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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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