Constraining the H${}_2$ column density distribution at z$\sim$3 from composite DLA spectra

Abstract

We present the detection of the average H${}_2$ absorption signal in the overall population of neutral gas absorption systems at $z\sim 3$ using composite absorption spectra built from the Sloan Digital Sky Survey-III damped Lyman-$\alpha$ catalogue. We present a new technique to directly measure the H${}_2$ column density distribution function $f_{{\mathrm{H}}_2}(N)$ from the average H${}_2$ absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta =-1.29\pm 0.06(\mathrm{stat})\pm 0.10(\mathrm{sys})$ and an incidence rate of strong H${}_2$ absorptions (with $N$(H${}_2)\gtrsim 10^{18}$cm${}^{-2}$) to be $4.0\pm 0.5(\mathrm{stat})\pm 1.0(\mathrm{sys})\%$ in HI absorption systems with $N($HI)$\ge 10^{20}$cm${}^{-2}$. Assuming the same inflexion point where $f_{{\mathrm{H}}_2}(N)$ steepens as at $z=0$, we estimate that the cosmological density of H${}_2$ in the column density range $\log N({\mathrm{H}}_2)$(cm${}^{-2})=18-22$ is $\sim 15\%$ of the total. We find one order of magnitude higher H${}_2$ incident rate in a sub-sample of extremely strong DLAs ($\log N($HI)(cm${}^{-2})\ge 21.7$), which, together with the the derived shape of $f_{{\mathrm{H}}_2}(N)$, suggests that the typical HI-H${}_2$ transition column density in DLAs is $\log N(\mathrm{H})$(cm${}^{-2})\gtrsim 22.3$ in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-$z$.

6 pages, 3 figures, accepted for publication in MNRAS letters