Molecular Hydrogen in the Damped Lyα Absorber of Q1331+170
摘要
We used HST STIS to obtain the spectrum of molecular hydrogen associated with the damped Lyα system at z abs = 1.7765 toward the quasar Q1331+170 at z em = 2.084. Strong H 2 absorption was detected, with a total H 2 column density of N (H 2 ) = (4.45 ± 0.36) × 10 19 cm -2 . The molecular hydrogen fraction is f = 2 N /( N + 2 N ) = 5.6% ± 0.7%, which is the greatest value reported so far in any redshifted damped Lyα system. This results from the combined effect of a relatively high dust-to-gas ratio, a low gas temperature, and an extremely low ambient UV radiation field. Based on the observed population of J states, we estimate the photoabsorption rate to be R abs = (7.6 ± 2.4) × 10 -13 s -1 , corresponding to a local UV radiation field of J (1000 Å) ≈ 2.1 × 10 -3 J 1000 Å, ☉ , where J 1000 Å, ☉ is the UV intensity at 1000 Å in the solar neighborhood. This is comparable to the metagalactic UV background intensity at this redshift and implies an extremely low star formation rate in the absorber's environment. We construct a simple model to describe the structure of the H 2 absorber, with a best-fit total hydrogen number density of n (H) ≈ 0.2 cm -3 and an electron temperature of T e ≈ 140 K. Assuming spherical symmetry, the mass of the H 2 cloud is estimated to be ≈6.5 × 10 7 M ☉ , larger than the masses of most giant molecular clouds in the Milky Way and nearby galaxies. The extinction of Q1331+170 due to the intervening DLA is E B-V ≈ 0.037, and we also find that the extinction by DLAs with firm H 2 detections is significantly greater than those for which only upper limits of f have been determined. The observed CO-to-H 2 column density ratio is N CO / N < 2.5 × 10 -7 , which is similar to the value measured for diffuse molecular clouds in the Galactic ISM. Finally, applying the inferred physical conditions to the observed C I fine structure excitation, we estimate the cosmic microwave background temperature to be T CMB = (7.2 ± 0.8) K at z = 1.77654, consistent wit