Gravitational interactions and mergers are shaping and reshaping galaxies throughout the observable Universe. While observations of interacting galaxies at low redshifts yield detailed information about the processes at work, observations at high redshifts suggest that interactions and mergers were much more frequent in the past. Major mergers of nearby disc galaxies form remnants that share many properties with ellipticals and are, in essence, present-day proto-ellipticals. There is also tantalizing evidence that minor mergers of companions may help build bulges in disc galaxies. Gas plays a crucial role in such interactions. Because of its dissipative nature, it tends to get crunched into molecular form, turning into fuel for starbursts and active nuclei. Besides the evidence for ongoing interactions, signatures of past interactions and mergers in galaxies abound: tidal tails and ripples, counter-rotating discs and bulges, polar rings, systems of young globular clusters, and ageing starbursts. Galaxy formation and transformation is clearly a prolonged process, occurring up to the present-day. Overall, the currently available observational evidence points towards Hubble's morphological sequence being mainly a sequence of decreasing merger damage.

We investigate trends of the cold and hot gas content of early-type galaxies with the presence of optical morphological peculiarities, as measured by the fine-structure index Sigma. H I mapping observations from the Literature are used to track the cold gas content, and archival ROSAT Position Sensitive Proportional Counter data are used to quantify the hot gas content. We find that E and SO galaxies with a high incidence of optical peculiarities are exclusively X-ray underluminous and, therefore, deficient in hot gas. In contrast, more relaxed galaxies with little or no signs of optical peculiarities span a wide range of X-ray luminosities. That is, the X-ray excess anticorrelates with Sigma. There appears to be no similar trend of cold! gas content with either fine-structure index or X-ray content. The fact that only apparently relaxed E and SO galaxies are strong X-ray emitters is consistent with the hypothesis that after strong disturbances, such as a merger, hot gas halos build up over a timescale of several gigayears. This is consistent with the expected mass loss from stars.

Globular clusters formed in galactic mergers (e.g., The Antennae) can now be studied at different stages of their evolution. In young merger remnants (e.g., NGC 7252) these "second-generation" globulars appear by the hundreds as young halo clusters of roughly solar metallicity. While at first bluer and much more luminous than old metal-poor globulars, they become redder after 1 - 1.5 Gyr and can then be observed as still overluminous red clusters of intermediate age in perturbed-looking E and SO galaxies (e.g., NGC 1316, 1700, 3610). There is evidence from the color distributions, projected radial distributions, and perhaps also luminosity functions that these clusters eventually assume the properties of red metal-rich globulars observed in many giant ellipticals. Studies of globular clusters in ongoing mergers and young remnants suggest that second-generation globulars form from giant molecular clouds shocked by the rapid pressure increase in the merger-induced starburst. This pressure-induced formation lends credence to Cen's (2001) argument that the general pressure increase during cosmological reionization at z approximate to 7-15 triggered the near-simultaneous formation of the universal population of first-generation metal-poor globulars observed in galaxies of all types.

Collisions and, mergers of gas-rich galaxies trigger bursts of star and cluster formation. Of the thousands of clusters typically formed during a major merger, only the most massive and compact survive for Gigayears as globular clusters (GCs). In less than or equal to 1 Gyr old merger remnants, these 'second-generation' GCs appear by the hundreds as young halo clusters of similar tosolar metallicity. Their likely descendants, metal-rich GCs of intermediate age (2 - 5 Cyr), have recently been found in - 10 E galaxies, where they appear as slightly overluminous red GCs With a still power-law-like luminosity function. Their color and radial distributions suggest that they evolve into the red metal-rich GCs observed in old ellipticals. There is. good evidence that second-generation GCs form from giant molecular clouds shocked by the rapid pressure increase in merger-induced starbursts. This mechanism supports the view that the universal pressure increase during cosmological reionization. may have triggered the formation of the metal-poor globulars observed in galaxies of all types.

The Chandra monitoring observations of "The Antennae'' (NGC 4038/4039) have led to the discovery of a variable, luminous, supersoft source (SSS). This source is detected only at energies below 2 keV and, in 2002 May, reached count rates comparable to those of the nine ultraluminous X-ray sources (ULXs) detected in these galaxies. Spectral fits of the SSS data give acceptable results only for a similar to90 - 100 eV blackbody spectrum with an intrinsic absorption column of N-H similar to (2-3) x 10(21) cm(-2). For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7 x 10(38) ergs s(-1) in 1999 December to 8.0 x 10(38) ergs s(-1) in 2002 May. The intrinsic, absorption-corrected, best-fit luminosity reaches 1.4 x 10(40) ergs s(-1) in 2002 May. The assumption of unbeamed emission would suggest a black hole of greater than or similar to 100M(.). However, if the emission is blackbody at all times, as suggested by the steep soft spectrum, the radiating area would have to vary by a factor of similar to10(3), inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. Viable explanations for the observed properties of the SSS are provided by anisotropic emission from either an accreting nuclear-burning white dwarf or an accreting stellar-mass black hole.

Young massive clusters differ markedly from old globular clusters in featuring extended, rather than tidally truncated, envelopes. Their projected-luminosity profiles are well fit by Elson-Fall-Freeman models with core radii 0.3 pc, r <= 8 pc and power-law envelopes of negative exponent 2.0 <= gamma <= 3.8. These envelopes form within the first few 10(6) yr and last similar to 10(8) -10(9.5) yr, depending on the environment. Many YMCs show clumpy substructure that may accelerate their initial relaxation. The cores of Magellanic-Cloud clusters show universal expansion from r(c) < 1 pc at birth to r(c) = 2 - 3 pc after 10(8) yr, but then seem to evolve along two bifurcating branches in a r, - log(age) diagram. The lower branch can be explained by mass-loss driven core expansion during the first 109 yr, followed by slow core contraction and the onset of core collapse due to evaporation. The upper branch, which shows continued core expansion proportional to logarithmic age, remains unexplained. There is strong evidence for rapid mass segregation in young clusters, yet little evidence for top-heavy IMFs or primordial mass segregation. Finally, YMCs show similar structure throughout the Local Group and as far away as we can resolve them (<= 20 Mpc).

We have determined the dynamical mass of the most luminous stellar cluster known to date, i.e. object W3 in the merger remnant galaxy NGC 7252. The dynamical mass is estimated from the velocity dispersion measured with the high-resolution spectrograph UVES on VLT. Our result is the astonishingly high velocity dispersion of sigma = 45 +/- 5 km s(-1). Combined with the large cluster size R-eff = 17.5 +/- 1.8 pc, this translates into a dynamical virial mass for W3 of (8 +/- 2) x 10(7) M-.. This mass is in excellent agreement with the value (similar to7.2 x 10(7) M-.) we previously estimated from the: cluster luminosity (M-v = -16.2) by means of stellar M/L ratios predicted by Simple Stellar Population models (with a Salpeter IMF) and confirms the heavyweight nature of this object. This results points out that the NGC 7252-type of mergers are able to form stellar systems with masses up to similar to 10(8) M-.. We find that W3, when evolved to similar to10 Gyr, lies far from the typical Milky Way globular clusters, but appears to be also separated from omegaCen in the Milky Way and G1 in M31, the most massive old stellar clusters of the Local Group, because it is too extended for a given mass, and from dwarf elliptical galaxies because it is much more compact for its mass. Instead the aged W3 is amazingly close to the compact objects named ultracompact dwarf galaxies (UCDGs) found in the Fornax cluster (Hilker et al. 1999; Drinkwater et al. 2000), and to a miniature version of the compact elliptical M 32. These objects start populating a previously deserted region of the fundamental plane.

We present the integrated 411 ks Chandra ACIS-S exposure of the Antennae galaxies (NGC 4038/39). Besides a rich population of pointlike sources, this spectacular image reveals a spatially and spectrally complex hot diffuse gaseous component. For the first time we detect intense line emission from Fe, Ne, Mg, and Si in the Antennae and obtain a detailed picture of spatially varied metal abundances in the hot interstellar medium (ISM) of a galaxy. In certain regions, the abundances of alpha-elements may be many times solar, while the Fe abundance is subsolar or near-solar. The differences in the local metal enrichment of the hot ISM may be related to the local star formation rates and to the degree of confinement of the enriched hot ISM. We also report large-scale gaseous features, including two gigantic, similar to10 kpc scale "loops" extending to the south of the merging disks and a low surface brightness hot halo, extending out to similar to18 kpc. These features may be related to superwinds from the starburst in the Antennae or result from the merger hydrodynamics. Their long cooling times suggest that they may persist to form the hot X-ray halo of the emerging elliptical galaxy.

This paper presents new images and spectroscopic observations of NGC 34 (Mrk 938) obtained with the du Pont 2.5 m and Baade 6.5 m telescopes at Las Campanas, plus photometry of an archival V image obtained with Hubble Space Telescope. This MV = - 21.6 galaxy has often been classified as a Seyfert 2, yet recently published infrared spectra suggest a dominant central starburst. We find that the galaxy features a single nucleus, a main spheroid containing a blue central disk and much outer fine structure, and tidal tails indicative of two former disk galaxies. At present these galaxies appear to have completed merging. The remnant shows three clear optical signs that the merger was gas-rich ('' wet '') and accompanied by a starburst: (1) It sports a rich system of young star clusters, of which 87 have absolute magnitudes - 10: 0 - MV >= - 15.4. Five clusters with available spectra have ages in the range 0.1-1.0Gyr and photometric masses of 2 x 10(6) M circle dot less than or similar to M less than or similar to 2 x 10(7) M circle dot; they are gravitationally bound young globular clusters. ( 2) The blue central disk appears to be young. It is exponential, can be traced to greater than or similar to 10 kpc radius, and has a smooth structure and colors suggesting that its optical light is dominated by a similar to 400 Myr old poststarburst population. And ( 3), the center of NGC 34 drives a strong outflow of cool, neutral gas, as revealed by broad blueshifted Na I D lines. The center-of-line velocity of this gas is -620 km s (-1), while the maximum detected outflow velocity reaches - 1050 km s (-1). Assessing all available evidence, we suggest that NGC 34 stems from two recently merged gas-rich disk galaxies with an estimated mass ratio of 1/3 less than or similar to m/M less than or similar to 2/ 3. The remnant seems to have first experienced a galaxy-wide starburst that then shrank to its current central and obscured state. The strong gaseous outflow came last.

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the giant elliptical galaxy NGC 3610, a well-established remnant of a dissipative galaxy merger. These observations supersede previous, shallower observations that revealed the presence of a population of metal-rich globular clusters of intermediate age ( similar to 1.5-4 Gyr). We detect a total of 580 cluster candidates, 46% more than from the previous WFPC2 images. The new photometry strengthens the significance of the previously found bimodality of the color distribution of clusters. Peak colors in V-I are 0.93 +/- 0.01 and 1.09 +/- 0.01 for the blue and red subpopulations, respectively. The luminosity function of the inner 50% of the metal-rich ("red'') population of clusters differs markedly from that of the outer 50%. In particular, the luminosity function of the inner 50% of the red clusters shows a flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue clusters. This is consistent with predictions of recent models of cluster disruption for the age range mentioned above and for metallicities that are consistent with the peak color of the red clusters, as predicted by population synthesis models. The radial surface density profile of red clusters follows that of the underlying galaxy light more closely than in "normal'' elliptical galaxies, which is consistent with the intermediate-age nature of the red clusters. We determine the specific frequency of clusters in NGC 3610 and find a present-day value of S-N = 1.4 +/- 0.6. Using published age estimates for the diffuse light of NGC 3610, as well as cluster disruption models, we estimate that this value will increase to S-N = 3.8 +/- 1.7 at an age of 10 Gyr, which is consistent with typical SN values for "normal'' elliptical galaxies. Our findings constitute further evidence in support of the notion that metal-rich cluster populations formed during major mergers involving gas-rich galaxies can evolve dynamically ( through disruption processes) into the red, metal- rich cluster populations that are ubiquitous in normal giant ellipticals.