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

We analyse Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M⋆) based on the rest-frame K-band luminosities and constrain their unobscured star-formation rates (SFR) based on the rest-frame UV continuua. Further, we compute a mean M⋆/LKrest = 0.45M⊙/L⊙. We find that the hosts are low M⋆, star-forming systems. The median M⋆ in our sample (〈M⋆〉 = 10 M⊙) is lower than that of “field” galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M⋆ is 10 M⊙ < M⋆ < 10 M⊙, while the range spanned by the unobscured SFR is 10M⊙ yr −1 < SFR < 10M⊙ yr . There is no evidence for intrinsic evolution in the distribution of M⋆ 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 lower, UV-based, specific SFR (φ ≡ SFR/M⋆) with previous claims that GRBs have some of the highest φ values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on φ. Subject headings: cosmology: observations — dust, extinction — galaxies: fundamental parameters — galaxies: ISM — gamma rays: bursts — infrared: galaxies