The Evolution of Globular-Cluster Systems Formed in Galactic Mergers

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

AST 0205994 Schweizer The formation and evolution of galaxies is of central interest in present-day astrophysics. There is mounting evidence that many galaxies grew from smaller units, and that galactic mergers may have played a major role in determining the shapes and dynamics of certain remnants, and perhaps even of elliptical galaxies. However, the details of this assembly remain unclear, and there are several competing hypotheses for the formation of ellipticals. Fortunately, a few galactic mergers still occur in the local universe and o.er valuable clues to the past assembly process. The 1992 discovery that globular star clusters form in large numbers during mergers of gas-rich galaxies has opened new avenues to studying the formation and evolution of globular clusters and of their host galaxies. Questions of current interest are: Is there an evolutionary link between globular clusters formed in mergers and old globular clusters in giant elliptical galaxies? How do systems of globular clusters formed in mergers evolve? Specifically, how do their luminosity functions, metallicity distributions, and kinematics evolve? Do galactic mergers imprint cluster systems with kinematic signatures (e.g., fast outer rotation) that can be traced along a sequence of merger remnants of increasing age? The Principal Investigator, Francois Schweizer, proposes carefully designed observational studies to address these questions. He points out that-whereas the birth of young clusters in ongoing mergers and the properties of old globular clusters in ellipticals have been studied widely in recent years-there is a surprising lack of knowledge about globular-cluster systems of intermediate age. Yet, these systems hold great promise for directly addressing the above issues. Dr. Schweizer and collaborators were among the .rst to study globular clusters formed in mergers with the Hubble Space Telescope and to follow up with ground-based spectroscopic observations to determine their ages and chemical abundances. He will now search for globular clusters of intermediate age (1 - 7 Gyr) in about a dozen galaxies thought to be transition objects between ongoing mergers (e.g., NGC 4038/39) and old ellipticals. He will use both the Baade 6.5-m telescope at Las Campanas Observatory and the Advanced Survey Camera on the Hubble Space Telescope to obtain some of the deepest images ever taken of these transition objects and their globular-cluster systems. Photometric analysis of these images, as well as of Hubble archival images, will yield the chronology of these cluster systems from their broad-band colors. The same deep images will also be used to study the evolution of the globular-cluster luminosity function with time. The luminosity functions of fresh-born cluster systems are power laws, while those of ancient systems are close to lognormal. Theory predicts that as globular-cluster systems age, their luminosity functions should evolve from the .rst to the second shape through preferential erosion of low-mass clusters. Deep luminosity functions reaching past the predicted turnover magnitudes will be measured from the images to subject this prediction to a critical test. A parallel program of multi-slit spectroscopy mainly with the Magellan 6.5-m telescopes, but also with the northern Gemini 8-m telescope, will yield metallicities and more accurate ages for some key globular-cluster systems of intermediate age. Measurements of radial velocities for typically 40 - 200 clusters per galaxy will permit comparative studies of the kinematics of metal-poor and metal-rich globulars within the same galaxy. This will yield valuable clues about the origins of what are thought to be .rst- and 2nd-generation cluster subpopulations. These observational studies will test all proposed formation mechanisms for ellipticals and will-specifically-focus on possible evidence for an evolutionary link between 2nd-generation globulars formed in mergers and metal-rich globulars in old ellipticals. The results from this project will help improve our understanding not only of globular-cluster formation and evolution, but also of galaxy formation and growth and, ultimately, of the origin of Hubble's morphological sequence of galaxy types. ***