We present the results of ROSAT PSPC X-ray observations of NGC 4125 and NGC 3610. These two galaxies are among the best representatives of a newly recognized class of elliptical galaxies that may be the product of recent mergers, NGC 4125 shows two plumes or disks crossing at right angles; NGC 3610 has the richest fine structure of all elliptical galaxies cataloged by Schweizer and Seltzer. Our X-ray observations show that these galaxies do not retain large gaseous halos, contrary to the suggested link between merging and X-ray brightness. Their X-ray luminosities (0.1-2 keV) are in the range 10(40-41) ergs s(-1), low compared with those of hot gas-rich elliptical galaxies. The X-ray to optical ratio of NGC 3610 is in the range of those of X-ray faint elliptical galaxies, consistent with the value found for the bulge of M31, where the X-ray emission is dominated by low-mass binaries; its X-ray spectrum is also comparable with those of X-ray faint elliptical galaxies, presenting significant very soft excess emission. The X-ray to optical ratio of NGC 4125 is larger and might suggest the presence of some hot interstellar medium. This suggestion is supported by the spectrum of NGC 4125, which shows evidence of Fe L emission. This spectrum can be fitted with either a low-abundance single-temperature Raymond model or with a two-temperature model with solar abundances. Further work is needed to firmly resolve the question of gaseous versus stellar X-ray emission in NGC 4125.

This paper presents imaging, photometric, and spectroscopic observations of NGC 3921=Mrk 430 gathered over many years with five optical telescopes. This luminous galaxy (M(V)=-22.8 for H-0=50) at cz(hel)=5926+/-15 km s(-1) features a single nucleus, a main body with complex fine structure (ripples, loops, fan-shaped protrusions), and a pair of similar to 100 kpc long, crossed tidal tails indicative of two former disk galaxies of near-equal mass. These galaxies have essentially merged. The main body of the remnant shows a typical post-starburst spectrum dominated in the blue by A3-5 V stars. The inferred burst age is 0.5-1 Gyr and the burst strength similar to 10% (by mass). Surrounding the nucleus is extremely centrally concentrated ionized gas that can be traced out to similar to 12(n) (7 kpc), emits greater than or similar to 1.5x10(41) ergs s(-1) in H alpha and shows signs of both rotational and chaotic motions. The bright semistellar nucleus appears strikingly off-centered relative to the main body, which itself features ''sloshing'' isophotes. That is, the centers of successive isophotes shift position by greater than or similar to 2 kpc, causing the nucleus to appear eccentric by up to 23% relative to a nearly half-light isophote. The luminous matter has clearly not yet equilibrated, and this merger remnant is dynamically young. Nevertheless, the mean light distribution of the main body is already well described by an r(1/4) law. This distribution plus the luminosity, UBV colors, color gradients, velocity dispersion, spectroscopic line strengths, and fine-structure index all agree with the notion that NGC 3921, which is a member of a small, tight group of four galaxies, is a 0.7+/-0.3 Gyr old protoelliptical (reckoned since dose passage that started the merger). Both it and its kin NGC 7252 are nearby analogs of distant galaxies with ''E+A''-type spectra in Butcher-Oemler clusters. A search for star clusters and associations in NGC 3921 reveals 19 candidate OB associations, but only five candidate young globular clusters with M(V)=-12 to -14. Thus, NGC 3921 appears to have distinctly fewer and certainly less luminous young globular clusters than NGC 7252. This less extreme population of young globulars may reflect a paucity of gas in one of the two merging component disks of this suspected SO-Sc or Sa-Sc merger (Hibbard & van Gorkom, AJ, in press). Such gas paucity may explain the weaker starburst and may have supplied fewer giant molecular clouds for globular cluster formation. Hence, the Hubble types and gas contents of component galaxies appear to play an important role in determining the cluster populations in merger remnants. (C) 1996 American Astronomical Society.

New, high-resolution images of NGC 3921 obtained with the Wide Field and Planetary Camera 2 of the refurbished HST are presented. NGC 3921 is the remnant of two merged disk galaxies and thought to be a 0.7+/-0.3 Gyr old protoelliptical. A search for star clusters has yielded 102 candidate globular clusters (GC) and 49 separate, more extended stellar associations (A) brighter than V=26.0, the estimated completeness limit (corresponding to M(v)=-8.5 for H-0=75 km s(-1) Mpc(-1)). Most of these objects are newly discovered. Among the best-observed objects [V less than or similar to 25, error sigma(V-I)less than or equal to 0.15], the majority are blue and have remarkably uniform colors, with median (V-I)(Cousins)=0.65 for GCs and 0.56 for As. The color indices of the GCs yield estimates of 250-750 Myr for the median age, depending on the adopted metallicity (1.0-0.2 Z.), and suggest a prolonged formation period (several 100 Myr) roughly in agreement with the estimated duration of the merger. The most luminous candidate GCs have absolute magnitudes M(v)=-12 to -13 and are predicted to fade by 3-4 mag over the next 15 Gyr, whence they will then have luminosities typical of luminous old GCs. The median half-light radius of candidate GCs is R(eff)less than or similar to 5 pc, compared with a median R(eff)=3 pc for Milky Way globulars. Since from their colors most of these clusters appear to be dozens of core crossing times old, they must be gravitationally bound. The luminosity functions of both the GCs and the As are well represented by a single power law, phi(L)dL proportional to L(-2.1+/-0.3)dL (for M(v) less than or equal to-8.5). Various ents suggest that the progenitors of these objects may have been normal Giant Molecular Clouds rather than Supergiant Molecular Clouds. The projected radial distribution of GCs is strongly centrally concentrated and follows closely the V-light profile of the galaxy (similar to r(1/4) law), suggesting that these clusters or their progenitors experienced the same violent relaxation as did the average luminous matter of the two merging galaxies. In contrast, the radial distribution of As appears relatively flat, We estimate that the total number of GCs in NGC 3921 has increased by greater than or similar to 40% during the merger, from about 270+/-60 GCs before to greater than or similar to 380 GCs after the merger. The inferred specific GC frequency is S-N greater than or similar to 0.7 at present and will, barring further mergers or significant cluster destruction, increase to S-N greater than or similar to 1.4, 1.8, and 2.9 over the next 2, 5, and 15 Gyr as the galaxy fades. Hence, within less than or similar to 7 Gyr NGC 3921 may begin to appear as a normal elliptical with a GC population typical for Es in low-density environments. (C) 1996 American Astronomical Society.

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.