GRB host galaxies

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GRB host galaxies A legacy approach
Daniele Malesani
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
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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
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What we already know
Djorgovski et al. 1998 Fruchter et al. 1999
Sokolov et al. 2001 Bloom et al. 2001, 2002
Fynbo et al. 2003 Le Floch 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 (lt L) and mass

• Blue colors - lack of EROs

• Large specific star formation rates

• Low metallicity? (line of sight / emission)

• Lack of grand-design spirals

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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
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Our sample
Definition of an unbiased sub-sample

• Only long-duration Swift bursts
• Low Galactic extinction (AV lt 0.5 mag)
• XRT early localization (lt 12 hr)
• Suitable VLT observability (-70 lt ? lt 23)

• X-ray based error radius lt 2" (small bias)

71 GRBs
55 with afterglow (77) 40 with redshift
(56) 0.033 lt z lt 6.295
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Our VLT Large Program
PI Jens Hjorth Time awarded 272 hr Time span
2006-2008 Data reduction 90 Soon available
throughthe ESO archive
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Chasing hosts (1)
The simplest but crucial task in host search is
to find them
Optically bright GRB
Accurate astrometry from afterglow images
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Chasing hosts (2)
Optically dark GRB
X-ray positionusually goodenough(Butler,
Evans)
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Large detection rate
R-band detection 58/71 80 Detection at z gt
3 6/13 45 K-band detection 23/65 35
Hjorth et al.in preparation
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Basic host properties (1)
-15 gt MB gt -21.5
Brighter hosts atlarger redshifts?
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Basic host properties (2)
Blue colors 2 lt R - K lt 5 1/51 possible ERO
Red with afterglow Blue no afterglow
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Redshift determination

• Filling the gaps spectra of all objects with R lt
  25

• Completion waiting for X-shooter

• Optical spectroscopy - blue and red grisms

z 0.444
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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 gt 2Implication for dark bursts
Jakobsson et al. in preparation
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Redshift distribution
Originaldistribution ?z? 2.39 After
ourprogram ?z? 2.13 Filling the redshift
desert (see Fiore et al. 2007)
Originaldistribution ?z? 2.39
Preliminary!
Jakobsson et al. in preparation
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Lyman alpha survey
Pre-Swift all targetted hosts have detected Ly?
23 systems in our sample all hosts with 2 lt z lt
4.5(host detection not required)
Analysis in progress, detection in 20 of the
cases
Milvang-Jensen et al. in preparation
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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 lt z lt 1.2 (Work in
progress!)
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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