Possible Atomic-to-Molecular Gas Transition in the Center of Merging Galaxies

Summary

ABSTRACT

We examine the distribution and kinematics of atomic and molecular gas mapped in a number of galaxies suspected to be in the process of merging. In most cases, the nuclear region of the merger has a high concentration of molecular gas, and a deficiency of atomic gas as compared with larger radii. Thus the total surface mass density of gas often has a minimum at an intermediate radius. In cases where the gas rotation curve is measured, the transition from regions dominated by molecular gas to those of atomic gas corresponds to abrupt changes in rotation characteristics. We propose that the merger is efficiently converting ISM from atomic into molecular form in central region of these galaxies, and that the dense clouds are experiencing radial accretion at a higher rate than diffuse gas.

INTRODUCTION

Since the early work of Toomre and Toomre (1972), much progress has been made in understanding dynamical processes of merger galaxies (cf. Barnes and Hernquist 1993). One result that appears consistently in theoretical studies and numerical simulations alike is that the end product of the merger is an early type galaxy. Increasingly, this scenario has acquired observational support as well. For example, the K-band light profiles of many Arp galaxies, mostly advanced mergers, show r^1/4 law typical of ellipticals (e.g., Wright et al. 1990; Stanfrd and Bushouse 1991). Yet, the process in which the merging disks shed their abundant gas mass remains unclear, and numerical simulations are far from adequately resolving this problem given the enormous dynamical range required to mimic the changes in the ISM.