We used HST/STIS to obtain the spectrum of molecular hydrogen associated with the damped Ly \alpha system at z _ { abs } = 1.7765 toward the quasar Q1331+170 at z _ { em } = 2.084 .
Strong { H } _ { 2 } absorption was detected , with a total { H } _ { 2 } column density of N ( { H } _ { 2 } ) = ( 4.45 \pm 0.36 ) \times 10 ^ { 19 } \ > { cm ^ { -2 } } .
The molecular hydrogen fraction is f _ { { H } _ { 2 } } = \frac { 2 N _ { H _ { 2 } } } { N _ { HI } +2 N _ { H _ { 2 } } } = ( 5.6 \pm 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 photo-absorption rate to be R _ { abs } = ( 7.6 \pm 2.4 ) \times 10 ^ { -13 } \ > { s ^ { -1 } } , corresponding to a local UV radiation field of J ( 1000 { \AA } ) \approx 2.1 \times 10 ^ { -3 } J _ { 1000 { \AA } , \odot } , where J _ { 1000 { \AA } , \odot } is the UV intensity at 1000 \ > \AA in the solar neighborhood .
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 .
We construct a simple model to describe the structure of the { H } _ { 2 } absorber , with a best-fit total hydrogen number density of n ( { H } ) \approx 0.2 \ > { cm ^ { -3 } } and an electron temperature of T _ { e } \approx 140 \ > { K } .
Assuming spherical symmetry , the mass of the { H } _ { 2 } cloud is estimated to be \approx 6.5 \times 10 ^ { 7 } \ > { M _ { \odot } } , larger than the masses of most giant molecular clouds ( GMCs ) in the Milky Way and nearby galaxies .
The extinction of Q1331+170 due to the intervening DLA is E _ { B - V } \approx 0.037 , and we also find that the extinction by DLAs with firm { H } _ { 2 } detections is significantly greater than those for which only upper limits of f _ { H _ { 2 } } have been put .
The observed CO-to-H _ { 2 } column density ratio is \frac { N _ { CO } } { N _ { H _ { 2 } } } < 2.5 \times 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 ( Songaila et al . 1994 ) , we estimate the cosmic microwave background temperature to be T _ { CMB } = ( 7.2 \pm 0.8 ) \ > { K } at z = 1.77654 , consistent with the predicted value of 7.566 \ > { K } from the standard cosmology .