On the (Non)Evolution of HI Disks over Cosmic Time

TL;DR

This study analyzes the distribution and evolution of high-redshift HI disks using DLA data, revealing self-similar distribution over 12 billion years and a decrease in gas covering fraction from z=4 to 2.2, with implications for galaxy evolution.

Contribution

It provides the first detailed measurement of the HI column density distribution function over z=2.2 to 5, showing invariance in shape and linking high-redshift HI properties to present-day disks.

Findings

f(N,X) follows a double power-law with a fixed shape over z=2.2 to 5

Normalization of f(N,X) decreases by a factor of two from z=4 to 2.2

HI mass density remains constant from z~2 to present day

Abstract

We present new results on the frequency distribution of projected HI column densities f(N,X), total comoving covering fraction, and integrated mass densities rho_HI of high redshift, HI `disks' from a survey of damped Lya systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full sample spanning z=2.2 to 5 [738 DLAs], f(N,X) is well fitted by a double power-law with a break column density N_d = 10^(21.55 +/- 0.04) and low/high-end exponents alpha = -2.00 +/- 0.05, -6.4^{+1.1}_{-1.6}. The shape of f(N,X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day HI disks as inferred from 21cm observations. We conclude that HI gas has been distributed in a self-similar fashion for the past 12Gyr. The normalization of f(N,X), in contrast, decreases by a factor of two during the ~2Gyr interval from z=4 to 2.2 giving corresponding decreases in both the total covering fraction and rho_HI. At z~2, these quantities match the present-day values suggesting no evolution during the past ~10Gyr. 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 antecedants of current early-type galaxies. The perceived constancy of rho_HI, meanwhile, implies that HI gas is a necessary but insufficient pre-condition for star formation and that the global star-formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.