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.

We present Gemini optical spectroscopy of 23 young star clusters in NGC 3256. We find that the cluster ages range from few to similar to 150 Myr. All these clusters are relatively massive [( 2-40) x 10(5) M(circle dot)] and appear to be of roughly 1.5 Z(circle dot) metallicity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may have formed in tidal-tail gas falling back into the system. We discuss our observations in light of other known cluster populations in merging galaxies, and suggest that NGC 3256 is similar to Arp 220, and hence may become an ultraluminous infrared galaxy as the merger progresses and the star formation rate increases. Some of the clusters that appeared as isolated in our ground-based images are clearly resolved into multiple subcomponents in the HST ACS images. The same effect has been observed in the Antennae galaxies, showing that clusters are often not formed in isolation, but instead tend to form in larger groups or cluster complexes.

We present deep HSTACS images and Keck spectroscopy of MC2 1635+119, a QSO hosted by a galaxy previously classified as an undisturbed elliptical galaxy. Our new images reveal dramatic shell structure indicative of a merger event in the relatively recent past. The brightest shells in the central regions of the host are distributed alternately in radius, with at least two distinct shells on one side of the nucleus and three on the other, out to a distance of similar to 13 kpc. The light within the five shells comprises similar to 6% of the total galaxy light. Lower surface brightness ripples or tails and other debris extend out to a distance of similar to 65 kpc. Asimple N-body model for a merger reproduces the inner shell structure and gives an estimate for the age of the merger of between similar to 30 Myr and similar to 1.7 Gyr, depending on a range of reasonable assumptions. While the inner shell structure is suggestive of a minor merger, the total light contribution from the shells and extended structures is more indicative of a major merger. The spectrum of the host galaxy is dominated by a population of intermediate age (similar to 1.4 Gyr), indicating a strong starburst episode that may have occurred at the time of the merger event. We speculate that the current QSO activity may have been triggered in the recent past by either aminor merger, or by debris from an older (similar to Gyr) major merger that is currently "raining'' back into the central regions of the merger remnant.

We present results from a pilot HSTACS deep imaging study in broadband V of five low- redshift QSO host galaxies classified in the literature as ellipticals. The aim of our study is to determine whether these early- type hosts formed at high redshift and have since evolved passively, or whether they have undergone relatively recent mergers that may be related to the triggering of the nuclear activity. We perform two- dimensional modeling of the light distributions to analyze the host galaxies' morphology. We find that, while each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the majority of them ( 4/ 5) reveal significant fine structure such as shells and tidal tails. These structures contribute between similar to 5% and 10% to the total V- band luminosity of each host galaxy within a region of r similar to 3r(eff) and are indicative of merger events that occurred between a few hundred Myr and a Gyrago. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy. Our results thus support a consistent scenario in which most of the QSO host galaxies suffered mergers with accompanying starbursts that likely also triggered the QSO activity in some way, but we are also left with considerable uncertainty on physical mechanisms that might have delayed this triggering for several hundred Myr after the merger.

We have mapped the key mid-IR diagnostics in eight major merger systems of the Toomre sequence (NGC 4676, NGC 7592, NGC 6621, NGC 2623, NGC 6240, NGC 520, NGC 3921, and NGC 7252) using the Spitzer Infrared Spectrograph. With these maps, we explore the variation of the ionized-gas, polycyclic aromatic hydrocarbon (PAH), and warm gas (H-2) properties across the sequence and within the galaxies. While the global PAH interband strength and ionized gas flux ratios ([Ne III]/[Ne II]) are similar to those of normal star-forming galaxies, the distribution of the spatially resolved PAH and fine structure line flux ratios is significantly different from one system to the other. Rather than a constant H-2/PAH flux ratio, we find that the relation between the H2 and PAH fluxes is characterized by a power law with a roughly constant exponent (0.61 +/- 0.05) over all merger components and spatial scales. While following the same power law on local scales, three galaxies have a factor of 10 larger integrated (i.e., global) H-2/PAH flux ratio than the rest of the sample, even larger than what it is in most nearby active galactic nuclei. These findings suggest a common dominant excitation mechanism for H2 emission over a large range of global H2/PAH flux ratios in major mergers. Early-merger systems show a different distribution between the cold (CO J = 1-0) and warm (H-2) molecular gas components, which is likely due to the merger interaction. Strong evidence for buried star formation in the overlap region of the merging galaxies is found in two merger systems (NGC 6621 and NGC 7592) as seen in the PAH, [Ne II], [Ne III], and warm gas line emission, but with no apparent corresponding CO (J = 1-0) emission. The minimum of the 11.3/7.7 mu m PAH interband strength ratio is typically located in the nuclei of galaxies, while the [Ne III/[Ne II] ratio increases with distance from the nucleus. Our findings also demonstrate that the variations of the physical conditions within a merger are much larger than any systematic trends along the Toomre sequence.

Black hole (BH) masses that have been measured by reverberation mapping in active galaxies fall significantly below the correlation between bulge luminosity and BH mass determined from spatially resolved kinematics of nearby normal galaxies. This discrepancy has created concern that one or both techniques suffer from systematic errors. We show that BH masses from reverberation mapping are consistent with the recently discovered relationship between BH mass and galaxy velocity dispersion. Therefore, the bulge luminosities are the probable source of the disagreement, not problems with either method of mass measurement. This result underscores the utility of the BH mass-velocity dispersion relationship. Reverberation mapping can now be applied with increased confidence to galaxies whose active nuclei are too bright or whose distances are too large for BH searches based on spatially resolved kinematics.

We report the discovery of a new double-image quasar that was found during a search for gravitational lenses in the southern sky. Radio source PMN J1838-3427 is composed of two flat-spectrum components with separation 1."0, flux density ratio 14:1, and matching spectral indices in VLA and VLBA images. Ground-based BRI images show the optical counterpart (total I = 18.6) is also double, with the same separation and position angle as the radio components. An HST/WFPC2 image reveals the lens galaxy. The optical flux ratio (27: 1) is higher than the radio value, probably because of differential extinction of the components by the lens galaxy. An optical spectrum of the bright component contains quasar emission lines at z = 2.78 and several absorption features, including prominent Ly alpha absorption. The lens galaxy redshift could not be measured, but it is estimated to be z = 0.36 +/- 0.08. The image configuration is consistent with the simplest plausible models for the lens potential. The flat radio spectrum and observed variability of PMN J1838-3427 suggest that the time delay between flux variations of the components is measurable and could thus provide an independent measurement of H-o.