Shrinking Galaxy Disks with Fountain-Driven Accretion from the Halo

TL;DR

This paper explores how fountain-driven accretion from the halo causes galaxy disks to shrink over time, affecting their structure and star distribution, especially in Blue Compact Dwarfs.

Contribution

It introduces a model showing halo gas condensation preserves disk shape while causing a gradual shrinkage, a novel insight into galaxy evolution.

Findings

Disk shape preservation during accretion

Gradual disk shrinkage over time

Potential significance for Blue Compact Dwarfs

Abstract

Star formation in most galaxies requires cosmic gas accretion because the gas consumption time is short compared to the Hubble time. This accretion presumably comes from a combination of infalling satellite debris, cold flows, and condensation of hot halo gas at the cool disk interface, perhaps aided by a galactic fountain. In general, the accretion will have a different specific angular momentum than the part of the disk that receives it, even if the gas comes from the nearby halo. Then the gas disk expands or shrinks over time. Here we show that condensation of halo gas at a rate proportional to the star formation rate in the fountain model will preserve an initial shape, such as an exponential, with a shrinking scale length, leaving behind a stellar disk with a slightly steeper profile of younger stars near the center. This process is slow for most galaxies, producing imperceptible radial speeds, and it may be dominated by other torques, but it could be important for Blue Compact Dwarfs, which tend to have large, irregular gas reservoirs and steep blue profiles in their inner stellar disks.