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J. Cui, J. Bechtold (Steward Observatory, University of Arizona), J. Ge (Dept. of Astronomy and Astrophysics, Pennsylvania State University), D.M. Meyer (Dept. of Physics and Astronomy, Northwestern University)
We used HST/STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly\alpha system at z\rm abs=1.7765 toward the quasar Q1331+170 at z\rm em=2.084. Strong {\rm H}2 absorption was detected, with a total {\rm H}2 column density of N({\rm H}2)=(4.45± 0.36)\times 1019\:{\rm cm-2}. The total molecular hydrogen fraction is f{\rm H}_2=2N_{\rm H_2}/N_{\rm HI}+2N_{\rm H_2}=(5.6±0.7)%, which is the largest 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 photo-absorption rate to be \beta0=(7.6±2.4)\times 10-13\:{\rm s-1}, corresponding to a local UV radiation field at the Lyman limit of j(912{\rm Å})\approx 5.0\times 10-22\:{\rm erg\:cm-2\:s-1\:Hz-1\:Sr-1}. This is comparable with the metagalactic UV background intensity at this redshift, and implies an extremely low star formation rate in the absorber's environment. The observed Ar underabundance of [{\rm Ar}/\alpha]=-0.46±0.05 also suggests that the ambient UV radiation originates externally from the cloud. We construct a simple model to describe the structure of the {\rm H}2 absorber, with a best-fit total hydrogen number density of n({\rm H})\approx 0.2\:{\rm cm-3} and an electron temperature of T\rm e\approx 140\:{\rm K}. The observed CO-to-H2 column density ratio is N_{\rm CO}/N_{\rm H_2}<2.5\times 10-7, 3 orders of magnitude lower than locally measured values. Finally, applying the inferred physical conditions to the observed C~I fine structure excitation (Songaila et al. 1994), we estimate the cosmic microwave background temperature to be T\rm CMB=(7.2±0.8)\:{\rm K} at z=1.77654, consistent with the predicted value of 7.566\:{\rm K} from the standard cosmology.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.