CO in Toomre Sequence

Do All Mergers Undergo an Ultraluminous Phase?

M. S. Yun & J. E. Hibbard, to be submitted to ApJ.
Original paper submitted to ApJ 3/26/99 (astro-ph/9903463);
referee requested that we split this paper in two. This is the second part

We examine high-resolution CO data of both IR and optically selected mergers in order to address if there are any fundamental differences between them. The optically selected mergers fall on the same relationship between the mean gas density and FIR luminosity defined by the IR luminous mergers, extending it to lower luminosity and lower mean gas density. This is similar to the ``global Schmidt law" of Kennicutt, and all merger systems appear to follow the same star formation ``law" as the field galaxies. If AGN fueling is largely decoupled from the gas distribution on kpc scales, then deviations from this relationship may constrain the contribution of AGN to the FIR luminosity. The response of the gas during the merger process is quantified by comparing the specific angular momentum, J/M, of the merger systems to normal spiral disks. We find that the molecular gas disks of the merger systems have ~5 times less J/M than field spirals with the same gas mass. There is little difference between the optically selected mergers and IR selected mergers suggesting that angular momentum shedding by molecular gas is a universal process among all major merger systems. Lastly, an examination of the radial gas density profiles suggests that some of the less luminous mergers such as NGC7252 may have had a dense central molecular core in the past, which has since been eroded by starburst activity.

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