Abstract

We search for the signature of molecular hydrogen in damped Lyman-? systems at high redshift. In the course of this survey, we constructed a representative sample of 18 DLA/sub-DLA systems with log N(H I) > 19.5 at high redshift (zabs > 1.8) and with metallicities relative to solar [X/H] > -1.3 (with [X/H] = log N(X)/N(H) log(X/H)? and X either Zn, S or Si). We show that the presence of molecular hydrogen at high redshift is strongly correlated with metallicity. The comparison of H2 transition wavelengths observed at high redshift to those measured in the laboratory can be used to constrain the variation with time of the proton-to-electron mass ratio, ?. Using the two best cases, we obtain the most stringent limit on the variation of ? over the last 12 Gyrs ever obtained, ??/? = 1.65±0.7×10-5. © 2008 Springer-Verlag Berlin Heidelberg.