SESSION III: INDIVIDUAL SYSTEMS

Anne Pellerin

Texas A&M University

We present a detailed study of the star cluster population detected in the galaxy NGC922, one of the closest collisional ring galaxies known to date, using HST/WFPC2 UBVI photometry, population synthesis models, and N-body/SPH simulations. We find that 69% of the clusters are younger than 7Myr, and that most of them are located in the ring or along the bar, consistent with the strong H_alpha emission. The cluster luminosity function slope of 2.1-2.3 for NGC922 is in agreement with those of young clusters in nearby galaxies. The observed age distribution displays a slope that is not consistent with a plain star formation history typical of ring galaxies. However if we take into account for cluster disruption, we find that the age distribution slope is more generally consistent with the simulations of a ring galaxy. We also find clusters with ages (>50Myr) and masses (>10⁵ M(sun)) that are excellent progenitors for faint fuzzy clusters. The best-fit simulation indicates that the collision and initial burst of star formation happened about 150Myr ago. The images also show a tidal plume pointing toward the companion. Its stellar age from our analysis is consistent with pre-existing stars that were stripped off during the passage of the companion. Finally, a comparison of the star-forming complexes observed in NGC922 with those of a distant ring galaxy from the GOODS field indicates very similar masses and sizes, suggesting similar origins.

Lou Nigra

University of Wisconsin, Madison

Understanding the hydrodynamic processes and conditions at the interface between the Magellanic Stream (MS) and the Galactic halo is critical to understanding the MS and by extension, tidal tails in other interacting galaxies. These processes not only help shape the clumpy stream, but also affect the neutral gas dynamics and transfer of mass from the stream to the halo, thus affecting the star-formation potential within these clumps and possibly serving as a metal enrichment and gas replenishment mechanism for the Galaxy, affecting future star formation there. We present new HI observations using the Green Bank Telescope in two separate MS regions with the highest sensitivity to date having resolution adequate to reveal the structure of this interface. We analyze the spatial and spectral structure of the HI clumps and enhance sensitivity even further through sophisticated spatial integration techniques, directly probing the neutral gas as it transitions from the clump centers to the halo interface. At clump centers, we search for cold cores, possible precursors to star formation. We then analyze the gas dynamics in the clumps for consistency with models of various hydrodynamic processes and possible halo gas properties. Since the two regions are significantly separated along the MS, we also gain insight into how these processes have evolved in time.

CONTRIBUTED TALK: Molecular Gas in the Early Stage of Interacting Galaxies NGC4567/4568 Pair

Hiroyuki Kaneko

The Graduate University for Advanced Studies (SOKENDAI)/ Nobeyama Radio Observatory, NAOJ

We present a 12CO (J=1-0) mapping observation of the interacting galaxy pair NGC4567/8 performed with the Nobeyama 45m telescope. The aims for this study are to investigate the response of cold molecular gas during the early stage of collision of galaxies, and to understand why the interacting systems are stimulated to violent star formation. Combining with previous HI observation, we found high molecular fractions f_mol, a ratio of the molecular to total gas surface densities) in NGC4567 and the overlap region in spite of their low total gas column densities. The large f_mol is interpreted as being due to the large pressure induced by the collision of galaxies, although the possibility of ram pressure that may selectively strip HI gas cannot be ruled out. Though NGC4567/4568 system shows calm star formation as a whole, its overlap region also has high star formation efficiency (star formation rate per solar mass of total gas). These results suggest that explosive star formation of the latter stage of collision begins not only at the nuclei of each progenitors as predicted by Barnes & Hernquist (1996) but at the collision area.

Mark Hancock

University of California Riverside

To search for Tidal Dwarf Galaxies (TDGs) and to study star formation in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the GALEX telescope. As part of that study, we present a GALEX UV and SDSS and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and star formation outside the disks. This includes a luminous star forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages of about 6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.