Recently, it has been suggested that the metallicity aversion of long-duration gamma-ray bursts (LGRBs) is not intrinsic to their formation, but rather a consequence of the anti-correlation between star-formation and metallicity seen in the general g
alaxy population. To investigate this proposal, we compare the metallicity of the hosts of LGRBs, broad-lined Type Ic (Ic-bl) supernovae (SNe), and Type II SNe to each other and to the metallicity distribution of star-forming galaxies using the SDSS to represent galaxies in the local universe and the TKRS for galaxies at intermediate redshifts. The differing metallicity distributions of the LGRB hosts and the star formation in local galaxies forces us to conclude that the low-metallicity preference of LGRBs is not primarily driven by the anti-correlation between star-formation and metallicity, but rather must be overwhelmingly due to the astrophysics of the LGRBs themselves. Three quarters of our LGRB sample are found at metallicities below 12+log(O/H) < 8.6, while less than a tenth of local star-formation is at similarly low metallicities. However, our SN samples are statistically consistent with the metallicity distribution of the general galaxy population. Using the TKRS population of galaxies, we are able to exclude the possibility that the LGRB host metallicity aversion is caused by the decrease in galaxy metallicity with redshift. The presence of the strong metallicity difference between LGRBs and Ic-bl SNe largely eliminates the possibility that the observed LGRB metallicity bias is a byproduct of a difference in the initial mass functions of the galaxy populations. Rather, metallicity below half-solar must be a fundamental component of the evolutionary process that separates LGRBs from the vast majority of Ic-bl SNe and from the bulk of local star-formation.
We present late-time Hubble Space Telescope imaging of the fields of six Swift GRBs lying at 5.0<z<9.5. Our data includes very deep observations of the field of the most distant spectroscopically confirmed burst, GRB 090423, at z=8.2. Using the preci
se positions afforded by their afterglows we can place stringent limits on the luminosities of their host galaxies. In one case, that of GRB 060522 at z=5.11, there is a marginal excess of flux close to the GRB position which may be a detection of a host at a magnitude J(AB)=28.5. None of the others are significantly detected meaning that all the hosts lie below Lstar at their respective redshifts, with star formation rates SFR<4Mo/yr in all cases. Indeed, stacking the five fields with WFC3-IR data we conclude a mean SFR<0.17Mo/yr per galaxy. These results support the proposition that the bulk of star formation, and hence integrated UV luminosity, at high redshifts arises in galaxies below the detection limits of deep-field observations. Making the reasonable assumption that GRB rate is proportional to UV luminosity at early times allows us to compare our limits with expectations based on galaxy luminosity functions derived from the Hubble Ultra-Deep Field (HUDF) and other deep fields. We infer that a luminosity function which is evolving rapidly towards steeper faint-end slope (alpha) and decreasing characteristic luminosity (Lstar), as suggested by some other studies, is consistent with our observations, whereas a non-evolving LF shape is ruled out at >90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy luminosity function, and the supply of ionizing photons in the early universe.
We detect the optical afterglow and host galaxy of GRB 070714B. Our observations of the afterglow show an initial plateau in the lightcurve for approximately the first 5 to 25 minutes, then steepening to a powerlaw decay with index alpha= 0.86 +/- 0.
10 for the period between 1 to 24 hours post burst. This is consistent with the X-ray light-curve which shows an initial plateau followed by a similar subsequent decay. At late time, we detect a host galaxy at the location of the optical transient. Gemini Nod & Shuffle spectroscopic observations of the host show a single emission line at 7167 angstroms which, based on a grizJHK photometric redshift, we conclude is the 3727 angstrom [O II] line. We therefore find a redshift of z=0.923. This redshift, as well as a subsequent probable spectroscopic redshift determination of GRB 070429B at z=0.904 by two other groups, significantly exceeds the previous highest spectroscopically confirmed short burst redshift of z=0.546 for GRB 051221. This dramatically moves back the time at which we know short bursts were being formed, and suggests that the present evidence for an old progenitor population may be observationally biased.
We present spectroscopy of the host of GRB 051022 with GMOS nod and shuffle on Gemini South and NIRSPEC on Keck II. We determine a metallicity for the host of log(O/H)+12 = 8.77 using the R23 method (Kobulnicky & Kewley 2004 scale) making this the hi
ghest metallicity long burst host yet observed. The galaxy itself is unusually luminous for a LGRB host with a rest frame B band absolute magnitude -21.5 and has the spectrum of a rapidly star-forming galaxy. Our work raises the question of whether other dark burst hosts will show high metallicities.
Gemini Nod & Shuffle spectroscopy on the host of the short GRB 070714B shows a single emission line at 7167 angstroms which, based on a grizJHK photometric redshift, we conclude is the 3727 angstrom [O II] line. This places the host at a redshift of
z=.923 exceeding the previous record for the highest spectroscopically confirmed short burst redshift of z=.546 held by GRB 051221. This dramatically moves back the time at which we know short bursts were being formed, and suggests that the present evidence for an old progenitor population may be observationally biased.
