Speaker
Description
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 $f_{\rm H_2}(N)$ from the average H$_2$ absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta= −1.29 \pm 0.06 ({\rm stat}) \pm 0.10 ({\rm sys})$ and an incidence rate of strong H$_2$ absorptions (with $N(\rm H_2)\ge 10^{18}$ cm$^{−2}$) to be $4.0 \pm 0.5 ({\rm stat}) \pm 1.0 ({\rm sys})$ per cent in HI absorption systems with $N(\rm HI)\ge 10^{20}\,{\rm cm}^{−2}$. Assuming the same inflexion point where $f_{\rm H_2}(N)$ steepens as at $z=0$, we estimate that the cosmological density of diffuse molecular gas (with H$_2$ in the column density range $\log N(\rm H_2)(\rm cm^{−2}) = 18–22$) is $\sim$15 per cent of the total. We find one order of magnitude higher H$_2$ incidence rate in a subsample of extremely strong damped Lyman $\alpha$ absorption systems (DLAs) ($\log N(\rm HI)(cm^{−2}) \ge 21.7$), which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical HI–H$_2$ transition column density in DLAs is $\log N(\rm H)(cm^{−2})\sim22.3$ in agreement with theoretical expectations for the average (low) metallicity of DLAs at high $z$.
| Wish list question? | 1. Just how well do we know the cosmic evolution of Omega_HI(z) and Omega_H2(z)? |
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