Long Gamma-Ray Burst Host Galaxies and their Environments

In this book-chapter we first briefly discuss some basic observational issues related to what a GRB host galaxy is (whether they are operationally well defined as a class) and sample completeness. We then describe some of the early studies of GRB hosts starting with statistical studies of upper limits done prior to the first detections, the first host detection after the BeppoSAX breakthrough and leading up to the current Swift era. Finally, we discuss the status of efforts to construct a more complete sample of GRBs based on Swift and end with an outlook. We only consider the host galaxies of long-duration GRBs.Comment: 31 pages, 14 figures; Chapter 13 in "Gamma-Ray Bursts", eds. C. Kouveliotou, R. A. M. J. Wijers, S. E. Woosley, Cambridge University Press, 201

A study of purely astrometric selection of extragalactic point sources with Gaia

Selection of extragalactic point sources, e.g. QSOs, is often hampered by significant selection effects causing existing samples to have rather complex selection functions. We explore whether a purely astrometric selection of extragalactic point sources, e.g. QSOs, is feasible with the ongoing Gaia mission. Such a selection would be interesting as it would be unbiased in terms of colours of the targets and hence would allow selection also with colours in the stellar sequence. We have analyzed a total of 18 representative regions of the sky by using GUMS, the simulator prepared for ESAs Gaia mission, both in the range of $12\le G \le 20$ mag and $12\le G \le 18$ mag. For each region we determine the density of apparently stationary stellar sources, i.e. sources for which Gaia cannot measure a significant proper motion. The density is contrasted with the density of extragalactic point sources, e.g. QSOs, in order to establish in which celestial directions a pure astrometric selection is feasible. When targeting regions at galactic latitude $|b| \ge 30^\mathrm{o}$ the ratio of QSOs to apparently stationary stars is above 50\% and when observing towards the poles the fraction of QSOs goes up to about $\sim80$\%. We show that the proper motions from the proposed Gaia successor mission in about 20 years would dramatically improve these results at all latitudes. Detection of QSOs solely from zero proper motion, unbiased by any assumptions on spectra, might lead to the discovery of new types of QSOs or new classes of extragalactic point sources.Comment: 4 pages, 4 figures, sent in and accepted for publishing to A&

Consensus report on 25 years of searches for damped Lyα\alpha galaxies in emission: Confirming their metallicity-luminosity relation at z≳2z \gtrsim 2

Starting from a summary of detection statistics of our recent X-shooter campaign, we review the major surveys, both space and ground based, for emission counterparts of high-redshift damped Ly$\alpha$ absorbers (DLAs) carried out since the first detection 25 years ago. We show that the detection rates of all surveys are precisely reproduced by a simple model in which the metallicity and luminosity of the galaxy associated to the DLA follow a relation of the form, ${\rm M_{UV}} = -5 \times \left(\,[{\rm M/H}] + 0.3\, \right) - 20.8$, and the DLA cross-section follows a relation of the form $\sigma_{DLA} \propto L^{0.8}$. Specifically, our spectroscopic campaign consists of 11 DLAs preselected based on their equivalent width of SiII $\lambda1526$ to have a metallicity higher than [Si/H] > -1. The targets have been observed with the X-shooter spectrograph at the Very Large Telescope to search for emission lines around the quasars. We observe a high detection rate of 64% (7/11), significantly higher than the typical $\sim$10% for random, HI-selected DLA samples. We use the aforementioned model, to simulate the results of our survey together with a range of previous surveys: spectral stacking, direct imaging (using the `double DLA' technique), long-slit spectroscopy, and integral field spectroscopy. Based on our model results, we are able to reconcile all results. Some tension is observed between model and data when looking at predictions of Ly$\alpha$ emission for individual targets. However, the object to object variations are most likely a result of the significant scatter in the underlying scaling relations as well as uncertainties in the amount of dust which affects the emission.Comment: 25 pages (7 of which in appendix), accepted for publication in MNRA

GRBs as Probes of Massive Stars Near and Far

Long-duration gamma-ray bursts are the manifestations of massive stellar death. Due to the immense energy release they are detectable from most of the observable universe. In this way they allow us to study the deaths of single (or binary) massive stars possibly throughout the full timespan massive stars have existed in the Universe. GRBs provide a means to infer information about the environments and typical galaxies in which massive stars are formed. Two main obstacles remain to be crossed before the full potential of GRBs as probes of massive stars can be harvested: i) we need to build more complete and well understood samples in order not to be fooled by biases, and ii) we need to understand to which extent GRBs may be intrinsically biased in the sense that they are only formed by a limited subset of massive stars defined by most likely a restricted metallicity interval. We describe the status of an ongoing effort to build a more complete sample of long-duration GRBs with measured redshifts. Already now we can conclude that the environments of GRB progenitors are very diverse with metallicities ranging from solar to a hundredth solar and extinction ranging from none to A_V>5 mag. We have also identified a sightline with significant escape of Lyman continuum photons and another with a clear 2175AA extinction bump.Comment: Invited review - in "Massive Stars as Cosmic Engines", IAU Symp. 250 (Kauai), ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University Press), p. 443-456. Typos and refs correcte

The Optically Unbiased GRB Host (TOUGH) survey. IV. Lyman-alpha emitters

We report the results of a spectroscopic search for Lyman-alpha emission from gamma-ray burst host galaxies. Based on the well-defined TOUGH sample of 69 X-ray selected Swift GRBs, we have targeted the hosts of a subsample of 20 GRBs known from afterglow spectroscopy to be in the redshift range 1.8-4.5. We detect Lya emission from 7 out of the 20 hosts, with the typical limiting 3sigma line flux being 8E-18 erg/cm2/s, corresponding to a Lya luminosity of 6E41 erg/s at z=3. The Lya luminosities for the 7 hosts in which we detect Lya emission are in the range (0.6-2.3)E42 erg/s corresponding to star-formation rates of 0.6-2.1 Msun/yr (not corrected for extinction). The rest-frame Lya equivalent widths (EWs) for the 7 hosts are in the range 9-40A. For 6 of the 13 hosts for which Lya is not detected we place fairly strong 3sigma upper limits on the EW (<20A), while for others the EW is either unconstrained or has a less constraining upper limit. We find that the distribution of Lya EWs is inconsistent with being drawn from the Lya EW distribution of bright Lyman break galaxies at the 98.3% level, in the sense that the TOUGH hosts on average have larger EWs than bright LBGs. We can exclude an early indication, based on a smaller, heterogeneous sample of pre-Swift GRB hosts, that all GRB hosts are Lya emitters. We find that the TOUGH hosts on average have lower EWs than the pre-Swift GRB hosts, but the two samples are only inconsistent at the 92% level. The velocity centroid of the Lya line is redshifted by 200-700 km/s with respect to the systemic velocity, similar to what is seen for LBGs, possibly indicating star-formation driven outflows from the host galaxies. There seems to be a trend between the Lya EW and the optical to X-ray spectral index of the afterglow (beta_OX), hinting that dust plays a role in the observed strength and even presence of Lya emission. [ABRIDGED]Comment: ApJ accepted (v2: minor changes in the Subject headings and reference list

Reconciling the Metallicity Distributions of Gamma-ray Burst, Damped Lyman-alpha, and Lyman-break Galaxies at z=3

We test the hypothesis that the host galaxies of long-duration gamma-ray bursts (GRBs) as well as quasar-selected damped Lyman-alpha (DLA) systems are drawn from the population of UV-selected star-forming, high-z galaxies (generally referred to as Lyman-break galaxies). Specifically, we compare the metallicity distributions of the GRB and DLA populations to simple models where these galaxies are drawn randomly from the distribution of star-forming galaxies according to their star-formation rate and HI cross-section respectively. We find that it is possible to match both observational distributions assuming very simple and constrained relations between luminosity, metallicity and HI sizes. The simple model can be tested by observing the luminosity distribution of GRB host galaxies and by measuring the luminosity and impact parameters of DLA selected galaxies as a function of metallicity. Our results support the expectation that GRB and DLA samples, in contrast to magnitude limited surveys, provide an almost complete census of z=3 star-forming galaxies that are not heavily dust-obscured.Comment: 31 pages, 6 figures. Accepted for publication in Ap

On the mass-metallicity relation, velocity dispersion and gravitational well depth of GRB host galaxies

We analyze a sample of 16 absorption systems intrinsic to long duration GRB host galaxies at $z \gtrsim 2$ for which the metallicities are known. We compare the relation between the metallicity and cold gas velocity width for this sample to that of the QSO-DLAs, and find complete agreement. We then compare the redshift evolution of the mass-metallicity relation of our sample to that of QSO-DLAs and find that also GRB hosts favour a late onset of this evolution, around a redshift of $\approx 2.6$. We compute predicted stellar masses for the GRB host galaxies using the prescription determined from QSO-DLA samples and compare the measured stellar masses for the four hosts where stellar masses have been determined from SED fits. We find excellent agreement and conclude that, on basis of all available data and tests, long duration GRB-DLA hosts and intervening QSO-DLAs are consistent with being drawn from the same underlying population. GRB host galaxies and QSO-DLAs are found to have different impact parameter distributions and we briefly discuss how this may affect statistical samples. The impact parameter distribution has two effects. First any metallicity gradient will shift the measured metallicity away from the metallicity in the centre of the galaxy, second the path of the sightline through different parts of the potential well of the dark matter halo will cause different velocity fields to be sampled. We report evidence suggesting that this second effect may have been detected.Comment: 11 pages, 6 figures, 6 tables. Accepted for publication in MNRAS Main Journal. For the definitive version visit http://mnras.oxfordjournals.org