arXiv subjects

D. Malesani

Publications and source records attributed to D. Malesani.

At least 73 records · Page 4Linked to original sources

arXiv2012

The metal-enriched host of an energetic gamma-ray burst at z ~ 1.6

(Abridged) Long gamma-ray burst (GRB) host galaxies might open a short-cut to the characteristics of typical star-forming galaxies throughout the history of the Universe. Due to the absence of near-infrared (NIR) spectroscopy, however, detailed investigations, specifically a determination of the gas-phase metallicity of gamma-ray burst hosts, was largely limited to redshifts z < 1 to date. Here, we report observations of the galaxy hosting GRB 080605 at z = 1.64 using optical/NIR spectroscopy and high-resolution HST/WFC3 imaging. We avail of VLT/X-shooter spectroscopy to measure the metallicity, electron density, star-formation rate (SFR), and reddening of the host. Specifically, we use different strong-line diagnostics based on [N II] to robustly measure the gas-phase metallicity for the first time for a GRB host at this redshift. The host of the energetic (E_iso ~ 2 x 10^53 erg) GRB 080605 is a morphologically complex, vigorously star-forming galaxy with an Hα-derived SFR of 31 M_sun/yr. Its ISM is significantly enriched with metals with an oxygen abundance between 8.3 and 8.6 depending on the adopted strong-line calibrator. This corresponds to values in the range of 0.4-0.8 times the solar value. For its measured stellar mass of 8 x 10^9 M_sun and SFR this value is consistent with the fundamental metallicity relation defined by star-forming field galaxies. Our observations directly illustrate that GRB hosts are not necessarily metal-poor, both on absolute scales as well as relative to their stellar mass and SFR. GRB hosts could thus be fair tracers of the population of ordinary star-forming galaxies as a whole at high redshift.

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arXiv2012

The distribution of equivalent widths in long GRB afterglow spectra

The extreme brightness of gamma-ray burst (GRB) afterglows and their simple spectral shape make them ideal beacons to study the interstellar medium of their host galaxies through absorption line spectroscopy. Using 69 low-resolution GRB afterglow spectra, we conduct a study of the rest-frame equivalent width (EW) distribution of features with an average rest-frame EW larger than 0.5 A. To compare an individual GRB with the sample, we develop EW diagrams as a graphical tool, and we give a catalogue with diagrams for the 69 spectra. We introduce a line strength parameter (LSP) that allows us to quantify the strength of the absorption features as compared to the sample by a single number. Using the distributions of EWs of single-species features, we derive the distribution of column densities by a curve of growth (CoG) fit. We find correlations between the LSP and the extinction of the GRB, the UV brightness of the host galaxies and the neutral hydrogen column density. However, we see no significant evolution of the LSP with the redshift. There is a weak correlation between the ionisation of the absorbers and the energy of the GRB, indicating that, either the GRB event is responsible for part of the ionisation, or that galaxies with high-ionisation media produce more energetic GRBs. Spectral features in GRB spectra are, on average, 2.5 times stronger than those seen in QSO intervening damped Lyman-alpha (DLA) systems and slightly more ionised. In particular we find larger excess in the EW of CIV1549 relative to QSO DLAs, which could be related to an excess of Wolf-Rayet stars in the environments of GRBs. From the CoG fitting we obtain an average number of components in the absorption features of GRBs of 6.00(-1.25,+1.00). The most extreme ionisation ratios in our sample are found for GRBs with low neutral hydrogen column density, which could be related to ionisation by the GRB emission.

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arXiv2012

The optically unbiased GRB host (TOUGH) survey. VI. Radio observations at z<1 and consistency with typical star-forming galaxies

The objective of this paper is to determine the level of obscured star formation activity and dust attenuation in a sample of gamma-ray burst (GRB) hosts; and to test the hypothesis that GRB hosts have properties consistent with those of the general star-forming galaxy populations. We present a radio continuum survey of all z<1 GRB hosts in The Optically Unbiased GRB Host (TOUGH) sample supplemented with radio data for all (mostly pre-Swift) GRB-SN hosts discovered before October 2006. We present new radio data for 22 objects and have obtained a detection for three of them (GRB 980425, 021211, 031203; none in the TOUGH sample), increasing the number of radio-detected GRB hosts from two to five. The star formation rate (SFR) for the GRB 021211 host of ~825 Mo yr^-1, the highest ever reported for a GRB host, places it in the category of ultraluminous infrared galaxies. We found that at least 63% of GRB hosts have SFR < 100 Mo yr^-1 and at most 8% can have SFR > 500 Mo yr^-1. For the undetected hosts the mean radio flux (<35 uJy 3sigma) corresponds to an average SFR < 15 Mo yr^-1. Moreover, ~88% of the z<1 GRB hosts have ultraviolet dust attenuation A_UV < 6.7 mag (visual attenuation A_V < 3 mag). Hence we did not find evidence for large dust obscuration in a majority of GRB hosts. Finally, we found that the distributions of SFRs and A_UV of GRB hosts are consistent with those of Lyman break galaxies, Halpha emitters at similar redshifts and of galaxies from cosmological simulations. The similarity of the GRB population with other star-forming galaxies is consistent with the hypothesis that GRBs, a least at z<1, trace a large fraction of all star formation, and are therefore less biased indicators than once thought.

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arXiv2012

SN 2006oz: rise of a super-luminous supernova observed by the SDSS-II SN Survey

We study SN 2006oz, a newly-recognized member of the class of H-poor, super-luminous supernovae. We present multi-color light curves from the SDSS-II SN Survey, that cover the rise time, as well as an optical spectrum that shows that the explosion occurred at z~0.376. We fitted black body functions to estimate the temperature and radius evolution of the photosphere and used the parametrized code SYNOW to model the spectrum. We constructed a bolometric light curve and compared it with explosion models. The very early light curves show a dip in the g- and r-bands and a possible initial cooling phase in the u-band before rising to maximum light. The bolometric light curve shows a precursor plateau with a duration of 6-10 days in the rest-frame. A lower limit of M_u < -21.5 can be placed on the absolute peak luminosity of the SN, while the rise time is constrained to be at least 29 days. During our observations, the emitting sphere doubled its radius to 2x10^15 cm, while the temperature remained hot at 15000 K. As for other similar SNe, the spectrum is best modeled with elements including O II and Mg II, while we tentatively suggest that Fe III might be present. We suggest that the precursor plateau might be related to a recombination wave in a circumstellar medium (CSM) and discuss whether this is a common property of all similar explosions. The subsequent rise can be equally well described by input from a magnetar or by ejecta-CSM interaction, but the models are not well constrained owing to the lack of post-maximum observations, and CSM interaction has difficulties accounting for the precursor plateau self-consistently. Radioactive decay is less likely to be the mechanism that powers the luminosity. The host galaxy, detected in deep imaging with the 10 m GTC, is a moderately young and star-forming, but not a starburst, galaxy. It has an absolute magnitude of M_g = -16.9.

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arXiv2012

The optically unbiased GRB host (TOUGH) survey. III. Redshift distribution

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 > 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.

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arXiv2012

A nearby GRB host prototype for z~7 Lyman-break galaxies: Spitzer-IRS and X-shooter spectroscopy of the host galaxy of GRB031203

Gamma-ray burst (GRB) host galaxies have been studied extensively in optical photometry and spectroscopy. Here we present the first mid-infrared spectrum of a GRB host, HG031203. It is one of the nearest GRB hosts at z=0.1055, allowing both low and high-resolution spectroscopy with Spitzer-IRS. Medium resolution UV-to-K-band spectroscopy with the X-shooter spectrograph on the VLT is also presented, along with Spitzer IRAC and MIPS photometry, as well as radio and sub-mm observations. These data allow us to construct a UV-to-radio spectral energy distribution with almost complete spectroscopic coverage from 0.3-35 micron of a GRB host galaxy for the first time, potentially valuable as a template for future model comparisons. The IRS spectra show strong, high-ionisation fine structure line emission indicative of a hard radiation field in the galaxy, suggestive of strong ongoing star-formation and a very young stellar population. The selection of HG031203 via the presence of a GRB suggests that it might be a useful analogue of very young star-forming galaxies in the early universe, and hints that local BCDs may be used as more reliable analogues of star-formation in the early universe than typical local starbursts. We look at the current debate on the ages of the dominant stellar populations in z~7 and z~8 galaxies in this context. The nebular line emission is so strong in HG031203, that at z~7, it can reproduce the spectral energy distributions of z-band dropout galaxies with elevated IRAC 3.6 and 4.5 micron fluxes without the need to invoke a 4000A break.

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arXiv2011

Diversity of multiwavelength emission bumps in the GRB 100219A afterglow

Context. Multi-wavelength observations of gamma-ray burst (GRB) afterglows provide important information about the activity of their central engines and their environments. In particular, the short timescale variability, such as bumps and/or rebrightening features visible in the multi-wavelength light curves, is still poorly understood. Aims. We analyze the multi-wavelength observations of the GRB100219A afterglow at redshift 4.7. In particular, we attempt to identify the physical origin of the late achromatic flares/bumps detected in the X-ray and optical bands. Methods. We present ground-based optical photometric data and Swift X-ray observations on GRB100219A. We analyzed the temporal behavior of the X-ray and optical light curves, as well as the X-ray spectra. Results. The early flares in the X-ray and optical light curves peak simultaneously at about 1000 s after the burst trigger, while late achromatic bumps in the X-ray and optical bands appear at about 20000 s after the burst trigger. These are uncommon features in the afterglow phenomenology. Considering the temporal and spectral properties, we argue that both optical and X-ray emissions come from the same mechanism. The late flares/bumps may be produced by late internal shocks from long-lasting activity of the central engine. An off-axis origin for a structured jet model is also discussed to interpret the bump shapes. The early optical bump can be interpreted as the afterglow onset, while the early X-ray flare could be caused by the internal activity. GRB 100219A exploded in a dense environment as revealed by the strong attenuation of X-ray emission and the optical-to-X-ray spectral energy distribution.

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arXiv2011

GRB091127/SN2009nz and the VLT/X-shooter spectroscopy of its host galaxy: probing the faint end of the mass-metallicity relation

We perform a detailed study of the gamma-ray burst GRB091127/SN2009nz host galaxy at z=0.490 using the VLT/X-shooter spectrograph in slit and integral-field unit (IFU). From the analysis of the optical and X-ray afterglow data obtained from ground-based telescopes and Swift-XRT we confirm the presence of a bump associated with SN2009nz and find evidence of a possible jet break in the afterglow lightcurve. The X-shooter afterglow spectra reveal several emission lines from the underlying host, from which we derive its integrated properties. These are in agreement with those of previously studied GRB-SN hosts and, more generally, with those of the long GRB host population. We use the Hubble Space Telescope and ground based images of the host to determine its stellar mass (M_star). Our results extend to lower M_star values the M-Z plot derived for the sample of long GRB hosts at 0.3<z<1.0 adding new information to probe the faint end of the M-Z relation and the shift of the LGRB host M-Z relation from that found from emission line galaxy surveys. Thanks to the IFU spectroscopy we can build the 2D velocity, velocity dispersion and star formation rate (SFR) maps. They show that the host galaxy has a perturbed rotation kinematics with evidence of a SFR enhancement consistent with the afterglow position.

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arXiv2011

A Photometric Redshift of z ~ 9.4 for GRB 090429B

Gamma-ray bursts (GRBs) serve as powerful probes of the early Universe, with their luminous afterglows revealing the locations and physical properties of star forming galaxies at the highest redshifts, and potentially locating first generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal to noise spectroscopy, or photometry. Here we present a photometric redshift of z~9.4 for the Swift-detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight-lines), the 90% likelihood range for the redshift is 9.06 < z < 9.52, although there is a low-probability tail to somewhat lower redshifts. Adopting Milky Way or Large Magellanic Cloud dust laws leads to very similar conclusions, while a Maiolino law does allow somewhat lower redshift solutions, but in all cases the most likely redshift is found to be z>7. The non-detection of the host galaxy to deep limits (Y_AB >~ 28 mag, which would correspond roughly to 0.001 L* at z=1) in our late time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs, and suggest that its progenitor is not greatly different to those of lower redshift bursts.

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arXiv2011

Spectroscopic evidence for SN 2010ma associated with GRB 101219B

We report on the spectroscopic detection of supernova SN 2010ma associated with the long gamma-ray burst GRB 101219B. We observed the optical counterpart of the GRB on three nights with the X-shooter spectrograph at the VLT. From weak absorption lines, we measure a redshift of z = 0.55. The first epoch UV--near-infrared afterglow spectrum, taken 11.6 hr after the burst, is well fit by a power law consistent with the slope of the X-ray spectrum. The second and third epoch spectra (obtained 16.4 and 36.7 days after the burst), however, display clear bumps closely resembling those of the broad-lined type-Ic SN 1998bw if placed at z=0.55. Apart from demonstrating that spectroscopic SN signatures can be observed for GRBs at these large distances, our discovery makes a step forward in establishing a general connection between GRBs and SNe. In fact, unlike most previous unambiguous GRB-associated SNe, GRB 101219B has a large gamma-ray energy (E_iso = 4.2 x 10^{51} erg), a bright afterglow, and obeys the "Amati" relation, thus being fully consistent with the cosmological population of GRBs.

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arXiv2011

The properties of SN Ib/c locations

Aims: To gain better insight on the physics of stripped-envelope core-collapse supernovae through studying their environments. Methods: We obtained low-resolution optical spectroscopy with the New Technology Telescope (+ EFOSC2) at the locations of 20 Type Ib/c supernovae. We measure the flux of emission lines in the stellar-continuum-subtracted spectra from which local metallicities are computed. For the supernova regions we estimate both the mean stellar age, interpreting the stellar absorption with population synthesis models, and the age of the youngest stellar populations using the H-alpha equivalent width as an age indicator. These estimates are compared with the lifetimes of single massive stars. Results: Based on our sample, we detect a tentative indication that Type Ic supernovae might explode in environments that are more metal-rich than those of Type Ib supernovae (average difference of 0.08 dex), but this is not a statistically significant result. The lower limits placed on the ages of the supernova birthplaces are overall young, although there are several cases where these appear older than what is expected for the evolution of single stars more massive than 25-30 M_{sun}. This is only true, however, assuming that the supernova progenitors were born during an instantaneous (not continuous) episode of star formation. Conclusions: These results do not conclusively favor any of the two evolutionary paths (single or binary) leading to stripped supernovae. We do note a fraction of events for which binary evolution is more likely, due to their associated age limits. The fact, however, that the supernova environments contain areas of recent (< 15 Myr) star formation and that the environmental metallicities are, at least, not against the single evolutionary scenario, suggest that this channel is also broadly consistent with the observations.

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arXiv2011

The Afterglows of Swift-era Gamma-Ray Bursts II.: Type I GRB versus Type II GRB Optical Afterglows

We use a large sample of GRB afterglow and prompt-emission data (adding further GRB afterglow observations in this work) to compare the optical afterglows (or the lack thereof) of Type I GRBs with those of Type II GRBs. In comparison to the afterglows of Type II GRBs, we find that those of Type I GRBs have a lower average luminosity and show an intrinsic spread of luminosities at least as wide. From late and deep upper limits on the optical transients, we establish limits on the maximum optical luminosity of any associated supernova, confirming older works and adding new results. We use deep upper limits on Type I GRB optical afterglows to constrain the parameter space of possible mini-SN emission associated with a compact-object merger. Using the prompt emission data, we search for correlations between the parameters of the prompt emission and the late optical afterglow luminosities. We find tentative correlations between the bolometric isotropic energy release and the optical afterglow luminosity at a fixed time after trigger (positive), and between the host offset and the luminosity (negative), but no significant correlation between the isotropic energy release and the duration of the GRBs. We also discuss three anomalous GRBs, GRB 060505, GRB 060614, and GRB 060121, in the light of their optical afterglow luminosities. (Abridged)

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arXiv2011

Variable Ly alpha sheds light on the environment surrounding GRB 090426

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionisation state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z=2.609, whose optical spectrum indicates an almost fully ionised medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Ly alpha absorption line. Photoionisation modeling shows that we are probing material from the vicinity of the burst (~80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies.

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arXiv2010

Probing GRB environments with time variability: ULTRASPEC fast imaging of GRB 080210

We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the burst and lasting for 26.45 min. The light curve is smooth on both short (down to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On top of the fireball power-law decay, bumps and wiggles at different time scales can, in principle, be produced by density fluctuations in the circumburst medium, substructures in the jet or by refreshed shocks. Comparing our constraints with variability limits derived from kinematic arguments, we exclude under-density fluctuations producing flux dips larger than 1 per cent with time scales \Deltat > 9.2 min (2 per cent on \Deltat > 2.3 min for many fluctuating regions). In addition, we study the afterglow VLT/FORS2 spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fit with a broken power law with slopes βopt = 0.71 \pm 0.01 and βX = 1.59 \pm 0.07, in disagreement with the fireball model, suggesting a non-standard afterglow for GRB 080210. We find AV = 0.18 \pm 0.03 mag optical extinction due to SMC-like dust and an excess X-ray absorption of log (NH/cm-2) = 21.58 +0.18 -0.26 assuming Solar abundances. The spectral analysis reveals a damped Lyα absorber (log (NH I /cm-2) = 21.90 \pm 0.10) with a low metallicity ([X/H] = -1.21 \pm 0.16), likely associated with the interstellar medium of the GRB host galaxy (z = 2.641).

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arXiv2010

GRB 050502B optical afterglow: a jet break at high redshift

Aims: GRB 050502B is well known for the very bright flare displayed in its X-ray light curve. Despite extensive studies, however, the optical light curve has never been discussed and its redshift is unconstrained. Possible correlations between optical and X-ray data are analysed. Methods: Photometric data from TNG in the R and I bands were used to compare the optical afterglow with the X-ray light curve. The HyperZ package and a late time VLT host observation were used to derive redshift estimates. Results: The I-band afterglow decay followed a power-law of index α = 2.1 $\pm$ 0.6, after a late break at ~ $1.3 \times 10^5$ s. The R - I color is remarkably red and the broadband spectral index β_OX = $0.9 \pm 0.1$ is consistent with the X-ray spectral slope β_X. Although a photometric redshift of z > 4 is the most conservative result to consider, a photometric redshift of z = 5.2 $\pm$ 0.3 is suggested with no extinction in the host, based on which an isotropic energy E_γ,iso = $(3.8 \pm 0.7) \times 10^52$ erg and a jet opening angle θ ~ $3.7^{\circ}$ are subsequently derived. Conclusions: The combined X-ray and optical data suggest an achromatic break, which we interpret as a jet break. The post jet break slope obeys roughly the closure relation for the jet slow cooling model. Because of the afterglow's very red color, in order for the redshift to be low (z < 1), extinction, if present in the host, must be significantly high. Since the optical-to-X-ray index is consistent with the X-ray spectrum, and there is no XRT evidence for excess N_H, GRB 050502B was likely at high redshift.

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arXiv2010

Discovery of the nearby long, soft GRB 100316D with an associated supernova

We report the Swift discovery of nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low redshift host galaxy and associated supernova. We derive the redshift of the event to be z = 0.0591 +/- 0.0001 and provide accurate astrometry for the GRB-SN. We study the extremely unusual prompt emission with time-resolved gamma-ray to X-ray spectroscopy, and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini South, VLT and HST observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB--SN connection can be gleaned.

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arXiv2010

Do Wolf-Rayet stars have similar locations in hosts as type Ib/c supernovae and long gamma-ray bursts?

Aims: We study the distribution of Wolf-Rayet (WR) stars and their subtypes with respect to their host galaxy light distribution. We thus want to investigate whether WR stars are potential progenitors of stripped-envelope core-collapse supernovae (SNe) and/or long-duration gamma-ray bursts (LGRBs). Methods: We derived the relative surface brightness (fractional flux) at the locations of WR stars and compared with similar results for LGRBs and SNe. We examined two nearby galaxies, M 83 and NGC 1313, for which a comprehensive study of the WR population exists. These two galaxies contain a sufficiently large number of WR stars and sample different metallicities. To enable the comparison, the images of the galaxies were processed to make them appear as they would look at a higher redshift. The robustness of our results against several sources of uncertainty was investigated with the aid of Monte Carlo simulations. Results: We find that the WC star distribution favours brighter pixels than the WN star population. WC stars are more likely drawn from the same distribution as SNe Ic than from other SN distributions, while WN stars show a higher degree of association with SNe Ib. It can also not be excluded that WR (especially WC) stars are related to LGRBs. Some differences between the two galaxies do exist, especially in the subtype distributions, and may stem from differences in metallicity. Conclusions: Although a conclusive answer is not possible, the expectation that WR stars are the progenitors of SNe Ib/c and LGRBs survives this test. The trend observed between the distributions of WN and WC stars, as compared to those of SNe Ib and Ic, is consistent with the theoretical picture that SNe Ic result from progenitors that have been stripped of a larger part of their envelope.

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arXiv2010

On the distribution of stellar masses in gamma-ray burst host galaxies

We analyse Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M*) based on the rest-frame K-band luminosities (L_Krest) and constrain their star formation rates (SFRs, not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M*/L_Krest = 0.45 Msun/Lsun. We find that the hosts are low M*, star-forming systems. The median M* in our sample ( = 10^9.7 Msun) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M* is 10^7 Msun < M* < 10^11 Msun, while the range spanned by the dust-uncorrected UV SFR is 10^-2 Msun yr^-1 < SFR < 10 Msun yr^-1. There is no evidence for intrinsic evolution in the distribution of M* with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR (PHI = SFR/M*) with previous claims that GRBs have some of the highest PHI values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on PHI.

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