies; suggestion: these asymmetries probably resulted either from internal
causes, like bars, or from tidal interactions: subsequent studies have confirmed the latter
HST observations of quasar hosts have provided the strongest evidence to date of an
association between interactions and AGN, e.g Bahcall et al. (1997) studied 20 nearby,
luminous quasars (Bahcall, J. N., Kirhakos, S., Saxe, D. H., & Schneider, D. P. 1997, ApJ, 479, 642); three
objects appear to be in merging systems with extreme tidal distortions, and 13 have close
companions; since one of the merging systems does not have a close companion, at least 14
out of 20 systems may be involved in tidal interactions. Thus, it seems that quasar hosts
involved in recent collisions are not only common, but perhaps ubiquitous.
Difficulties in understanding the AGN-interactions connection:
The likelihood that the connection is only one part of a triangular relationship, in which
starbursts are the third player; the nature of the remaining part, the connection between
starbursts and AGN, is very unclear (despite a huge literature of studies of individual systems).
At one extreme there is the possibility that the only connection is accidental or indirect, that both
phenomena are triggered by similar conditions.
AGN in Collisional Galaxies
At the other extreme: the black hole engine is not necessary, but rather (radio quiet) AGN might
consist only of buried or unresolved starbursts and supernova remnants (e.g., Terlevich, R. J. 1994, in
Violent Star Formation from 30 Doradus to QSOs: Proceedings of the First IAC-RGO Mtg., (Cambridge, Cambridge
University Press) p. 329). AGN variability is accounted for by frequent supernova explosions and
interactions of supernova remnants with the dense, turbulent environment – controversially
discussed between Terlevich and Heckman (1994, in Mass-Transfer Induced Activity in Galaxies , ed. I.
Shlosman, (Cambridge, Cambridge University Press) p. 234)
Problem could be solved by new VLBI observations; for instance, in a study of 18 ULIRGs Smith,
Lonsdale and Lonsdale (Smith, H. E., Lonsdale, Colin J., & Lonsdale, Carol J. 1998, ApJ, 492, 137) found that
the compact radio cores in 7 out of the 11 that they modeled could be accounted for by starbursts
and radio supernovae; the remaining 4 objects could not be fit with starburst models, and "almost
certainly house" accreting black hole AGNs.
AGN in Collisional Galaxies
In conclusion, understanding the nature of the three-way relationship between interactions,
starbursts and AGN has proven extremely difficult. It is a problem to determine whether
we're watching accretion onto a supermassive black hole, or a pure starburst. However,
VLBI observations can help to answer these questions. Self-consistent interaction
simulations have shown that the merger process usually involves multiple collisions of two
or more galaxies, and that SF is probably induced by a variety of dynamical processes,
whose characteristic timescales span orders of magnitude. Thus, we cannot expect all
connections to be very obvious in most current statistical studies.
Pictures
Bridges, tails and plumes that stretch out to several galaxy diameters or more, are some
of the most spectacular structures formed in galaxy collisions; bridges link large galaxies
to small companions, while tails stretch far away from the galaxy causing the perturbation
Mice Galaxy NGC 4676
Antennae vs Superantennae IRAS
19254-7245
Pictures
Bridges, tails and plumes that stretch out to several galaxy diameters or more, are some
of the most spectacular structures formed in galaxy collisions; bridges link large galaxies
to small companions, while tails stretch far away from the galaxy causing the perturbation
Mice Galaxy NGC 4676
Antennae vs Superantennae IRAS
19254-7245
Pictures
Bridges, tails and plumes that stretch out to several galaxy diameters or more, are some
of the most spectacular structures formed in galaxy collisions; bridges link large galaxies
to small companions, while tails stretch far away from the galaxy causing the perturbation
Mice Galaxy NGC 4676
Antennae vs Superantennae IRAS
19254-7245
Atoms for Peace Galaxy NGC 7252
Image courtesy of NRAO/AUI and J. Hibbard & J. van Gorkom