The Host Galaxies of Dust-Obscured Gamma-Ray Bursts Daniel A. Perley (Caltech), A.J. Levan (Warwick), N.R. Tanvir (Leicester), S.B. Cenko, J. Bloom (UCB), J. Hjorth, D. Malesani (DARK) “Normal” galaxies (LIR < 1011 Lo) ULIRGs (LI R > 1012 Lo) 100% 0% Redshift LIRGs (1011 Lo < LI R < 1012 Lo) Modified from Perez-Gonzales et al. 2005 1 Contribution to total SFR 2 A significant fraction of high-redshift star formation is dust-obscured (Figure 1), a fact that should also be manifest in the properties of the GRB host sample if GRBs follow the cosmic star-formation rate. Yet, few dusty galaxies have been detected in pre-Swift host surveys1. GRBs from dusty galaxies should be “dark” or at least significantly reddened, since the dust that obscured the stars should also obscure the afterglow light. The lack of dusty host galaxies in pre-Swift samples may therefore reflect only the small number of well-localized dark GRBs at that time. (model assuming α=-1.7) Figure 1 – The sites of star-formation in the Universe as inferred by Spitzer. Highly IR-luminous galaxies become a predominant source of star formationat z>1. Traditional definitions of dark bursts (e.g. βox) are not ideal, since at high-z fairly mild extinction columns can extinguish significantly, while high-luminosity afterglows can occasionally be optically detected even through a thick dust screen. To isolate the most dust-obscured events, we calculate the minimum Av permitted by afterglow observations given a range of allowed redshifts and intrinsic synchrotron afterglow parameters. We have observed these events across the spectrum (optical, near-IR, mid-IR, submm, radio) to characterize their properties. Figure 2 (above) – IR-to-X-ray SEDs of several extremely extinguished afterglows, normalized to the XRT 1keV flux. Figure 3 (below) – Colors of dark and non-dark GRB host galaxies compared to other galaxy populations. Dark hosts are significantly redder, dustier and more massive than the hosts of their optically-bright counterparts. In optical-NIR observations. For example, very dark GRB host in our sample is detected by Spitzer at 4.5μm, indicating significant stellar mass (compared to optically selected Spitzer samples, which rarely detect hosts.) However: Blue hosts are also present, Including the hosts of many of the most unambiguously unobscured Swift GRBs (e.g. 061222A, 100621A). Evidently, highly-obscured sightlines are common even in relatively low-mass (<109 Mo) galaxies. However, submm/radio luminosities are “modest”. EVLA and CSO observation have not yet detected synchrotron or thermal emission from any source (with the possible exception of GRB 080207), limiting the star formation rate generally to a few hundred solar masses per year. Even among the darkest Swift GRBs, few events seem to be occuring in the most spectacular high-z SMGs and ULIRGs. Figure 4 (above) – Stellar masses for dark and non-dark GRB hosts. Curves indicate the derived mass as a function of z for events with unknown redshift. Redshift z Radio star-formation rate (Mo/yr) Figure 5 (left) – Limiting star-formation rates inferred from deep EVLA nondetections of the hosts of seven highly dust-extinguished GRBs. One event is detected at ~4σ.