Molecular Hydrogen in the Damped Ly(alpha) Absorber of Q1331+170

We used HST STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly(alpha) system at zabs = 1.7765 toward the quasar Q1331+170 at zem = 2.084. Strong H2 absorption was detected, with a total H2 column density of N(H2) = (4.45 +0.36) * 1019 cm-2. The molecular hydrogen fraction is f... = 2N.../(N... + 2N...) = 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 Rabs = (7.6 +2.4) * 10^-13 s^-1, corresponding to a local UV radiation field of J(1000 A) ~~2.1 * 103J1000 A, (circle dot operator), where J1000 A, (circle dot operator)is the UV intensity at 1000 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 absorberʹs environment. We construct a simple model to describe the structure of the H2 absorber, with a best-fit total hydrogen number density of n(H) ~~0.2 cm^-3 and an electron temperature of Te ~~140 K. Assuming spherical symmetry, the mass of the H2 cloud is estimated to be ~~6.5 * 10^7 M(circle dot operator), 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 ~~0.037, and we also find that the extinction by DLAs with firm H2 detections is significantly greater than those for which only upper limits of f... have been determined. The observed CO-to-H2 column density ratio is NCO/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 TCMB = (7.2 +- 0.8) K at z = 1.77654, consistent with the predicted value of 7.566 K from the standard cosmology.