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GRB Host GalaxiesS. R.Kulkarni, E. J. Berger & Caltech GRB group
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Back to the sixties!
• Parallel with quasar astronomy– By late sixties astronomers were interested in
• understanding how quasars quase
• exploiting quasars to understand the Universe
• Same with GRBs– Most squares (main stream astronomers) want to
exploit GRBs
– Cool people (SRK) want to understand how GRBs burst
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This Talk: A one-minute summary
• Long duration GRBs arise from the death of massive stars
• In almost all cases GRB afterglow shows strong ISM absorption (e.g. MgI) from the host galaxy
• Several examples of dusty hosts have already been seen (“dark” events)
• It appears that many GRB host galaxies are
sub-L* blue galaxies.
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GRB Host Galaxies: A Gallery
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Summing up several large HST efforts
• GRB host galaxies appear to be run-of-mill star-forming galaxies
• GRBs trace blue light (i.e.. massive stars)
• Thus GRBs not only (reasonably) trace star-formation but thus their afterglow can be used to trace the *disk* ISM.
Bloom (PhD thesis)
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Redshifts, Redshifts, Redshifts
• Obtaining redshifts is the key to the use of GRB host galaxies
• Redshifts are best obtained by absorption spectroscopy of the early optical afterglow
• Unlike Lyman-Break galaxies (LBG) one can obtain redshifts fainter than 25 mag (our record, 30 mag host)
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GRB 021004: OT Discovery(Fox et al.) 9 minutes after the GRB!
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GRB Hosts vs. QSOAbsorbers
Salamanca et al. 2002
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Dust & Gamma-rays
• Gamma-rays are penetrating.– Opacity due to Compton scattering
– Column density < 1024 atom cm-2
• Thus GRBs are detectable even if embedded in molecular clouds
– However the optical afterglow will be suppressed
“DARK BURSTS”
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A Prototype Dark Burst: GRB 970828
RT
Djorgovski et al.
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SCUBA
VLA
A Radio / Submillimeter Survey
Berger, Cowie, Kulkanri, et al. 2003
Using the VLA and SCUBA (18 hosts).
~ 20% detection rate above a 3 level of 3 mJy (350 GHz) and 30 Jy (8.5 GHz).
Inferred bolometric luminosities and star formation rates are typical of ULIRGs.
Statistically, F±0.35 mJy
F,8.5 ~ 14±2.5 Jy
SFR ~ 100 M /yr
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Hosts at Long Wavelengths: Summary
• A fraction of the host galaxies have been detected at long wavelengths (decimeter and sub-millimeter). These appear to be ULIRGS and similar to the Scuba sample.
• The fraction of GRBs without strong optical afterglow DIRECTLY traces dusty star formation in the distant universe. This ratio is less than 50% and perhaps as low as 10%.
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Keck GRB Host Program
• For the past seven years I have focussed my Keck time essentially on GRBs
• Systematic program of spectroscopy and near IR photometry
• We are in the process of releasing a comprehensive catalog (about 50 hosts)
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The Redshift Distribution of GRB Hosts
Comparison to Lyman-break galaxies: redshift determination effective well below L*
Comparison to galaxies in the HDF with known z: GRB selection allows us to reveal a population that is inaccessible to other methods
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GRB Host Galaxies – sub-L* galaxies
Berger et al. 2004 (in prep)
Selection bias? Dusty hosts will hide the optical afterglow no localizations.
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Swift Launch: October 2004Swift Launch: October 2004
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Palomar 60-inch: Now a robotic telescope
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Solving the GRB Mystery: An Experimental Approach
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GRB 021004: Host+OT Spectrum(Fox, Price, Barth, et al.)
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S. Kulkarni, E. Berger & Caltech GRB group
Gamma-Ray Burst Host Galaxies: A different diagnostic of high redshift
star formation
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A persistent radio source observed ~1 yr after the burst.
Afterglow emission is expected to be 1-2 orders of magnitude fainter during this time, and decaying
Berger,, Kulkarni & Frail, 2001
Radio Observations: GRB 980703
€
z = 0.966 R host = 22.6
o12
1o
L 103~L
yr M 300 ~ SFR
×
−
FIR
An ULIRG undergoing a nuclear starburst:
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GRBs as Light Houses
• Afterglow of GRB can be used to trace the ISM *within* the disk of the star-forming galaxy
• In contrast, quasar absorption spectroscopy informs us of only the halo
• Thus afterglow absorption spectroscopy offer an entirely new diagnostic as compared to quasar spectroscopy.
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QSO Mg II (metallic line) Absorbers
(Steidel)
GRB hosts:a few kpc?
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Offset & Extinction: A Mystery?
Berger,, Kulkarni & Frail, 2001
VLA / VLBA
GRB 980703 exploded near the center of a starburst galaxy
However, the optical afterglow indicates <1 mag of extinction:
• Dust destruction by GRB?• GRB progenitor prefer less dusty regions?• Young starburst destroys dust more effectively?
HST
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Two Mysteries
• What is the true fraction of dark bursts?
• Why are there no examples of a GRB embedded in a Compton thick GMC?– selection effect?– GRBs occur outside GMCs
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Location, location, location …
Bloom et al. 2001
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So How Do we Use GRBs as Lighthouses?
Within the first 3 hours ½ of all afterglows are brighter than typical high-z quasars.
A 30-60 minute spectrum on a large telescope will provide S/N ~ 10; A delayed response will require ~2 hours (ESI, LRIS, MIKE, IMACS)
Taking all considerations into account, the expected event rate for rapid spectroscopy from Swift is about one per 10-15 days.
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Neon Lines: A Direct Evidence for Massive Star Formation?
[Ne III] / [O II] line ratios: GRB host galaxies: mean = 0.24 median = 0.18
LMC H II regions: mean = 0.06 median = 0.04
Consistent with models with Te > 37000 K, low metallicities
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Distribution of Mg II 2796 Equivalent Widths
QSO Absorbers(Steidel & Sargent) GRBs
0009
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0102
22
9907
12
9705
0899
0510
9901
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