Gas Flow and Star Formation in the ‘ Antennae ’ Galaxies

The prominent interacting galaxy pair NGC 4038/9 contains many active star-forming regions and is continuously forming new star clusters. We present a self-consistent n-body model for this system which includes an SPH gas component. The model qualitatively explains the apparent concentration of gas in the so-called overlap region between the two nuclei as a bridge of gas connecting the two galaxies. Projected on the sky, the bridge appears as a dense spot of gas. We discuss some implications for the evolution of Ultra-luminous infrared galaxies. The stellar dynamics of the Antennae galaxies merger has been studied in detail by Toomre & Toomre (1972), Barnes (1988), and others. We are interested in this merger to study star formation under extreme conditions. The gas transport towards the galaxy centers in the early stages of the mergers may lead to circum-nuclear starbursts. Tidal forces during the first passage trigger the formation of bars or induce m = 2 spiral arm modes in the approaching galaxies. Once a bar is forming, the gas piles up at the inner Lindblad resonance which in turn undergoes rapid star formation. The distribution of young stellar clusters, is therefore linked to the dynamical history of the merger. With bolometric luminosities and space densities comparable to or even higher than those of quasars, Ultra-luminous Infrared Galaxies (ULIRGs) are the most luminous objects (L≥10 12 L ⊙) in the local Universe (Soifer et al. 1987). Deep ground based (Rigopoulou et al. 1999) and HST–NICMOS (Scoville et al. 2000) images reveal that ULIRGs are undergoing a major merger. Often the merging includes two similar size disk galaxies. However, the merging sequence for ULIRGs seems to be somewhat different from that of lower luminosity nearby mergers, implying that perhaps ULIRGs have a different type of progenitor galaxies or their gas properties are different. Rigopoulou et al. (1999) have shown that in fact the gas properties of ULIRGs are different than those of the less luminous (L≥10 11 L ⊙) LIRG systems. They found that although in LIRGs the CO content decreases with decreasing separation, ULIRGs appear to still be gas-rich even at advanced stages of the merger. Activity in ULIRGs is known to be in part due to star bursts and also due to AGN activity. One may speculate, that AGN activity is triggered by the merger event. However, no correlation between AGN activity and merger state has been found (Rigopoulou et …