We present rest-frame NIR luminosities and stellar masses for a large and uniformly-selected population of GRB host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ∼ 0.5 and z ∼ 1.5, but little variation between z ∼ 1.5 and z ∼ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high-redshift while low-mass star-forming galaxies retain little dust in their ISM. Comparing our luminosity distributions to field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star-formation is constant in galaxies with gas-phase metallicity below approximately the Solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously-reported "excess" in the GRB rate beyond z 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star-formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.

The binary neutron star merger GW170817 was the first multi-messenger event observed in both gravitational and electromagnetic waves. 1,2 The electromagnetic signal began ∼ 2 seconds post-merger with a weak, short burst of gamma-rays, 3 which was followed over the next hours and days by the ultraviolet, optical and near-infrared emission from a radioactivelypowered kilonova. [4][5][6][7][8][9][10][11] Later, non-thermal rising X-ray and radio emission was observed. 12,13 The low luminosity of the gamma-rays and the rising non-thermal flux from the source at late times could indicate that we are outside the opening angle of the beamed relativistic jet. Alternatively, the emission could be arising from a cocoon of material formed from the interaction between a jet and the merger ejecta. [13][14][15] Here we present late-time optical detections and deep near-infrared limits on the emission from GW170817 at 110 days post-merger. Our new observations are at odds with expectations of late-time emission from kilonova models, being too bright and blue. 16,17 Instead, the emission arises from the interaction between the relativistic ejecta of GW170817 and the interstellar medium. We show that this emission matches the expectations of a Gaussian structured relativistic jet, which would have launched a high luminosity short GRB to an aligned observer. However, other jet structure or cocoon models can also match current data -the future evolution of the afterglow will directly distinguish the origin of the emission.For the Hubble Space Telescope (HST), the end of Sun constraint for GW170817 was on 6 December 2017 (∼ 110 rest-frame days post-merger), and we immediately obtained deep observations in the optical and infrared (see Table 1 and Methods for details of the observations and reduction). The new images were astrometrically aligned to our earlier epoch HST data in order to accurately locate the merger site and perform photometry (see Methods). Images of the merger site in each of our filters are shown in Figure 1. We detect emission at the location of the merger in the optical F606W and F814W filters (central wavelengths, λ cen ∼ 589, 802 nm, respectively). For the near-IR filters F140W and F160W (λ cen ∼ 1392, 1527 nm, respectively) we could not establish significant detections and so can place only upper limits on the transient flux at these wavelengths. Optical and near-infrared light curves for the counterpart to GW170817, including our recent observations, are shown in Figure 2.A detection in the optical or near-IR at such late times is not expected from the family of kilonova models currently in use. Indeed, most detailed studies stop at ∼ 30 days where predicted luminosities correspond to 30 mag, 16, 18 undetectable for even HST. Alternative models of kilonova emission with a slower decay of the light curves 17 would nevertheless predict redder emission than we observe. Initially blue, with M r,AB − M H,AB 0.4 mag at 1.5 days 19 , GW170817 evolved to become very red, with M F606W,AB − M F160W,AB = 2.8 mag at 11 d...

We present observations of the afterglows and host galaxies of three short-duration gamma-ray bursts (GRBs): 100625A, 101219A and 110112A. We find that GRB 100625A occurred in a z = 0.452 early-type galaxy with a stellar mass of ≈ 4.6×10 9 M ⊙ and a stellar population age of ≈ 0.7 Gyr, and GRB 101219A originated in a starforming galaxy at z = 0.718 with a stellar mass of ≈ 1.4 × 10 9 M ⊙ , a star formation rate of ≈ 16 M ⊙ yr −1 , and a stellar population age of ≈ 50 Myr. We also report the discovery of the optical afterglow of GRB 110112A, which lacks a coincident host galaxy to i 26 mag and we cannot conclusively identify any field galaxy as a possible host. From afterglow modeling, the bursts have inferred circumburst densities of ≈ 10 −4 − 1 cm −3 , and isotropic-equivalent gamma-ray and kinetic energies of ≈ 10 50 − 10 51 erg. These three events highlight the diversity of galactic environments that host short GRBs. To quantify this diversity, we use the sample of 36 Swift short GRBs with robust associations to an environment (∼ 1/2 of 68 short bursts detected by Swift to May 2012) and classify bursts originating from four types of environments: late-type (≈ 50%), early-type (≈ 15%), inconclusive (≈ 20%), and "host-less" (lacking a coincident host galaxy to limits of 26 mag; ≈ 15%). To find likely ranges for the true late-and early-type fractions, we assign each of the host-less bursts to either the late-or early-type category using probabilistic arguments, and consider the scenario that all hosts in the inconclusive category are early-type galaxies to set an upper bound on the early-type fraction. We calculate most likely ranges for the late-and early-type fractions of ≈ 60 − 80% and ≈ 20 − 40%, respectively. We find no clear trend between gamma-ray duration and host type. We also find no change to the fractions when excluding events recently claimed as possible contaminants from the long GRB/collapsar population. Our reported demographics are consistent with a short GRB rate driven by both stellar mass and star formation.

Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 1050ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.