Missing Molecular Hydrogen and the Physical Conditions of GRB Host Galaxies

We examine the abundance of molecular hydrogen (H2) in the spectra of gamma ray burst afterglows (GRBs). In nearby galaxies, H2 traces the cold neutral medium (CNM) and dense molecular star-forming interstellar gas. Although H2 is detected in at least half of all sight lines toward hot stars in the Magellanic Clouds and in [?]25% of damped Lya systems toward quasars, it is not detected in any of the five GRB environments with a similar range of neutral hydrogen column density and metallicity. We detect no vibrationally excited H2 that would imply that the GRB itself has photodissociated its parent molecular cloud, so such models are ruled out unless the parent cloud was [?]4 pc in radius and was fully dissociated prior to the spectroscopic observations, or the star escaped its parent cloud during its main-sequence lifetime. The low molecular fractions for the GRBs are mysterious in light of their large column densities of neutral H and expectations based on local analogs, i.e., 30 Doradus in the LMC. This surprising lack of H2 in GRB damped Lya absorbers indicates that the destruction processes that suppress molecule formation in the LMC and SMC are more effective in the GRB hosts, most probably due to a combination of low metallicity and an FUV radiation field 10-100 times the Galactic mean field. These inferred conditions place strong constraints on the star-forming regions in these early galaxies.