Effects of Rotation on Thermal-Gravitational Instability in the Protogalactic Disk Environment

Abstract

Thermal-gravitational instability (TGI) is studied in the protogalactic environment. We extend our previous work, where we found that dense clumps first form out of hot background gas by thermal instability and later a small fraction of them grow to virialized clouds of mass M_c >~ 6X10^6 M_sun by gravitational infall and merging. But these clouds have large angular momentum, so they would be difficult, if not impossible, to further evolve into globular clusters. In this paper, through three-dimensional hydrodynamic simulations in a uniformly rotating frame, we explore if the Coriolis force due to rotation in protogalactic disk regions can hinder binary merging and reduce angular momentum of the clouds formed. With rotation comparable to the Galactic rotation at the Solar circle, the Coriolis force is smaller than the pressure force during the early thermal instability stage. So the properties of clumps formed by thermal instability are not affected noticeably by rotation, except increased angular momentum. However, during later stage the Coriolis force becomes dominant over the gravity, and hence the further growth to gravitationally bound clouds by gravitational infall and merging is prohibited. Our results show that the Coriolis force effectively destroys the picture of cloud formation via TGI, rather than alleviate the problem of large angular momentum.