Major mergers of gas-rich spiral galaxies trigger some of the strongest starbursts known and lead to the formation of thousands of new star clusters. Whereas in ongoing mergers it is unknown at present what fraction of these clusters will remain gravitationally bound, the situation is clearer in recent, 0.5-2 Gyr old merger remnants that resemble protoellipticals. There, star clusters numbering in the few hundreds are found to have luminosities (M-V = -8 to -16), median colors (V-I = 0.5-0.7), and median half-light radii (less than or similar to 5 pc) consistent with their being young globular clusters. Evidence concerning these clusters' nature, numbers, relation to globular clusters in ellipticals, and progenitors is reviewed.

This paper presents data extracted from an image of the merging galaxies NGC 4038/39 obtained with the ROSAT High-Resolution Imager (HRI). This 5 ''-resolution image reveals complex and intricate X-ray emission associated with both galaxies, including: (1) regions of almost filamentary emission, closely following the H alpha distribution and generally related with star-forming regions; (2) emission peaks coincident with H II regions; (3) three possibly pointlike super-Eddington sources with L(X) greater than or similar to 4 x 10(39) ergs s(-1); and (4) prominent nuclear emission peaks with L(X) near similar to 10(40) ergs s(-1). Estimates suggest that X-ray emission from early-type stars in these galaxies is a small component of the total emission. Most of the observed X-ray emission can probably be explained as a combination of emission from discrete luminous evolved X-ray sources (binaries and supernova remnants) and from a diffuse hot interstellar medium (ISM). The morphology of the image suggests that there may be both nuclear outflows analogous to those observed in other starburst galaxies (e.g., NGC 253) and superbubbles (as, e.g., in LMC). The latter, however, would be 100-400 times more luminous than analogous features observed in the LMC and in M101.

High-resolution images of the candidate dynamically young ellipticals NGC 1700 and NGC 3610 have been obtained with the WFPC2 of the Hubble Space Telescope. NGC 1700 contains approximately a dozen dust clouds arranged in a chaotic distribution near the center of the galaxy, suggestive of a merger or accretion event. NGC 3610 contains a remarkably twisted stellar disk within 2.7 '' (0.4 kpc for H-0 = 75) of the center, but no apparent dust features. The smooth, symmetric appearance of this disk and the lack of correlation with the color image suggest that it is a relaxed stellar component, possibly a disk twisted by differential precession. Its relevance to a past interaction remains unclear since the feature appears to be unique among ellipticals observed so far. A search for intermediate-age globular clusters formed during merger or accretion events reveals a population of clusters in NGC 3610 which are similar to 0.7 mag brighter in V, similar to 0.2 mag redder in V-I, and more centrally located than a typical old, metal-poor population. Simulations using Bruzual-Charlot (1996, in preparation) cluster evolution models suggest that the colors and magnitudes of these clusters are consistent with an age of 4 Gyr (assuming near-solar metallicity). However, spectroscopic measurements of the cluster metallicities will be needed to confirm this age estimate. In the case of NGC 1700 few if any new globular clusters seem to have formed during the accretion event that produced the loops and shells of this galaxy. Combining the present results with previous HST observations of other merger remnants shows generally good agreement between age estimates based on photometric, spectroscopic, and/or kinematic observations of the remnants, and age estimates based on the colors and luminosities of young and intermediate-age star clusters. (C) 1997 American Astronomical Society.

We present new U, B, V, and I images of the prototypical merger remnant NGC 7252 obtained with the WFPC2 instrument of the Hubble Space Telescope. The photometry reaches about 3 mag deeper than the previous observations with WF/PCl and we detect 499 cluster candidates, most of them previously undiscovered. We can distinguish three populations of star clusters. We confirm the existence of a very luminous, blue population of clusters with a narrow range in color. Comparisons with Bruzual & Charlot (1996, in preparation) models incorporating a Salpeter stellar IMF show that the mean ages of these clusters are 650 Myr for [Fe/H] = 0.0 and 750 Myr if [Fe/H] = -0.3. Therefore, these are the clusters that formed during the merger event. The upper limit to the effective radii of these objects is 4.8+/-0.4 pc (for D = 64.4 Mpc, H-0 = 75), suggesting that they have physical properties like Galactic globular clusters. The second population of clusters is associated with the inner disk. Their (U-B) colors and reddening-free Q values indicate that their light is dominated by O stars and that they have ages less than 10 Myr. However, with [R-eff] = 8.3 +/- 0.6 pc, these objects may be more like stellar associations and they may not survive for a significant time. Finally, there is also evidence for the presence of the most luminous of the old, metal-poor globular clusters that belonged to the progenitor galaxies. The cluster luminosity function is a single power law with slope alpha approximate to -1.8 down to a limiting magnitude of V = 26. The current specific cluster frequency is S-N = 0.6 +/- 0.3 for old clusters and young clusters with M > 10(5) M., but after the remnant has faded for about 15 Gyr S-N will have increased to about 2.5. The central disk is now resolved into mush finer detail than in earlier WF/PCl observations and the light profile of the nuclear region is consistent with a single power lay with slope gamma= -1.26+/-0.03. After both the main body and the cluster system of NGC 7252 have aged for several Gyr it may have the properties of a field elliptical. (C) 1997 American Astronomical Society. [S0004-6256(97)00512-8].

Ultraviolet-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the 4 m Blanco Telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3-15 kpc from the center and move with a velocity dispersion of 140 +/- 35 km s(-1) in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(H beta) = 6-13 Angstrom], while the eighth lies in a giant H II region and shows no detectable absorption features. Based on comparisons with model cluster spectra computed by Bruzual & Chariot and Bressan, Chiosi, & Tantalo, six of the absorption-line clusters have ages in the narrow range of 400-600 Myr, indicating that they formed early in the recent merger. These clusters, and probably also the seventh absorption-line cluster, are globular clusters, as judged by their small effective radii and ages corresponding to similar to 10(2) core crossing times. The one emission-line object is less than or similar to 10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within about +/-0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5-1 Gyr old protoelliptical galaxy, its second-generation solar-metallicity globular clusters provide direct evidence that giant elliptical galaxies with bimodal globular cluster systems can form through major mergers of gas-rich disk galaxies. A puzzling property of the observed young globular clusters are the high masses of (1-35)M(omega Cen) implied by their luminosities and ages (for an assumed Salpeter IMF). A spectrum of a candidate superluminous globular cluster in the elliptical galaxy NGC 1700, obtained with the Hiltner Telescope at MDM Observatory, shows this object to be a foreground star.

Gravitational interactions and mergers affect the morphologies and dynamics of galaxies from our Local Group to the limits of the observable universe. Observations of interacting galaxies at low redshifts (z less than or similar to 0.2) yield detailed information about many of the processes at work. I briefly review these processes and the growing evidence that mergers play a major role in the delayed formation of elliptical and early-type disk galaxies both in the field and in clusters. Low-z observations clearly contradict the notion of a single epoch of E formation at z greater than or similar to 2; instead, E and S0 galaxies continue forming to the present. The different rates of E and S0 formation inferred from observations of distant and nearby clusters may partially reflect the dependence of dynamical friction on mass: Major, E-forming; mergers may tend to occur earlier than minor, S0-forming mergers because the dynamical friction is strongest for equal-mass galaxies.

Star formation in starbursts appears to be biased toward compact clusters, with up to 20% of all stars formed in them. Observations with HST show that many of these clusters have luminosities (-9 > M-V > -16), UBVI colors, and half-light radii (R-eff approximate to 3 pc) consistent with their being young globular clusters (YGC). Although we know little about the long-term stability of the youngest clusters (<20 Myr), compact clusters older than similar to 10 t(cross) (20-40 Myr) are bound gravitationally and very likely YGCs. The present review concentrates on recent progress achieved in dating such clusters and on evidence that mergers of spiral galaxies can produce relatively rich subsystems of YGCs of solar metallicity in the remnants' halos. Studies of such subsystems suggest a scenario in which second-generation globulars form from giant molecular clouds squeezed into collapse by the high-pressure environment of starbursts. These bursts, often driven by mergers, may explain ongoing cluster formation in NGC 4038/39, the young halo globulars found around protoellipticals like NGC 3921 and NGC 7252, and the subpopulations of red metal-rich globulars observed in many giant ellipticals.

The Wide Field Planetary Camera 2 of the Hubble Space Telescope has been used to obtain high-resolution images of NGC 4038/4039 that go roughly 3 mag deeper in V than previous observations made during cycle 2. These new images allow us to measure the luminosity functions (LFs) of clusters and stars over a range of 8 mag (-14 < M-V < -6). To first order, the LF is a power law, with exponent alpha = -2.12 +/- 0.04. However, using a variety of different techniques to decouple the cluster and stellar LFs, which overlap in the range -9 less than or similar to M-V less than or similar to -6, we find an apparent bend in the young cluster LF at approximately M-V = -10.4. Brightward of this magnitude the LF has a power-law exponent alpha = -2.6 +/- 0.2, while faintward the slope is alpha = -1.7 +/- 0.2. The bend corresponds to a mass approximate to 1 x 10(5) M., only slightly lower than the characteristic mass of globular clusters in the Milky Way (approximate to 2 x 10(5) M.). It is currently not feasible to determine the cluster LF fainter than M-V approximate to -8, where individual stars are likely to dominate. The stellar LF in the range -9 < M, < -6 is much steeper, with alpha = -2.9 +/- 0.1, and is dominated by young red and blue supergiants. The star clusters of the Antennae appear slightly resolved, with median effective radii of 4 +/- 1 pc, similar to or perhaps slightly larger than those of globular clusters in our Galaxy. However, the radial extents of some of the very young clusters (ages less than 10 Myr) are much larger than those of old globular clusters (e.g., the outer radius of knot S exceeds 450 pc). This may indicate that the tidal forces from the galaxies have not had time to remove some of the outer stars from the young clusters. A combination of the UBVI colors, H alpha morphology, and Goddard High Resolution Spectrograph (GKRS) spectra enables us to age date the clusters in different regions of the Antennae. Star clusters around the edge of the dust overlap region appear to be the youngest, with ages less than or similar to 5 Myr, while clusters in the western loop appear to be 5-10 Myr old. Many star clusters in the northeastern star formation region appear to be similar to 100 Myr old, with an LF in V that has shifted faintward by similar to 1.0 mag relative to the younger (0-20 Myr) clusters that dominate over most of the rest of the galaxy. A third cluster population consists of intermediate-age clusters (similar to 500 Myr) that probably formed during the initial encounter responsible for ejecting the tails. A handful of old globular clusters from the progenitor galaxies are also identified. Most of these lie around NGC 4039, where the lower background facilitates their detection. Age estimates derived from GHRS spectroscopy yield 3 +/- 1 Myr for knot K (just south of the nucleus of NGC 4038) and 7 +/- 1 Myr for knot S in the western loop, in good agreement with ages derived from the UBVI colors. Effective gas out flow velocities from knots S and K are estimated to be about 25-30 km s(-1), based on the above cluster ages and the sizes of the surrounding H alpha bubbles. However, the measured widths of the interstellar absorption lines suggest dispersion velocities of similar to 400 km s(-1) along the lines of sight to knots S and K.