GRB host galaxies: A legacy approach Daniele Malesani Collaborators: Dark Cosmology Centre Collaborators: Jens Hjorth Johan Fynbo Páll Jakobsson Bo Milvang-Jensen Andreas Jaunsen + many many others 44th Rencontres de Moriond - Very High Energy Phenomena in the Universe - 2009 February 6

new population of high-redshift star-forming galaxies GRB host galaxies GRBs are connected with ongoing star formation They are bright and detected at all redshifts They select galaxies independently of their light Redshift and metallicities from afterglows GRB hosts form a new population of high-redshift star-forming galaxies

What we already know Low luminosity (< L*) and mass Djorgovski et al. 1998; Fruchter et al. 1999; Sokolov et al. 2001; Bloom et al. 2001, 2002; Fynbo et al. 2003; Le Floc’h et al. 2003, 2006; Tanvir et al. 2004; Jakobsson et al. 2005; Fruchter et al. 2006; Stanek et al. 2006; Chary et al. 2007; Courty et al. 2007; Prochaska et al. 2007; Wainwright et al. 2007; Wolf & Podsiadlowski 2007; Modjaz et al. 2008; Savaglio et al. 2008; Castro Cerón et al. 2008; Chen et al. 2008 Low luminosity (< L*) and mass Blue colors - lack of EROs Large specific star formation rates Low metallicity? (line of sight / emission) Lack of grand-design spirals

The need for an unbiased sample The potential of GRBs for cosmological studies is hampered by non-uniformity of samples Haphazard mixture of missions Too few optical detections (179/345 = 52%) Far too few redshifts (113/345 = 33%) Swift is now providing a uniform sample Precise X-ray localizations now available

Our sample Definition of an unbiased sub-sample Only long-duration Swift bursts Low Galactic extinction (AV < 0.5 mag) XRT early localization (< 12 hr) Suitable VLT observability (-70° <  < 23°) X-ray based: error radius < 2" (small bias) 71 GRBs 55 with afterglow (77%) 40 with redshift (56%): 0.033 < z < 6.295

Our VLT Large Program PI: Jens Hjorth Time awarded: 272 hr Time span: 2006-2008 Data reduction: ~90% Soon available through the ESO archive

Chasing hosts (1) The simplest but crucial task in host search is to find them Optically bright GRB Accurate astrometry from afterglow images

Chasing hosts (2) Optically dark GRB X-ray position usually good enough (Butler, Evans)

Large detection rate R-band detection: 58/71 = 80% Detection at z > 3: 6/13 = 45% K-band detection: 23/65 = 35% Hjorth et al. in preparation

Basic host properties (1) -15 > MB > -21.5 Brighter hosts at larger redshifts?

Basic host properties (2) Blue colors: 2 < R - K < 5 1/51 possible ERO Red: with afterglow Blue: no afterglow

Redshift determination Filling the gaps: spectra of all objects with R < 25 Completion waiting for X-shooter Optical spectroscopy - blue and red grisms [OII] H [OIII] z = 0.444

Redshift results Outcome: 9 new redshifts 4 known redshifts likely wrong! 8 constrained redshifts (redshift desert) 40/71 …  … 45/71 Overall 75% of bursts have now a redshift constraint None of the targetted hosts have z > 2 Implication for dark bursts Jakobsson et al. in preparation

Redshift distribution Original distribution: z = 2.39 After our program: z = 2.13 Filling the “redshift desert” (see Fiore et al. 2007) Original distribution: z = 2.39 Preliminary! Jakobsson et al. in preparation

Lyman alpha survey Pre-Swift: all targetted hosts have detected Ly 23 systems in our sample: all hosts with 2 < z < 4.5 (host detection not required) Analysis in progress, detection in ~20% of the cases Milvang-Jensen et al. in preparation

Short GRB - red sequence survey Do short GRBs eplode in galaxy clusters? Prototype: GRB 050509B (Bloom et al. 2005, Pedersen et al. 2005) Mini-sample of 19 short GRBs localized by BAT Looking for the red sequence in R vs z bands Probing clusters at 0 < z < 1.2 (Work in progress!)

Legacy value Large numbers of events: need to control systematics Continuing effort to create a high-quality sample The future: X-shooter, HST, JWST, Alma A sample good for many kinds of GRB science