SIMBAD references

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_ H₂(N) from the average H₂ absorption signal. Assuming a power-law column density distribution, we obtain a slope β= -1.29 ±0.06(stat) ±0.10 (sys) and an incidence rate of strong H₂ absorptions [with N(H₂) >= 10¹⁸ cm^–2] to be 4.0 ±0.5(stat) ±1.0 (sys) per cent in H I absorption systems with N(H I) >=10²⁰ cm^–2. Assuming the same inflexion point where f_ H₂(N) steepens as at z = 0, we estimate that the cosmological density of H₂ in the column density range log N(H₂)(cm^–2)= 18-22 is ∼15 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) [logN(H I) (cm^–2) ≥21.7], which, together with the derived shape of f_ H₂(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.