2005ApJ...633..649C

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 H₂absorption was detected, with a total H₂column density of N(H₂)=(4.45±0.36)x10¹⁹/cm2. The molecular hydrogen fraction is f_H2=2N_H2/(N_HI_+2NH₂_)=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)x10^–13/s, corresponding to a local UV radiation field of J(1000 Å)~2.1x10^–3J_1000Å,☉, where J_1000Å, ☉_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 H₂absorber, with a best-fit total hydrogen number density of n(H)~0.2/cm3 and an electron temperature of T_e~140 K. Assuming spherical symmetry, the mass of the H₂cloud is estimated to be ~6.5x10⁷ 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 H₂detections is significantly greater than those for which only upper limits of f_H2have been determined. The observed CO-to-H₂column density ratio is N_CO/N_H2<2.5x10^–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.