We present ultraviolet through near-infrared photometry and spectroscopy of the host galaxies of all superluminous supernovae (SLSNe) discovered by the Palomar Transient Factory prior to 2013 and derive measurements of their luminosities, star formation rates, stellar masses, and gas-phase metallicities. We find that TypeI (hydrogen-poor) SLSNe (SLSNe I) are found almost exclusively in low-mass ( * <´ M M 2 10 9) and metal-poor (12+log 10 [O/ H] < 8.4) galaxies. We compare the mass and metallicity distributions of our sample to nearby galaxy catalogs in detail and conclude that the rate of SLSNe I as a fraction of all SNe is heavily suppressed in galaxies with metallicities   Z 0.5 . Extremely low metallicities are not required and indeed provide no further increase in the relative SLSN rate. Several SLSN I hosts are undergoing vigorous starbursts, but this may simply be a side effect of metallicity dependence: dwarf galaxies tend to have bursty star formation histories. TypeII (hydrogen-rich) SLSNe (SLSNe II) are found over the entire range of galaxy masses and metallicities, and their integrated properties do not suggest a strong preference for (or against) low-mass/low-metallicity galaxies. Two hosts exhibit unusual properties: PTF 10uhf is an SLSN I in a massive, luminous infrared galaxy at redshift z=0.29, while PTF 10tpz is an SLSN II located in the nucleus of an early-type host at z=0.04.

We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 < z < 3.6, the largest sample of GRB host spectra available to date. Most of our GRBs were detected by Swift and 76% are at 0.5 < z < 2.5 with a median z med ∼ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (A V ), oxygen abundances (12 + log(O/H)), and emission-line widths (σ). We study GRB hosts up to z ∼ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFR med ∼ 0.6 M yr −1 atO ] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in A V and σ, with the highest values seen at z ∼ 1.5 (A V ) or z ∼ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log(O/H) = 7.9 and 12 + log(O/H) = 9.0 with a median 12 + log(O/H) med ∼ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ∼20% at z < 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ∼ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z 0.5 Z . Together with a high incidence of Z ∼ 0.5 Z galaxies at z ∼ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ∼ 3. Significant correlations exist between the hosts' physical properties. Oxygen abundance, for example, relates to A V (12 + log(O/H) ∝ 0.17 · A V ), line width (12 + log(O/H) ∝ σ 0.6 ), and SFR (12 + log(O/H) ∝ SFR 0.2 ). In the last two cases, the normalization of the relations shift to lower metallicities at z > 2 by ∼0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts' properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments.

We present 10 new gamma-ray burst (GRB) redshifts and another five redshift limits based on host galaxy spectroscopy obtained as part of a large program conducted at the Very Large Telescope (VLT). The redshifts span the range 0.345 ≤ z 2.54. Three of our measurements revise incorrect values from the literature. The homogeneous host sample researched here consists of 69 hosts that originally had a redshift completeness of 55% (with 38 out of 69 hosts having redshifts considered secure). Our project, including VLT/X-shooter observations reported elsewhere, increases this fraction to 77% (53/69), making the survey the most comprehensive in terms of redshift completeness of any sample to the full Swift depth, analyzed to date. We present the cumulative redshift distribution and derive a conservative, yet small, associated uncertainty. We constrain the fraction of Swift GRBs at high redshift to a maximum of 14% (5%) for z > 6 (z > 7). The mean redshift of the host sample is assessed to be z 2.2, with the 10 new redshifts reducing it significantly. Using this more complete sample, we confirm previous findings that the GRB rate at high redshift (z 3) appears to be in excess of predictions based on assumptions that it should follow conventional determinations of the star formation history of the universe, combined with an estimate of its likely metallicity dependence. This suggests that either star formation at high redshifts has been significantly underestimated, for example due to a dominant contribution from faint, undetected galaxies, or that GRB production is enhanced in the conditions of early star formation, beyond that usually ascribed to lower metallicity.

Context. Gamma-ray burst (GRB) afterglows probe sightlines to star-forming regions in distant star-forming galaxies. Here we present a study of the peculiar afterglow spectrum of the z = 0.889 Swift GRB 140506A. Aims. Our aim is to understand the origin of the very unusual properties of the absorption along the line of sight. Methods. We analyse spectroscopic observations obtained with the X-shooter spectrograph mounted on the ESO/VLT at two epochs 8.8 h and 33 h after the burst, and with imaging from the GROND instrument. We also present imaging and spectroscopy of the host galaxy obtained with the Magellan telescope. Results. The underlying afterglow appears to be a typical afterglow of a long-duration GRB. However, the material along the line of sight has imprinted very unusual features on the spectrum. First, there is a very broad and strong flux drop below 8000 Å (∼4000 Å in the rest frame), which seems to be variable between the two spectroscopic epochs. We can reproduce the flux-drops both as a giant 2175 Å extinction bump and as an effect of multiple scattering on dust grains in a dense environment. Second, we detect absorption lines from excited H i and He i. We also detect molecular absorption from CH + . Conclusions. We interpret the unusual properties of these spectra as reflecting the presence of three distinct regions along the line of sight: the excited He i absorption originates from an H ii-region, whereas the Balmer absorption must originate from an associated photodissociation region.The strong metal line and molecular absorption and the dust extinction must originate from a third, cooler region along the line of sight. The presence of at least three separate regions is reflected in the fact that the different absorption components have different velocities relative to the systemic redshift of the host galaxy.