1. 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 February 6

2. 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

3. 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

4. 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

5. 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%): < z < 6.295

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

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

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

9. 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

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

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

12. 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

13. 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

14. 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

15. 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

16. Short GRB - red sequence survey
Do short GRBs eplode in galaxy clusters?
Prototype: GRB B
(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!)

17. 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