Constraining the H2_2 column density distribution at z\sim3 from composite DLA spectra

Apr 12, 2018
6 pages
Published in:
  • Mon.Not.Roy.Astron.Soc. 478 (2018) 1, L7-L11
  • Published: Jul 21, 2018
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Abstract: (Oxford University Press)
We present the detection of the average H_2 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 H_2 column density distribution function fH2(N)f_{\rm H_2}(N) from the average H_2 absorption signal. Assuming a power-law column density distribution, we obtain a slope β=1.29±0.06(stat)±0.10(sys)\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys) and an incidence rate of strong H_2 absorptions [with N(H_2) ≳ 10^18 cm^−2] to be 4.0±0.5(stat)±1.0(sys) per cent4.0 \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 fH2(N)f_{\rm H_2}(N) steepens as at z = 0, we estimate that the cosmological density of H_2 in the column density range logN(H2)(cm2)=1822\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22 is 15 per cent{\sim } 15\hbox{ per cent} of the total. We find one order of magnitude higher H_2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) [logN(HI)(cm2)21.7\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7], which, together with the derived shape of fH2(N)f_{\rm H_2}(N), suggests that the typical H i–H_2 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.
Note:
  • 6 pages, 3 figures, accepted for publication in MNRAS letters
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