A study of 69 E and SO galaxies located mostly in the field and in groups reveals that the UBV colors become systematically bluer at any given luminosity as the amount of merger-induced fine structure increases. To quantify such fine structure, we define an index-SIGMA that measures ripples, jets of luminous matter, X structures, and boxy isophotes; it ranges between 0 and 7.6 for the above galaxies. The correlations between UBV colors and this index SIGMA closely resemble the correlations found earlier between CN, Mg2, and H-beta line strengths and the same SIGMA. in 36 ellipticals [Schweizer et al. ApJ, 364, L33 (1990)]. Because SIGMA is a rough measure of dynamical youth or rejuvenation, both sets of correlations are most likely due to systematic variations in the mean age of the stellar populations, rather than to variations in their mean metallicity. The new color correlations emphasize that these suggested age variations are not limited to the nuclei, but occur globally in the stellar populations of E and SO galaxies. These correlations also yield a rough ranking of E and SO galaxies by the date of their last major merger event. To calibrate this chronology, we develop a simple two-burst model of evolving stellar populations in mergers and apply it to derive heuristic merger ages (HMA) from UBV colors for each galaxy. These HMAs vary mainly as a function of two parameters: the Hubble type of the premerger components and the gas-to-star conversion efficiency. For representative ranges of these parameters, the HMAs of our 69 E and SO galaxies spread over at least 5 Gyr and up to 10 Gyr. Hence the scatter in color-magnitude relations-though relatively small-is fully compatible with the hypothesis that in the field such galaxies formed, or were seriously modified, by major mergers during at least 1/3 to 2/3 the age of the Universe. A mean HMA of approximately 8 Gyr is suggested for E's with no fine structure and 4.6 Gyr for those with the most fine structure. Good candidates for dynamically young ellipticals having formed through mergers of disk galaxies during the last 7 Gyr are NGC 3610, 1700, 4125, 4915, and 5322; significant rejuvenation seems to have occurred also in NGC 596, 3640, and 5018.

We report 2".4 resolution interferometric observations of the CO emission in the merger galaxy NGC 7252. The molecular gas concentration around the nucleus of this galaxy is well resolved and shows solid body rotation in a direction opposite to the motion of the ionized gas in the outer parts of the galaxy. The CO surface brightness distribution is complex, yet the kinematics of the molecular gas bears striking resemblance to that of the inner ionized gas disk. The CO can be traced farther into the galactic center than the H-alpha emission, but a small central hole (approximately 300 pc radius) apparently free of gas remains. At larger radii, there is a gap between the molecular gas disk and the outlying atomic gas found in H I maps. The molecular gas observations are discussed in light of recent numerical simulations showing the formation of a kinematically decoupled gas concentration at the center of a merger remnant. Details of the gas distribution as a function of radius may have been produced by powerful winds generated in the active star-forming phase of the recent past. Based on these observations, we suggest that NGC 7252 is well on its way to becoming an elliptical with a counterrotating stellar core.

New, high-resolution images of the central region of NGC 7252 obtained with the Planetary Camera of the Hubble Space Telescope are presented. NGC 7252 (sometimes referred to as the ''Atoms-for-Peace'' Galaxy) is a prototypical example of a remnant of two merged disk galaxies. Our most striking result is the discovery of a population of about 40 blue pointlike objects in this galaxy. The mean absolute magnitude of these objects is M(V) = -13 mag; the mean color is V-I=0.7 mag; and the mean effective radius is 10 pc (for H-1 = 50 km s-1 Mpc-1). The luminosities, colors, projected spatial distribution, and sizes are all compatible with the hypothesis that these objects formed within the last 1 Gyr following the collision of two spiral galaxies, and that they are young globular clusters. It therefore appears that the number of globular clusters may increase during the merger of gas-rich galaxies. This weakens van den Bergh's objection against ellipticals being formed through disk mergers, based mainly on the fact that disk galaxies have fewer globular clusters per unit luminosity than ellipticals do. Other findings are: (1) NGC 7252 shows a single, semistellar nucleus; (2) relatively bright spiral structure is seen within 3''.5 (1.6 kpc) of the center, presumably formed through the continued infall of gas into a disk around the center of the galaxy; (3) dust lanes and very weak spiral structure are seen out to about 9'' (4.2 kpc), primarily on the NE side; and (4) a ripple is found on the west side, 5''.0 from the center.

Spectra of the two brightest ''bluish knots'' surrounding the nucleus of the merger remnant NGC 7252 are presented. These knots were among a half dozen discovered from the ground and belong to a population of approximately 40 similar blue objects found and resolved by Whitmore et al. with the Hubble Space Telescope. The spectra, obtained with the Hale 5 m telescope, show Halpha, Hbeta, Mg I, and Fe I absorption lines characteristic of late A-type to mid-F-type stars at the redshift of NGC 7252 (within +/- 150 km s-1), but no emission lines. These spectra demonstrate that the two knots are relatively young (approximately 10-500 Myr), populous star clusters of exceptional luminosity (M(V) = -17.1 and -15.6 for H-0 = 50). Because of their age and small effective radii (approximately 10 pc), these clusters must be gravitationally bound and are most likely young globular clusters. They formed during the merger of two gas-rich spirals and will fade by 3 1/2-7 mag during the next 15 Gyr. We discuss the implications of finding young globular clusters in a merger remnant likely to evolve into an elliptical galaxy.

We present the first observations of the neutral hydrogen distribution and x-ray emission in the prototypical merger remnant NGC 7252, the ''Atoms-for-Peace'' galaxy. These data are supplemented by accurate B and R surface photometry, reaching a limit of mu(B) = 26.5 mag arcsec-2, and images taken through a narrow-band Ha filter. We find all of the 2 X 10(9) h-2 M. of atomic gas to be restricted to the outer, tidal regions of this system (H-0=100 h km s-1 Mpc-1). By contrast, the molecular gas traced by the (CO(1-->0))-C-12 map of Wang et al. [ApJ, 396, 510 (1992)] is confined to an inner rotating disk of radius 7'' and has an Halpha counterpart. The gap between the atomic and molecular gas distributions is filled in by diffuse Halpha emission and perhaps by x-ray emission. The velocity field of the atomic gas in the tidal tails indicates that they are swinging through space in the same sense as the rotation of the inner gas disk. The H I at the apparent base of the northwestern tail seems to be falling back toward the main body of the galaxy, yet there is no H I associated with this main stellar body: This suggests ongoing efficient conversion of the atomic gas into other phases in this region. The Halpha velocity anomalies previously found in the remnant body may be produced in part by the combination of tail-related, noncircular motions and the inner gas-disk rotation. Both tidal tails have bluer B - R colors than the main body of the remnant, with the bluest regions coinciding with peaks in the gas column density. Each tail contains one giant H II region near the end of its optical light distribution. These H II regions are associated with large concentrations of gas and stars that approach the sizes and gas contents of dwarf galaxies. The H I extends beyond the end of the optical tails and reaches projected distances of 62 h-1 kpc east and 120 h-1 kpc northwest from the center. We discuss the possible relevance of these data to: (1) the transformation of merged spirals into ellipticals; (2) the generation of ripples by returning tidal material; and (3) the formation of bound stellar systems from tidally tom material.

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.