We present results from high spatial resolution (FWHM ≈ 0.3–0.5″) near-IR (1.6 and 2.1μm) imaging of a complete sample of ultraluminous infrared galaxies (ULIGs) chosen to have “warm” mid-IR colors (f25/f60 > 0.2) characteristic of AGN. In conjunction with our WFPC2 imaging program (Surace et al. 1998), we have found that nearly all of these systems are advanced mergers with complex nuclear morphologies. The extended underlying galaxies are detected in each system at H and K′, and are found to have luminosities of a few L∗, similar to quasars (McLeod & Rieke 1994). Many of the circumnuclear star-forming knots seen at optical wavelengths have been detected. Based on model SEDs, their bolometric luminosities appear similar to those of the extended nuclear starbursts seen in other, less-luminous interacting systems (i.e. NGC 4038/9). Each ULIG is increasingly dominated at long wavelengths by a compact source which we identify as a putative active nucleus. The optical/near-IR colors of these putative nuclei are more extreme than the most infrared-active starburst galaxies, yet are identical to “far-IR loud” quasars which are in turn similar to optical quasars with significant hot (800 K) dust emission. Half of the ULIGs have dereddened nuclear near-IR luminosities comparable to those of QSOs, while the others resemble Seyferts; this may be an effect of patchy extinction and scattering. Similarities between the putative ULIG nuclei and QSO nuclei, the underlying host galaxies, and the apparent young age of the ULIGs (as evidenced by their compact star-forming knots) support the evolution of “warm” ULIGs into optical QSOs.

We have completed spectroscopic observations (Wu et al. 1997a) of a sample of 73 very luminous infrared galaxies (log(LIR/L⊙) ≥ 11.5;H0 = 50 km s−1 Mpc−1) from the 2-Jy catalogue (Strauss et al. 1992) using the 2.16m telescope at the Beijing Astronomical Observatory. Spectral and interacting classifications are performed for the sample (Wu et al. 1997b). These statistical results provide strong evidence for the idea that interactions trigger nuclear activity and enhance the infrared luminosity. With the decrease of nuclear separation, relative velocity and specific angular momentum decrease rapidly, while on the contrary, both infrared luminosity and Hα equivalent width increase. Dynamical friction plays an important role even when two galaxies have large separation. This provides a favorable condition for strong star formation. We construct a simple merger sequence, from interaction class 1 to 4, to class 5 and 6 and then to the class 0 regarded as being in the stage of advanced merger. Along this sequence, spectral types change from HII-like to AGN-like. Considering the strong correlation of very luminous infrared galaxies in spectral classification schemes, it is reliable that infrared luminous galaxies evolve from HII-like galaxies to AGN-like galaxies. The different properties of infrared luminous Seyfert 1s and optically selected Seyfert 1s suggest that infrared luminous galaxies could evolve into optical Seyfert 1s in the last stage.

RXJ 13434+0001 is a rare example of radio-quiet type-2 quasars at high redshift. It was discovered through deep ROSAT observations and identified with a galaxy with a strong but narrow Lyα emission line at z = 2.35. In order to constrain the hard-X-ray properties we observed RXJ 13434+0001 with ASCA. The main purpose is to study the origin of the X-ray emission observed with ROSAT. If it is a scattered component from a strongly absorbed AGN, we could see it much brighter in the hard X-ray band.

This paper reviews some of the outstanding questions concerning compact groups of galaxies. These relate to the physical nature and dynamical status of the groups, their formation and evolution, and their role in galaxy evolution. The picture that emerges is that compact groups are generally physically dense systems, although often contaminated by optical projections. Their evolution is likely a continuous process of infall, interaction and merging. As new galaxies are added, and previous ones merge, the membership of the group evolves. I suggest that while the size of the group changes little, other physical properties such as total mass, gas mass, velocity dispersion, fraction of early-type galaxies increase with time. This picture is at least qualitatively consistent with observations and provides a natural explanation for the strongest correlations found in compact group samples.

VLA images of the neutral hydrogen (HI) in the direction of HCG 2, 16, 33, 88, and 92 (Stephan's Quintet) are examined. In HCG 2 and 16, the HI gas is bound to the individual galaxies but shows definite signs of tidal interaction; while in HCG 92, the more compact configuration, the HI gas is contained within a few prominent cloud features well displaced from the optical positions of any of the spiral members. In every case, the motions of the gas are consistent with the motions of the galaxies within the Hickson groups. A range of kinematical properties is observed for the HI gas, from well-ordered rotation, to small-scale systematic gradients within the cloud features.

The Hickson Compact Groups (Hickson 1982) are relatively isolated systems of galaxies with projected separations comparable to the diameters of the galaxies themselves and are an ideal laboratory for the study of the effects of interactions on the various properties of galaxies. Roughly one third of the galaxies in HCG show clear signs of interaction such as tidal distortion, truncation, and peculiar rotation curves. Galaxies that are not now interacting may have suffered past interactions or mergers. My study is based on a VLA survey of galaxies of different morphologies in 65 Compact Groups from Hickson's (1982, 1993) catalogue and only groups with minimum of 4 accordant velocities were included in the study. This final sample consisted of 298 galaxies (80E, 92S0, 126S). The observations were carried out using the VLA in various configurations at wavelengths of 20cm and 6cm over a period of several years. The angular resolutions ranged from 20″ to 0.3″ and non-detection sensitivities ranged from 1 mJy to 0.3 mJy for different observations. A total of 86 galaxies were detected as radio sources (32ES0, 54S).

Many types of galaxy interactions have been posited to occur in clusters, although it remains unclear which processes actually occur, and which ones might help explain the tendency for early type galaxies to inhabit high density environments, or cause the rapid evolution of cluster galaxies (e.g., Dressler et al. 1997). With these questions in mind, we have been conducting an environmental inventory of galaxies in the Virgo Cluster. Our approach is to combine surveys of spirals and S0s with detailed studies of the most interesting and peculiar galaxies. In this paper, we describe two main points. 1.) There is a population of spiral galaxies in the Virgo cluster with the small central light concentrations (bulge-to-disk ratios, or B/D's) characteristic of isolated Sb and Sc galaxies, but global star formation rates lower than those of isolated spirals of any Hubble class (Sa-Sc). These Virgo galaxies are generally classified as “early type” (e.g. Sa), and thus contribute to the morphology-density relationship. 2.) There are several types of environmental interactions occurring in Virgo, including low velocity tidal interactions and mergers, high velocity tidal interactions and collisions, HI accretion, and ICM-ISM stripping. We discuss examples of some of these interactions.

The origin of the Hubble sequence remains a long-standing puzzle in astronomy. Giant galaxies range from slowly-rotating dense ellipticals to thin late-type spiral disks. At the faint end, there are two distinct classes of “ellipticals/spheroids” that are easily separated in plots of nearly any two of their properties, such as central surface brightness versus luminosity (Ferguson and Binggeli 1994; Kormendy 1985). The elliptical class includes the bright giants and extends to the rare high surface brightness “dwarf ellipticals”, M32 being the prototype. The “spheroidal” galaxies have low surface brightnesses and are all ≳3 magnitudes fainter than L∗, the characteristic break in the luminosity function. The dwarf spheroidal galaxies (dSph) in our Local Group of galaxies with magnitudes in the range −8≳MB≳ - 12 are often considered to be the low luminosity extreme of this sequence, but nearly all other known galaxies in this class reside in clusters.

Using the ROSAT All-Sky-Survey (RASS), we examine the X-ray properties of a statistically complete sample of 294 nearby (z < 0.09) Abell clusters from our VLA 20cm survey (Ledlow & Owen 1995) and 49 Poor Groups (z < 0.03) (Burns et al. 1996). Our analysis includes a catalog of all significant (> 3σ) X-ray peaks, an analysis of the X-ray extents, identification of ICM emission, comparison to optical cluster properties, and a cross-correlation with our radio galaxy catalogue. We will make optical/X-ray overlays of the cluster fields available over the WWW in the near future (see http://astro.nmsu.edu/~mledlow for updates).

In order to investigate cluster evolutionary effects we present a new chemo-dynamical model. The model takes into account N-body simulations, modified for galaxy-galaxy binary encounters. The model incorporates also the codes for spectrophotometric and chemical evolution of galaxies. In the same time the calculations include the interaction between two merging subclusters.

In this paper we present a new indicator of the evolution of galaxies in clusters (both galactic and intrinsic cluster evolutionary phenomena) – the colour gradient in clusters of galaxies.

We analyze the K-band Hubble diagram for a sample of brightest cluster galaxies (BCGs) in the redshift range 0 < z < 1. We confirm that the scatter in the absolute magnitudes of the galaxies is small (0.3 magnitudes). The BCGs exhibit very little luminosity evolution in this redshift range: if q0 = 0.0 we detect no luminosity evolution; for q0 = 0.5 we measure a small negative evolution (i.e., BCGs were about 0.5 magnitudes fainter at z = 1 than today). If the mass in stars of these galaxies had remained constant over this period of time, substantial positive luminosity evolution would be expected: BCGs should have been brighter in the past since their stars were younger. A likely explanation for the observed zero or negative evolution is that the stellar mass of the BCGs has been assembled over time through merging and accretion, as expected in hierarchical models of galaxy formation. The colour evolution of the BCGs is consistent with that of an old stellar population (zform > 2) that is evolving passively. We can thus use evolutionary population synthesis models to estimate the rate of growth in stellar mass for these systems. We find that the stellar mass in a typical BCG has grown by a factor ≃ 2 since z ≃ 1 if q0 = 0.0 or by factor ≃ 4 if q0 = 0.5. These results are in remarkably good agreement with the predictions of semi-analytic models of galaxy formation and evolution set in the context of a hierarchical scenario for structure formation.

We are interested in examining the influence of nearest neighbor galactic encounters in gravitationally clustered systems by using cosmological N-body simulation.

We have started a survey of the molecular gas content of spiral galaxies in high and low density regions (HDS and CS), selected according to well-defined criteria (Maia et al. 1994, ApJS, 93, 425). The HDS sample is formed by galaxies that are in groups of three or more members. The groups are defined such that they have a density contrast δρ/ρ ≥ 500. This is equivalent to densities larger than 18 galaxies/Mpc3. The CS sample is made up of galaxies which are not members of any group and which are situated in a region with a density contrast δρ/ρ ≤ 0.01, i.e. less than 0.0004 galaxies/Mpc3.

We analyze a sample of galaxies in a region of three rich Abell Clusters. The data are taken from the Edinburgh Catalogue of the cluster. First, we divide the whole cluster area into different parts according to the assumed subclusters. Now we can find the position angles of the cluster and subclusters. We find strong evidence that the position angles of galaxies within our clusters are aligned to a large extent. For the cluster A754, position angles of galaxies tend to be perpendicular to the direction of the position angle of the cluster. Consequently, the angular momentum of galaxies are preferentially perpendicular to the cluster plane. For the cluster A14, position angles of galaxies tend to be parallel to the direction of the position angle of the cluster. Consequently, the angular momentum of galaxies are preferentially parallel to the cluster plane. For the cluster A3667 we obtain a more complicated picture suggesting that the alignment of galaxies in this cluster may have a different shape. From the distribution of the positions angles of galaxies we also found evidence for possible subclustering inside the whole cluster. This result is confirmed by the investigation of the distribution of the vectors normal to the galactic planes. Moreover we confirm the existence of a “line of sight” effect, originally found by Godłowski &, Ostrowski (1996) for galaxies belonging to the clusters in the Tully Catalogue (1988), for the clusters in our basic catalog.

We have used a sample of 15749 galaxies taken from the Las Campanas Redshift Survey to investigate the effects of environment on the rate of star formation (SFR) in galaxies. For each galaxy we measure SFR by [OII] emission, while a concentration index (C) is used to decouple the effect of the “morphology-environment” relation from the SFR. Galactic environment is characterized both by the 3-space local density (ρ) of galaxies and by membership in groups and clusters.

Compact Groups (CGs) of galaxies represent an extreme class of objects. They typically contain 4-8 galaxies with high space density (as in the centers of rich clusters) but with low velocity dispersions, being an excellent laboratory for the study of galaxy interactions and their effects. Up to now no unbiassed sample of CGs was available in the literature, and therefore several of the classical problems and paradoxes involving CGs could not be properly addressed.

This paper reports two studies on the distribution of the spin angular momenta of galaxies in physical clusters of galaxies.

Compact groups are ideal sites to study the influence of strong dynamical evolution due to environment on molecular cloud formation and star formation efficiency. We have observed 70 galaxies belonging to 45 Hickson compact groups (HCGs) in the 12CO(1→0) and 12CO(2→1) lines, in order to determine their molecular content. We compare the gas content relative to blue and LFIR luminosities of galaxies in compact groups with respect to other samples in the literature, including various environments and morphological types. We find that there is some hint, of enhanced MH2/LB and Mdust/LB ratios in the galaxies from compact group with respect to our control sample, especially for the most compact groups, suggesting that tidal interactions can drive the gas component inwards, by removing its angular momentum, and concentrating it in the dense central regions, where it is easily detected. The threshold at 20–30 kpc in mean galaxy separation for the enhancement of H2 suggests that it must correspond to an acceleration of the merging process and a significant inward gas flow. The molecular gas content in compact group galaxies is similar to that in pairs and starburst samples. However, the total LFIR luminosity of HCGs is quite similar to that of the control sample, and therefore the star formation efficiency appears lower than in the control galaxies. However this assumes that the FIR spatial distributions are similar in both samples which is not the case at radio frequencies. Higher spatial resolution FIR data are needed to make a valid comparison. Given their short dynamical friction time-scale, it is possible that some of these systems are in the final stage before merging, leading to ultraluminous starburst phases. We also find for all galaxy samples that the H2 content (normalized to blue luminosity) is strongly correlated with LFIR, while the total gas content (H2+HI) is not.