Molecular Hydrogen in the Damped Lyα Absorber toward Q1331+170

Details Molecular Hydrogen in the Damped Lyα Absorber toward Q1331+170 Molecular Hydrogen in the Damped Lyα Absorber toward Q1331+170

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aas...20514501c&link_type=abstract

American Astronomical Society Meeting 205, #145.01; Bulletin of the American Astronomical Society, Vol. 36, p.1589

Astronomy and Astrophysics

Astronomy

Scientific paper

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 H2 absorption was detected, with a total H2 column density of N(H2)=(4.45± 0.36)× 1019\:cm-2. The total molecular hydrogen fraction is f{ H2}=(2N_ H2)/(N_ HI+2N H2)=(5.6± 0.7)%, which is the largest 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 photo-absorption rate to be β 0=(7.6± 2.4)× 10-13\:s-1, corresponding to a local UV radiation field at the Lyman limit of j(912Å)≈ 5.0× 10-22\: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 [Ar/α ]=-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 H2 absorber, with a best-fit total hydrogen number density of n(H)≈ 0.2\:cm-3 and an electron temperature of T e≈ 140\:K. The observed CO-to-H2 column density ratio is (N CO)/(N H2)<2.5× 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 CMB=(7.2± 0.8)\:K at z=1.77654, consistent with the predicted value of 7.566\:K from the standard cosmology.

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