2010ApJ...721.1919C

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_{Krest}}) 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_☉/L_☉. We find that the hosts are low M_{sstarf}, star-forming systems. The median M_{sstarf} in our sample (〈M_{sstarf}〉 = 10^9.7 M_☉) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M_{sstarf} is 10⁷ M_☉< M_{sstarf}< 10¹¹ M_☉, while the range spanned by the dust-uncorrected UV SFR is 10^–2 M_☉/yr < SFR < 10 M_☉/yr. 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 (φ ≡ SFR/M _{sstarf}) 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.