Molecular hydrogen in the damped Ly alpha absorber of Q1331+170

作者：Cui J^*; Bechtold J; Ge J; Meyer DM

来源：Astrophysical Journal, 2005, 633(2): 649-663.

DOI：10.1086/444368

• galaxies : ISM
• ISM : molecules
• quasars : absorption lines
• quasars : individual (Q1331+170)

We used HST STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly alpha 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) x 10(19) cm(-2). The molecular hydrogen fraction is f(H2) = 2N(H2)/(N-H I + 2N(H2)) = 5.6% +/- 0.7%, which is the greatest value reported so far in any redshifted damped Ly alpha 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) x 10(-13) s(-1), corresponding to a local UV radiation field of J (1000 angstrom) approximate to 2.1 x 10(-3) J(1000) (angstrom, .), where J(1000 angstrom, .) is the UV intensity at 1000 angstrom 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) approximate to 0.2 cm(-3) and an electron temperature of T-e approximate to 140 K. Assuming spherical symmetry, the mass of the H-2 cloud is estimated to be approximate to 6.5 x 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 EB-V approximate to 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(H2) have been determined. The observed CO-to-H-2 column density ratio is N-CO/NH2 < 2.5 x 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 with the predicted value of 7.566 K from the standard cosmology.

• 出版日期2005-11-10
• 单位西北大学