AAS 205th Meeting, 9-13 January 2005
Session 145 Intergalactic Media
Poster, Thursday, January 13, 2005, 9:20am-4:00pm, Exhibit Hall

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[145.01] Molecular Hydrogen in the Damped Ly\alpha Absorber toward Q1331+170

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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