On the Distribution of Stellar Masses in Gamma-ray Burst Host Galaxies

We analyze Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M sstarf) based on the rest-frame K-band luminosities (L_{K_{rest}}) and constrain their star formation rates (SFRs; not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M_⋆ /L_{K_{rest}} = 0.45 M sun/L sun. We find that the hosts are low M sstarf, star-forming systems. The median M sstarf in our sample (langM sstarfrang = 109.7 M sun) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M sstarf is 107 M sun <M sstarf <1011 M sun, while the range spanned by the dust-uncorrected UV SFR is 10-2 M sun yr-1 <SFR <10 M sun yr-1. There is no evidence for intrinsic evolution in the distribution of M sstarf with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR (phi ≡ SFR/M sstarf) with previous claims that GRBs have some of the highest phi values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on phi. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.