SESSIONS XI and XII: MERGERS

Daniel H. McIntosh

University of Missouri-Kansas City

Billions of years of star formation and mass assembly have produced red and blue galaxy populations with unique characteristics. Major mergers between galaxies of comparable mass are dramatic examples of hierarchical structure formation, a key tenet of the standard cosmological model. These gravitational interactions play an important role in modern theories of galaxy evolution, and are a leading explanation for a growing population of old, non-star-forming, spheroid galaxies. Despite recent progress in our understanding of major mergers both observationally and theoretically, we lack a complete picture of the overall role that this dynamic evolutionary process has played in producing the bimodal galaxy population. In particular, a debate continues regarding what portion of the elliptical and spheroid galaxy population originated from major mergers. With the advent of modern surveys of local and distant galaxies, better merger statistics should help resolve this debate soon, and address other unknowns like the environmental dependencies of mergers. I will review the status of constraints on important merger demographics such as mass, environment and progenitor makeup.

Eva Juette

AIRUB, Germany

It has become clear by recent theoretical studies that unequal-mass mergers play an important role to build up the current galaxy population. In particular, they might be the progenitors of S0 galaxies. Furthermore, based on the observations of the SAURON project of early-type galaxies, it has been suggested that unequal-mass mergers might form fast rotating early-types. Despite the potential importance of these unequal-mass mergers, observations are rare, however. We present a multiwavelength investigation of a sample of 15 moderate luminosity mergers, having a FIR luminosity at least an order of magnitude below that of an ULIRG. That implies a moderate starburst induced by the merger. Optical observation (imaging and spectroscopy) are used to investigate the stellar populations. HI and CO observations were obtained to investigate the merger history from a dynamical point of view and to determine the amount of gas available for star formation. We found that most of the sample galaxies have a significant amount of gas despite the moderate star formation rate. In a more detailed case study of two sample galaxies, NGC 4194 and NGC 4441, we investigated the gas properties of these unequal-mass mergers. The comparison with ULIRGs can help to explain the differences in induced star formation found in the different kind of mergers.

Petri Vaisanen

South African Astronomical Observatory

Luminous IR galaxies (LIRGs) dominate the star-formation (SF) at higher redshifts. Understanding of their physical details may be acquired from local LIRG samples, which are mostly interacting and merging systems. We have used adaptive optics (AO) in the near-IR to survey a sample of LIRGs with <0.1 arcsec (30 to 100 pc) resolution. We detect core-collapse SNe in the heavily extincted nuclear regions to compare with other star-formation estimates. The data are merged with SALT and AAT spectroscopic follow-up and HST and Spitzer archival imaging to further study the details of LIRG SF and its spatial distribution, history, triggering and quenching, and correlate these with LIRG dynamical histories. The first AO detected SNe are reported as well as details of the first studied LIRGs. NIR AO images have been absolutely crucial to be able to interpret the kinematic and dynamical data, and to characterize the SF by bridging the gap between optical and MIR data. The unique combination of data has revealed surprises. One galaxy showed an unexpected third component in the interaction, which moreover turned out to host the most active star formation. Another target showed evidence in the NIR of a very rare case of leading spiral arms, rotating in the same direction as the arms open. Both of these LIRGs involve unusual high-velocity (>400 km/s) encounters.

Santiago Arribas

IEM-CSIC

We are carrying out a project devoted to the study of the internal structure of a representative sample of about 60 low-redshift LIRGs and ULIRGs using several optical and near-IR IFS facilities. Here we present the main characteristics of this project, and discuss recent results derived from optical data obtained with the VIMOS / VLT instrument. In particular, the kinematics and physical structure of different galaxy components (i.e. star forming regions, old stellar population, warm ionized gas, neutral gas) will be characterized and discussed in terms of fundamental properties of the systems (e.g. L_ir), and their dynamical status (isolated, pairs, or merger remnants).

Lihwai Lin

Academia Sinica Institute, Taiwan

The evolution of the galaxy merger rate and its impact on galaxy properties have been studied intensively over the last decade. It becomes clear now that types of mergers, i.e. gas-rich (wet), gas-poor (dry), or mixed, affect the merger products. The epoch when each type of merger dominates also differ. In my talk, I will review the recent progress on the measurements of galaxy merger rates out to z~3 and the level of interaction-triggered star formation using large samples from various redshift surveys. These results provide insights to the importance of mergers in the mass assembly history of galaxies and in the evolution of galaxy properties. I will also present new results in characterizing the environment of galaxy mergers, and discuss their implications of the built up of red-sequence galaxies in the framework of hierarchical structure formation.