Cold Molecular Gas as a Possible Component of Dark Matter in the Outer Parts of Disk Galaxies

Abstract

In the last few years new evidence has been presented for the presence of ongoing massive star formation in the outer HI disks of galaxies. These discoveries strongly suggest that precursor molecular gas must also be present in some physical state which is escaping detection by the usual means (CO(1-0), IR, etc.). We present a model for such a gas in a framework which views the HI as the result of an ongoing ``photodissociation <--> dust grain reformation&#39;&#39; equilibrium in a cold, clumpy molecular medium with a small area filling factor.