Molecular Hydrogen in the Damped Ly alpha Absorber of Q1331+170

Abstract

We used HST/STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly$\hat{I}\pm$ system at $z_{\mathrm{abs}}=1.7765$ toward the quasar Q1331+170 at $z_{\mathrm{em}}=2.084$. Strong ${\mathrm{H}}_2$ absorption was detected, with a total ${\mathrm{H}}_2$ column density of $N({\mathrm{H}}_2)=(4.45\pm 0.36)\times 10^{19}\mathrm{c}{\mathrm{m}}^{-2}$.The molecular hydrogen fraction is $f_{{\mathrm{H}}_2}=\frac{2N_{{\mathrm{H}}_2}}{N_{\mathrm{HI}}+2N_{{\mathrm{H}}_2}}=(5.6\pm 0.7)$, which is the greatest value reported so far in any redshifted damped Ly$\hat{I}\pm$ system. This results from the combined effect of a relatively high dust-to-gas ratio, a low gas temperature, and an extremely low ambient UV radiation field. Based on the observed population of $J$ states, we estimate the photo-absorption rate to be $R_{\mathrm{abs}}=(7.6\pm 2.4)\times 10^{-13}{\mathrm{s}}^{-1}$, corresponding to a local UV radiation field of $J(1000\tilde{\mathrm{A}})\approx 2.1\times 10^{-3}{J}_{1000\tilde{\mathrm{A}},\odot }$, where $J_{1000\tilde{\mathrm{A}},\odot }$ is the UV intensity at $1000\tilde{A}$ in the solar neighborhood. This is comparable with the metagalactic UV background intensity at this redshift, and implies an extremely low star formation rate in the absorber's environment. The observed CO-to-H${}_2$ column density ratio is $\frac{N_{\mathrm{CO}}}{N_{{\mathrm{H}}_2}}<2.5\times 10^{-7}$, which is similar to the value measured for diffuse molecular clouds in the Galactic ISM. Finally, applying the inferred physical conditions to the observed C I fine structure excitation (Songaila {\it et al.} 1994), we estimate the cosmic microwave background temperature to be $T_{\mathrm{CMB}}=(7.2\pm 0.8)\mathrm{K}$ at $z=1.77654$, consistent with the predicted value of $7.566\mathrm{K}$ from the standard cosmology.

Accepted for publication, Astrophysical Journal. Abstract abbreviated