On the (Non)Evolution of H I Gas in Galaxies Over Cosmic Time
Abstract
We present new results on the frequency distribution of projected H I column densities f(N H I , X), total comoving covering fraction, and integrated mass densities ρH I of high-redshift, H I galactic gas from a survey of damped Lyα systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full sample spanning z = 2.2-5 (738 DLAs), f(N H I , X) is well fitted by a double power law with a break column density Nd = 1021.55±0.04 cm-2 and low/high-end exponents α = -2.00 ± 0.05, - 6.4+1.1 -1.6. The shape of f(N H I , X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day H I disks as inferred from 21 cm observations. We conclude that H I gas has been distributed in a self-similar fashion for the past 12 Gyr. The normalization of f(N H I , X), in contrast, decreases by a factor of 2 during the ≈2 Gyr interval from z = 4-2.2 with coincident decreases in both the total covering fraction and ρH I . At z ≈ 2, these quantities match the present-day values suggesting no evolution during the past ≈10 Gyr. We argue that the evolution at early times is driven by "violent" processes that removes gas from nearly half the galaxies at z ≈ 3 establishing the antecedents of current early-type galaxies. The perceived constancy of ρH I , meanwhile, implies that H I gas is a necessary but insufficient precondition for star formation and that the global star formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2009
- DOI:
- 10.1088/0004-637X/696/2/1543
- arXiv:
- arXiv:0811.2003
- Bibcode:
- 2009ApJ...696.1543P
- Keywords:
-
- galaxies: evolution;
- intergalactic medium;
- quasars: absorption lines;
- Astrophysics
- E-Print:
- 6 pages, 3 figures. Submitted to ApJL. Comments/criticism encouraged. See also http://www.ucolick.org/~xavier/SDSSDLA/index.html