Fynbo, J.

Gorosabel, J.

Hjorth, J.

Jakobsson, P.

Jaunsen, A.O.

English

University of Hertfordshire Research Archive

arXiv:astro-ph/0611561v1  17 Nov 2006February 5, 2008 14:29 WSPC - Proceedings Trim Size: 9.75in x 6.5in main1FUNDAMENTAL PROPERTIES OF GRB-SELECTED GALAXIES:A SWIFT/VLT LEGACY SURVEYPA´LL JAKOBSSONCentre for Astrophysics Research, University of Hertfordshire, College Lane,Hatfield, Herts, AL10 9AB, UKJENS HJORTH and JOHAN P. U. FYNBODark Cosmology Centre, Niels Bohr Institute, University of Copenhagen,Juliane Maries Vej 30, 2100 Copenhagen, DenmarkJAVIER GOROSABELInstituto de Astrof´ısica de Andaluc´ıa (CSIC), Apartado de Correos 3004, 18080 Granada, SpainANDREAS O. JAUNSENInstitute of Theoretical Astrophysics, PO Box 1029, 0315 Oslo, NorwayWe present the motivation, aims and preliminary result from the Swift/VLT legacysurvey on gamma-ray burst host galaxies. This survey will produce a homogeneous andwell-understood host sample covering more than 95% of the lookback time to the BigBang, and allow us to characterize their fundamental properties.1. IntroductionWith a very broad redshift distribution and a mean redshift of around z = 2.8,1gamma-ray bursts (GRBs) are becoming extremely useful tracers of star-forminggalaxies. Long-duration GRBs are known to be associated with the deaths of short-lived massive stars2 and thus have the essential advantage that their detectionrequires only a single stellar progenitor. Therefore, their detection is in principleindependent of host galaxy luminosity.The Swift satellite and a suite of ground-based observatories are detecting, lo-calizing and studying a large homogeneous sample of GRBs. To take advantage ofthis unique sample, we have launched a dedicated programme aimed at buildingup a sample of host galaxies, based on Swift detections and VLT follow-up. This isa Large Programme to be executed over a period of two years. The resulting hostsample will be largely unaffected by dust extinction and entirely independent ofhost galaxy luminosity. A more thorough description of the survey and preliminaryresults are presented in Hjorth et al. (in prep).The details of the sample selection are relatively straight-forward, i.e. the GRBsFebruary 5, 2008 14:29 WSPC - Proceedings Trim Size: 9.75in x 6.5in main2 P. Jakobssonhave to be well-placed for optical follow-up observations: (i) Detected by Swift after1 March 2005 when it was fully operational and automatically slewing. (ii) An X-ray position is available, obtained by the Swift XRT detector. (iii) The Galacticextinction is less than AV < 0.5mag. (iv) Declination favorable for VLT and not ata polar declination, i.e. −70◦ < dec < 25◦.2. AimsThe concrete goals of the programme are to: (i) Identify the GRB hosts, reaching alimit of around R = 27.0 and K = 21.5, which will allow us to detect extremely redobjects. For non-detections of hosts we will spend additional time to reach a limit ofaround R = 28.0. While hosts have been detected for nearly all pre-Swift localizedGRBs, almost none have been detected in the Swift era. (ii) Measure redshifts forGRBs without absorption redshifts. (iii) Search for the Lyα emission line whenpossible, i.e. for bursts with a known redshift z & 2. (iv) Study the effects of dustreddening within hosts. (v) Determine the host luminosity function. Finally, we willperform detailed studies of particularly interesting targets, e.g. short-duration GRBhosts and very bright hosts. Specifically, we will carry out emission line diagnostics,e.g. metallicity estimates via the R23 method.33. ResultsThe final host sample is expected to consist of approximately 70 galaxies of whicha major fraction will have redshifts. The programme so far has consisted mostly oftarget build-up, observational preparation, data taking and preliminary analysis. Todate, only six months after the start of the programme, we have completed roughlyhalf of item (i) above; R- and K-band imaging of three of the hosts is displayed inFig. 1 as an example. The current average and median R-band magnitude of thesample is fainter than 25.5.With this programme, we hope to detect a number of faint galaxies (such asthe GRB 030323 host4) that possibly dominate5 the total star-formation density atz & 2, but are impossible to find and study by other methods than GRB selection.But most importantly, we will produce a coherent sample of GRB host galaxies forfuture follow-up with the HST, Spitzer, VLT, and later with ALMA and JWST.References1. P. Jakobsson, et al. A&A, 447, 897 (2006).2. J. Hjorth, et al. Nature, 423. 847 (2003).3. J. Gorosabel, et al. A&A, 444, 711 (2005).4. P. Vreeswijk, et al. A&A. 419, 927 (2004).5. P. Jakobsson, et al. MNRAS, 362, 245 (2005).6. N. R. Butler, AJ, submitted, astro-ph/0611031.February 5, 2008 14:29 WSPC - Proceedings Trim Size: 9.75in x 6.5in mainGRB-Selected Galaxies: A Swift/VLT Legacy Survey 3(z = 0.65)GRB 050416A (R) GRB 050416A (K)GRB 050915A (R) GRB 050915A (K)Fig. 1. A mosaic of three of the targets; left column displays the R-band while the K-band is inthe right column. The host galaxy is detected in both bands for all targets, and is located insidethe revised6 XRT error circle in each case (solid circle). Each host galaxy also coincides with thecorresponding optical afterglow. The GRB 050915A host and all the K-band host detections havenot been reported before. North is up and east left in each panel which is 20′′ on a side.

Fundamental Properties of GRB-Selected Galaxies: A Swift/VLT Legacy Survey

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