Evidence for Rotation in the Galaxy at z=3.15 Responsible for a Damped Lyman-alpha Absorption System in the Spectrum of Q2233+1310

Abstract

Proof of the existence of a significant population of normal disk galaxies at redshift z>2 would have profound implications for theories of structure formation and evolution. We present evidence based on Keck HIRES observations that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may well be such an example. Djorgovski et al have recently detected the Lyman-alpha emission from the absorber, which we assume is at the systemic redshift of the absorbing galaxy. By examining the profiles of the metal absorption lines arising from the absorbing galaxy in relation to its systemic redshift, we find strong kinematical evidence for rotation. Therefore the absorber is likely to be a disk galaxy. The inferred circular velocity for the galaxy is >200 km/s. With a separation of ~17 kpc between the galaxy and the quasar sightline, the implied dynamic mass for the galaxy is >1.6x10(11) solar mass. The metallicity of the galaxy is found to be [Fe/H]=-1.4, typical of damped Lyman-alpha galaxies at such redshifts. However, in another damped galactic rotation is evident. In the latter case, the damped Lyman-alpha absorber occurs near the background quasar in redshift so its properties may be influenced by the background quasar. These represent the only two cases at present for which the technique used here may be applied. Future applications of the same technique to a large sample of damped Lyman-alpha galaxies may allow us to determine if a significant population of disk galaxies already existed only a few billion years after the Big Bang.