Galaxy interactions expel a significant amount of stars and gas into the surrounding environment. I review the formation and evolution of the tidal debris spawned during these collisions, and describe how this evolution depends on the large scale environment in which the galaxies live. In addition to acting as a long-lived tracer of the interaction history of galaxies, the evolution of this material - on both large scales and small - has important ramifications for galactic recycling processes, the feeding of the intracluster light and intracluster medium within galaxy clusters, and the delayed formation of galactic disks and dwarf galaxies.

The amount, timing and ultimate location of mass transfer and induced star formation in galaxy collisions are sensitive functions of orbital and galaxy structural parameters. I discuss the role of detailed case studies and describe the results for two systems, Arp 284 and NGC 2207/IC 2163, that have been studied with both multiwaveband observations, and detailed dynamical models. The models yield the mass transfer and compressional histories of the encounters and the “probable causes” or triggers of individual star-forming regions.

The possibility of a gaseous halo stream which was stripped from the Sagittarius dwarf galaxy is presented. The total mass of the neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic Anti-Center is 4 − 10 × 106 M⊙ (at 36 kpc, the distance to the stellar debris in this region). Both the stellar and gaseous components have negative velocities in this part of the sky, but the gaseous component extends to higher negative velocities. We suggest this gaseous stream was stripped from the main body of the dwarf 0.2 – 0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density > 10−4 cm−3. The gas would then represent the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.

We report on the properties of the hot gaseous halos of 10 nearby ultraluminous IRAS galaxies from Chandra observations. There exists diffuse soft X-ray emission surrounding the central nucleus within 10 kpc of the nuclear region with a temperature of about 0.7 keV and metallicity about Z ~ 1.0Z⊙. Also, emission lines from α elements and ions for are seen in all nearby ULIRGs in our sample. Outside the central region, the Chandra observations reveal a more extended hot gaseous halo with a temperature of about 0.6 keV and low metallicity (Z ~ 0.1Z⊙) for some of the ULIRGs. We discuss the nature and the implications of our results for the origin of X-ray halos in elliptical galaxies and the constraints this places on the feedback process.

We propose that the giant HI ring recently discovered by HIPASS for S0 galaxy NGC 1533 is formed by unequal-mass merging between gas-rich LSB (low surface brightness: “ghost”) galaxies and HSB disks. The NGC 1533 progenitor HSB spiral is transformed into a barred S0 during merging and the outer HI gas disk of the LSB is transformed into the giant HI ring. We also discuss two different possibilities for the origin of isolated star-forming regions (“ELdot” objects) in the giant gas ring.

Head-on collisions of spiral galaxies can bring large quantities of gas out of spiral disks and into the intergalactic medium. Only two clear cases (UGC 12914/5 and UGC 813/6) of such collisions are known (Condon et al. 1993, 2002) and in both cases several 109 M⊙ of neutral gas is found in the bridge between the two galaxies which are now separating. About half of the gas is molecular. The gas, atomic or molecular, is brought out by collisions between clouds, which then acquire an intermediate velocity and end up between the galaxies. The bridges contain no old stars and in each case only one HII region despite the large masses of molecular gas, such that the star formation efficiency is very low in the bridges. The collisions occurred 20 – 50 million years ago, much greater than the collapse time for dense cores. We (Braine et al. 2003, 2004) show that collisions between molecular clouds, and not only between atomic gas clouds, bring gas into the bridges. It is not currently known whether the galaxies and bridges are bound or whether they will continue to separate, releasing several 109 M⊙ of neutral gas into the intergalactic medium.

The luminous barred galaxy NGC 6872 is one of the largest spiral galaxies known. Star formation occurs all along the arms, which extend over more than 100 kpc. The galaxy experiences tidal perturbations from the nearby companion IC 4970 passing by on a low-inclination, prograde orbit. We have mapped the large-scale distribution and kinematics of the atomic gas (HI) in the NGC 6872/IC 4970 system and carried out N-body simulations with stars and gas. HI is absent from the central region; on the other hand, large gas concentrations are found at the tip of the tidal arms, spatially coincident with the blue stellar clusters and with the peaks of the Hα distribution. We use that remarkable system to investigate the evolution of gas and stars in a close prograde encounter, examine the influence of a dark matter halo on the length of the tidal tails, and test models of collisionally induced star formation.

Galaxy-galaxy interactions is one of the leading candidates for triggering nuclear activity in galaxies. Such interactions are thought to be able to bring a fresh supply of gas to the center of a galaxy to fuel its resident supermassive black hole. Optical observations, however, do not reveal direct evidence for interactions among the majority of active galaxies. on the other hand, neutral atomic hydrogen (HI) gas has proven to be an exquisite tracer of galaxy-galaxy interactions, even when no such interactions are visible in the optical. Here, we describe our systematic HI imaging surveys of galaxies hosting active galactic nuclei (AGNs) and quasi-stellar objects (QSOs), and present a few illustrative results.

We present a bi-dimensional Fourier analysis of near-infrared images of KPG 404 (NGC 5394/95). The 2D Fourier analysis shows that NGC 5394 is a H2β galaxy in the DP classification (Block & Puerari 1999). In contrast, NGC 5395 displays a very complex structure which needs a number of Fourier coefficients to be explained. A tightly wound m = 1 (DP class H1α) is the main structure, but other m = 1 and m = 2 coefficients (suggesting spiral arm modulation) are also present in the Fourier spectra. The complex structure of NGC 5395 also suggests a strong interaction in the pair. The m = 1 coefficients can represent a pseudo ring-type structure, resulting of a collision rather than a grazing passage. We conclude that the most probable scenario of the interaction in this pair should take into account a crossing of NGC 5394 through the disk of NGC 5395.

We present near infrared observations in J, H and K' pass-bands of the interacting pair of galaxies KPG 404 (NGC 5394/95). We calculate total magnitudes, surface brightnesses and colour profiles. We present the growth curve to compare our photometry against earlier works in which circular aperture estimations were published. We also perform a structural (disk + bulge) analysis to get insight on the morphology of the pair. The disk+bulge fit shows that NGC 5394 (KPG 404A) is more compact than normal galaxies, while NGC 5395 (KPG 404B) is less concentrated. This suggests an interacting scenario in which NGC 5394 passes through the disk of NGC 5395 (a Cartwheel-type collision), rather than a passage as M51-type pairs.

We are studying a sample of nearly 20 nearby (cz≲13,000 km s−1) IR-luminous interacting/merging galaxies observed with the ACIS in the Chandra archive (e.g., Gao et al. 2003; Ptak et al. 2003). Most galaxies in the sample are luminous infrared galaxies (LIGs) and nearly half of them are ultraluminous infrared galaxies (ULIGs) with bolometric luminosities comparable to QSOs. Based on multiwavelength data and numerical simulations of gas-rich galaxy mergers, we have attempted to arrange them in a merger sequence perspective and compared their broadband X-ray emission here with the optical images (mostly DSS).

We discuss our ongoing multi-instrument HST investigation of the nuclear regions of the 11 interacting and merging galaxies in the Toomre Sequence. We are studying the nuclear kinematics using STIS (G750M) spectra, the nuclear stellar populations using STIS (G430L) spectra, and the nuclear morphology using NICMOS and WFPC2 images. The results will provide new insight into the physical processes that operate during galaxy interactions.

We analyzed the effects of having a close companion on the star formation activity of galaxies in the 8K galaxy pair catalog selected from the 2dFGRS. We found that, statistically, galaxies with rp < 25h−1 kpc and ΔV < 100km s−1 have enhanced star formation with respect to isolated galaxies with the same luminosity and redshift distribution. Our results suggest that the physical processes at work during tidal interactions can overcome the effects of environment, except in dense regions.

We obtained VLA radio total power and polarization maps of a merging pair of galaxies NGC4038/39. The whole system is filled with bright extended radio emission with a high thermal fraction of ⋍ 50% at 10.45 GHz and very strong magnetic fields of ⋍ 20μG. A star-forming region at the southern end of the dark cloud complex extending between the galaxies has highly tangled magnetic fields reaching the strength of ⋍ 30μG, even larger than in both individual galaxies. The polarized components associated with the HI ridge at the base of one of the tidal tails and with the northern complex of dark clouds (weakly forming stars) have been identified. A relic magnetic spiral coincident with the northern galaxy has been found, too. Our data on the magnetic fields combined with those in HI, Hα, X-rays and in far infrared are used as a diagnostic tool to distinguish several physically distinct regions.

There are several physical processes to remove gas from galaxies in clusters, with subsequent starvation and star formation quenching: tidal interactions between galaxies, or tidal stripping from the cluster potential itself, interactions with the hot intra-cluster medium (ICM) through ram pressure, turbulent or viscous stripping, or also outflows from star formation of nuclear activity, We review the observational evidence for all processes, and numerical simulations of galaxies in clusters which support the respective mechanisms. This allows to compare their relative efficiencies, all along cluster formation.

Results are presented from a wide area, high resolution HI synthesis survey of the outer regions of the nearby M81 group, where internal (galactic) and external (group-related) evolution processes can be studied simultaneously in great detail. The survey encompasses the star forming dwarf galaxies M81dwA, UGC4483, and HoII, where evidence of ram pressure stripping was recently discovered. The data do not reveal any intergalactic HI, but the outer parts of HoII are reminiscent of tidal tails. We argue however that those structures are equally consistent with the latest ram pressure models including cooling. The case for a hot intergalactic medium in this poor, spiral-only group is thus still open. The survey also puts tight constraints on possible counterparts to the local high velocity cloud population in an external group, reaching a 3σ column density of 1019 atom cm−2 and a 6σ limiting mass of 1.5 × 105M⊙.

It is known that among active galaxies (AG) with strong emission lines (UV-galaxies, Sy 1 and Sy2, Markarian and Kazarian galaxies, Radio-galaxies, QSO's host galaxies and so on) there are large per cent of objects with double and multiple nucleus. The common sizes and volumes of these nuclei are on the order of a few hundred parsecs or kilo-parsecs. In fact these are not double galaxies or clusters of galaxies as many of astronomers believe, but just the complicated nucleus of AG. The problem is: what are the nature and the birth of these objects? There are in fact two basic suppositions in the subject: (a) The complicated nuclei are the result of merging or colliding of two or more galaxies, or: (b) They are the results of nuclear activity. The results of detailed spectroscopic observations of a number of ”tidal galaxies”, carried out with the 5m Palomar telescope, 2.6m telescope of Ambartsumian Byurakan astrophysical observatory (multi-pupil spectroscopy with Tiger receiver) and 6m telescope of Special Astrophysical observatory of Russia are presented. It is shown that in many cases the ”tidal dwarf galaxies ”(or actually complicated nucleus) are the result of galactic nuclear activity.

We present numerical simulations of the dynamical and chemical evolution of galaxy clusters. from X-ray spectra it is evident that the Intra-Cluster Medium (ICM) is abundant in metals. As heavy elements are only produced in stars the processed material must have been ejected by cluster galaxies into the ICM. Several different mechanisms for interaction between the cluster galaxies and the ICM are possible but their efficiency of metal enrichment of the cluster and their time evolution are still not known. Suggested processes which remove enriched material from the galaxies are ram-pressure stripping, galactic winds, superwinds driven by starburst activity and jets from active galaxies. First results of high resolution hydrodynamic simulations on cluster scales are presented to investigate the effect of these different pollution mechanisms.

We present preliminary results of numerical modeling of the combined effects of ICM stripping and galaxy collisions on galaxies in clusters. We use the Hydra adaptive mesh, P3M + SPH code of Couchman et al. (1995). Generally, the formation of extended tidal and splash structures in galaxy interactions will facilitate ram pressure stripping. ICM compression of massive clouds stripped from tails and plumes may produce significant populations of free-floating star clusters or dwarf galaxies.

We present results from an ongoing survey of the ionized gas kinematics in Virgo cluster spiral galaxies using Fabry-Perot (FP) interferometry. Our goal is to study the environmental effects on galaxy evolution in the Virgo cluster. We report here on the Hα distribution map and velocity field of NGC 4438, an interacting galaxy near the centre of the cluster.