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Register a new userAre LGRBs biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs. II: star formation rates and metallicities at z < 1
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Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars and could thus be a potentially powerful tool to trace cosmic star formation. However, especially at low redshifts (z < 1.5) LGRBs seem to prefer particular types of environment. Our aim is to study the host galaxies of a complete sample of bright LGRBs to investigate the impact of the environment on GRB formation. We study host galaxy spectra of the Swift/BAT6 complete sample of 14 z < 1 bright LGRBs. We use the detected nebular emission lines to measure the dust extinction, star formation rate (SFR) and nebular metallicity (Z) of the hosts and supplement the data set with previously measured stellar masses M$_{star}$. The distributions of the obtained properties and their interrelations (e.g. mass-metallicity and SFR-M$_{star}$ relations) are compared to samples of field star-forming galaxies.We find that LGRB hosts at z < 1 have on average lower SFRs than if they were direct star-formation tracers. By directly comparing metallicity distributions of LGRB hosts and star-forming galaxies, we find a good match between the two populations up to Z $sim 8.4-8.5$, after which the paucity of metal-rich LGRB hosts becomes apparent. The LGRB host galaxies of our complete sample are not inconsistent with the mass-metallicity relation at similar mean redshift and stellar masses. The cutoff against high metallicities (and high masses) can explain the low SFR values of LGRB hosts. We find a hint of increased incidence of starburst galaxies in the Swift/BAT6 z < 1 sample with respect to that of a field star-forming population. Given that the SFRs are low on average, the latter is ascribed to low stellar masses. Nevertheless the limits on the completeness and metallicity availability of current surveys, coupled with the limited number of LGRB host galaxies, prevent us from reaching more quantitative conclusions.
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(Abridged) Long gamma-ray bursts (LGRB) have been suggested as promising tracers of star formation owing to their association with the core-collapse of massive stars. The goal of this work is to characterise the population of host galaxies of LGRBs at 1 < z < 2, investigate the conditions in which LGRBs form at these redshifts and assess their use as tracers of star formation. We perform a spectro-photometric analysis to determine the stellar mass, star formation rate, specific star formation rate and metallicity of the complete, unbiased host galaxy sample of the Swift/BAT6 LGRB sample at 1 < z < 2. We compare the distribution of these properties to the ones of typical star-forming galaxies from the MOSDEF and COSMOS2015 Ultra Deep surveys, within the same redshift range. We find that, similarly to z < 1, LGRBs do not directly trace star formation at 1 < z < 2, and they tend to avoid high-mass, high-metallicity host galaxies. We also find evidence for an enhanced fraction of starbursts among the LGRB host sample with respect to the star-forming population of galaxies. Nonetheless we demonstrate that the driving factor ruling the LGRB efficiency is metallicity. The LGRB host distributions can be reconciled with the ones expected from galaxy surveys by imposing a metallicity upper limit of 12+logOH ~ 8.55. Metallicity rules the LGRB production efficiency, which is stifled at Z > 0.7 Zsun. Under this hypothesis we can expect LGRBs to trace star formation at z > 3, once the bulk of the star forming galaxy population are characterised by metallicities below this limit. The moderately high metallicity threshold found is in agreement with the conditions necessary to rapidly produce a fast-rotating Wolf-Rayet star a in close binary system, and could be accommodated by single star models under chemically homogeneous mixing with very rapid rotation and weak magnetic coupling.
LGRBs are associated with massive stars and are therefore linked to star formation. The conditions necessary to produce LGRBs can affect the relation between the LGRB rate and star formation. By using the power of a complete LGRB sample, our aim is to understand whether such a bias exists and, if it does, what is its origin. In this first paper, we build the SED of the z<1 host galaxies of the BAT6 LGRB sample, and determine their stellar masses from SED fitting. We compare the resulting stellar mass distribution (i) with star-forming galaxies observed in deep surveys (UltraVISTA); (ii) with semi-analitical models of the z<1 star forming galaxy population and (iii) with numerical simulations of LGRB hosts having different metallicity thresholds for the progenitor star environment. We find that at z<1 LGRBs tend to avoid massive galaxies and are powerful in selecting faint low-mass star-forming galaxies. The stellar mass distribution of the hosts is not consistent with that of the UltraVISTA star-forming galaxies weighted for their SFR. This implies that, at least at z<1, LGRBs are not unbiased tracers of star formation. To make the two distributions consistent, a much steeper faint-end of the mass function would be required, or a very shallow SFR-Mass relation for the low mass galaxy population. GRB host galaxy simulations indicates that, to reproduce the stellar mass distribution, a metallicity threshold of the order of Z_th=0.3-0.5Z_sun is necessary. Models without a metallicity threshold or with an extreme threshold of Z_th = 0.1Z_sun are excluded at z<1. The use of the BAT6 complete sample makes this result not affected by possible biases which could have influenced past results based on incomplete samples. The preference for low metallicities (Z<~0.5Z_sun) can be a consequence of the particular conditions needed for the progenitor star to produce a GRB. (Abridged)
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 galaxy 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 the results of a photometric redshift analysis designed to identify z>6 galaxies from the near-IR HST imaging in three deep fields (HUDF, HUDF09-2 & ERS). By adopting a rigorous set of criteria for rejecting low-z interlopers, and by employing a deconfusion technique to allow the available IRAC imaging to be included in the candidate selection process, we have derived a robust sample of 70 Lyman-break galaxies (LBGs) spanning the redshift range 6.0<z<8.7. Based on our final sample we investigate the distribution of UV spectral slopes (beta), finding a variance-weighted mean value of <beta>=-2.05 +/- 0.09 which, contrary to some previous results, is not significantly bluer than displayed by lower-redshift starburst galaxies. We confirm the correlation between UV luminosity and stellar mass reported elsewhere, but based on fitting galaxy templates featuring a range of star-formation histories, metallicities and reddening we find that, at z>=6, the range in mass-to-light ratio (M*/L_UV) at a given UV luminosity could span a factor of ~50. Focusing on a sub-sample of twenty-one candidates with IRAC detections at 3.6-microns we find that L* LBGs at z~6.5 have a median stellar mass of M* = (2.1 +/- 1.1) x 10^9 Msun and a median specific star-formation rate of 1.9 +/- 0.8 Gyr^-1. Using the same sub-sample we have investigated the influence of nebular continuum and line emission, finding that for the majority of candidates (16 out of 21) the best-fitting stellar-mass estimates are reduced by less than a factor of 2.5. Finally, a detailed comparison of our final sample with the results of previous studies suggests that, at faint magnitudes, several high-redshift galaxy samples in the literature are significantly contaminated by low-redshift interlopers (abridged).
The assembly of galaxies can be described by the distribution of their star formation as a function of cosmic time. Thanks to the WFC3 grism on HST it is now possible to measure this beyond the local Universe. Here we present the spatial distribution of Halpha emission for a sample of 54 strongly star-forming galaxies at z~1 in the 3D-HST Treasury survey. By stacking the Halpha emission we find that star formation occurred in approximately exponential distributions at z~1, with median Sersic index of n=1.0+-0.2. The stacks are elongated with median axis ratios of b/a=0.58+-0.09 in Halpha, consistent with (possibly thick) disks at random orientation angles. Keck spectra obtained for a subset of eight of the galaxies show clear evidence for rotation, with inclination-corrected velocities of 90 to 330 km/s. The most straightforward interpretation of our results is that star formation in strongly star-forming galaxies at z~1 generally occurred in disks. The disks appear to be scaled-up
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