Galaxies with Spiral Structure up to Z ~ 0.87: Limits on M/L and the Stellar Velocity Dispersion

Abstract

We consider seven distant galaxies with clearly evident spiral structure from Hubble Space Telescope images. We place upper limits on their mass-to-light ratios (M/L) by computing M/L_B for a maximal disk. We find that these galaxies have maximal-disk mass-to-light ratios M/L_B = 1.5-3.5 (M/L_B)_⊙ at the low end, but within the range seen in nearby galaxies. The mass-to-light ratios are low enough to suggest that the galaxies contain a young, rapidly formed stellar population. By using a Toomre stability criterion for formation of spiral structure, we place constraints on the ratio of M/L to the stellar velocity dispersion. If these galaxies have maximal disks, they would have to be nearly unstable so as to have small enough velocity dispersions that their disks are not unrealistically thick. This suggests that either a fraction of the light originates from a component of stars in a thick disk or bulge not strongly affected by the spiral structure, or there is a substantial amount of dark matter present in the luminous regions of the galaxy.