GRB Host Galaxies S. R.Kulkarni, E. J. Berger

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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. QSO Absorbers
Salamanca et al. 2002
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Dust Gamma-rays

• Gamma-rays are penetrating.
• Opacity due to Compton scattering
• Column density lt 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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A Radio / Submillimeter Survey
Using the VLA and SCUBA (18 hosts). 20
detection rate above a 3s level of 3 mJy (350
GHz) and 30 mJy (8.5 GHz). Inferred bolometric
luminosities and star formation rates are typical
of ULIRGs. Statistically, Fn,350 0.350.35
mJy Fn,8.5 142.5 mJy ?
?SFR? 100 M? /yr
SCUBA
VLA
Berger, Cowie, Kulkanri, et al. 2003
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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
Selection bias? Dusty hosts will hide the
optical afterglow ? no localizations.
Berger et al. 2004 (in prep)
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Swift 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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(No Transcript)
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GRB 021004 HostOT Spectrum(Fox, Price, Barth,
et al.)
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Gamma-Ray Burst Host Galaxies A different
diagnostic of high redshift star formation
S. Kulkarni, E. Berger Caltech GRB group
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Radio Observations GRB 980703
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
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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
GRB hosts a few kpc?
(Steidel)
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Offset Extinction A Mystery?

• GRB 980703 exploded near the center of a
  starburst galaxy
• However, the optical afterglow indicates lt1 mag
  of extinction
• Dust destruction by GRB?
• GRB progenitor prefer less dusty regions?
• Young starburst destroys dust more effectively?

HST
VLA / VLBA
Berger,, Kulkarni Frail, 2001
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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 gt 37000 K,
low metallicities
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Distribution of Mg II 2796 Equivalent Widths
QSO Absorbers (Steidel Sargent)
GRBs
000926
010222
990712
970508
990510
990123