The Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) have been used to obtain new Hubble Space Telescope images of NGC 4038/4039 ("The Antennae"). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function by approximately two magnitudes over our previous WFPC2 results, and find that it continues as a single power law, dN/dL ∝ L α with α = −2.13 ± 0.07, down to the observational limit of M V ≈ −7. Similarly, the mass function is a single power law dN/dM ∝ M β with β = −2.10 ± 0.20 for -2clusters with ages < 3 × 10 8 yr, corresponding to lower mass limits that range from 10 4 to 10 5 M ⊙ , depending on the age range of the subsample. Hence the power law indices for the luminosity and mass functions are essentially the same. The luminosity function for intermediate-age clusters (i.e., ∼100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to M V ≈ −6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until M V ≈ −4. An analysis of individual ∼0.5-kpc sized areas over diverse environments shows good agreement between values of α and β, similar to the results for the total population of clusters in the system. There is tentative evidence that the values of both α and β are flatter for the youngest clusters in some areas, but it is possible that this is caused by observational biases. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with an ∼10-50 Myr old cluster of estimated mass ∼ 10 6 M ⊙ having apparently triggered the formation of several younger, more massive (up to 5 × 10 6 M ⊙ ) clusters along a dust lane. A comparison with new NICMOS observations reveals that only 16 ± 6% of the IR-bright clusters in the Antennae are still heavily obscured, with values of A V > 3 mag.

We present K-band photometry for 51 candidate merger remnants to assess the viability of whether spiral-spiral mergers can produce bona fide elliptical galaxies. Using both the de Vaucouleurs r 1/4 and Sérsic r 1/n fitting laws, it is found that the stellar component in a majority of the galaxies in the sample has undergone violent relaxation. However, the sample shows evidence for incomplete phase mixing. The analysis also indicates the presence of ''excess light'' in the surface brightness profiles of nearly one-third of the merger remnants. Circumstantial evidence suggests that this is due to the effects of a starburst induced by the dissipative collapse of the gas. The integrated light of the galaxies also shows that mergers can make L* elliptical galaxies, in contrast to earlier infrared studies. The isophotal shapes and related structural parameters are also discussed, including the fact that 70% of the sample show evidence for disky isophotes. The data and results presented are part of a larger photometric and spectroscopic campaign to thoroughly investigate a large sample of mergers in the local universe.

We study the origin and properties of "extra" or "excess" central light in the surface brightness profiles of remnants of gas-rich mergers. By combining a large set of hydrodynamical simulations with data on observed mergers that span a broad range of profiles at various masses and degrees of relaxation, we show how to robustly separate the physically meaningful extra light -i.e. the stellar population formed in a compact central starburst during a gas-rich merger -from the outer profile established by violent relaxation acting on stars already present in the progenitor galaxies prior to the final stages of the merger. This separation is sensitive to the treatment of the profile, and we demonstrate that certain fitting procedures can yield physically misleading results. We show that our method reliably recovers the younger starburst population, and examine how the properties and mass of this component scale with e.g. the mass, gas content, and other aspects of the progenitors. We consider the time evolution of the profiles in different bands, and estimate the biases introduced by observational studies at different phases and wavelengths. We show that, when appropriately quantified, extra light is ubiquitous in both observed and simulated gas-rich merger remnants, with sufficient mass (∼ 3 − 30% of the stellar mass) to explain the apparent discrepancy in the maximum phase-space densities of ellipticals and their progenitor spirals. The nature of this central component provides a powerful new constraint on the formation histories of observed systems and can inform both our studies of their progenitors and our understanding of the global kinematics and structure of spheroids.

A direct consequence of hierarchical galaxy formation is the existence of dual supermassive black holes, which may be preferentially triggered as active galactic nuclei (AGNs) during galaxy mergers. Despite decades of searching, however, dual AGNs are extremely rare, and most have been discovered serendipitously. Using the all-sky WISE survey, we identified a population of over 100 morphologically identified interacting galaxies or mergers that display red mid-infrared colors often associated in extragalactic sources with powerful AGNs. The vast majority of these advanced mergers are optically classified as star-forming galaxies, which suggests that they may represent an obscured population of AGNs that cannot be found through optical studies. In this work, we present Chandra/ACIS observations and near-infrared spectra with the Large Binocular Telescope of six advanced mergers with projected pair separations less than ∼10 kpc. The combined X-ray, near-infrared, and mid-infrared properties of these mergers provide confirmation that four out of the six mergers host at least one AGN, with four of the mergers possibly hosting dual AGNs with projected separations less than ∼10 kpc, despite showing no firm evidence for AGNs based on optical spectroscopic studies. Our results demonstrate that (1) optical studies miss a significant fraction of single and dual AGNs in advanced mergers, and (2) mid-infrared pre-selection is extremely effective in identifying dual AGN candidates in late-stage mergers. Our multi-wavelength observations suggest that the buried AGNs in these mergers are highly absorbed, with intrinsic column densities in excess of~> N 10 H 24 cm −2, consistent with hydrodynamic simulations.