Constraining the H2 column density distribution at z ∼ 3 from composite DLA spectra. (21st April 2018) - Balashev, S A
Abstract: We present the detection of the average H2 absorption signal in the overall population of neutral gas absorption systems at z ∼ 3 using composite absorption spectra built from the Sloan Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H2 column density distribution function $f_{\rm H_2}(N)$ from the average H2 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 H2 absorptions [with N (H2 ) ≳ 10 18 cm −2 ] to be $4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$ in H i absorption systems with N (H i ) ≥10 20 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 H2 in the column density range $\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$ is ${\sim } 15\hbox{ per cent}$ of the total. We find one order of magnitude higher H2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) [$\log N(\rm{H\, \small {I}}) ({\rm cm}^{-2}) \ge 21.7$ ], which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical H i –H2 transition column density in DLAs is log N (H)(cm −2 ) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high- z .