1. 1 GRB Host Galaxies S. R.Kulkarni, E. J. Berger & Caltech GRB group

2. 2 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

3. 3 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.

4. 4 GRB Host Galaxies: A Gallery

5. 5 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)

6. 6 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)

7. 7 GRB 021004: OT Discovery (Fox et al.) 9 minutes after the GRB!

8. 8 GRB Hosts vs. QSO Absorbers Salamanca et al. 2002

9. 9 Dust & Gamma-rays Gamma-rays are penetrating. –Opacity due to Compton scattering –Column density < 10 24 atom cm -2 Thus GRBs are detectable even if embedded in molecular clouds –However the optical afterglow will be suppressed “DARK BURSTS”

10. 10 A Prototype Dark Burst: GRB 970828 RT Djorgovski et al.

11. 11 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

12. 12 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%.

13. 13 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)

14. 14 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

15. 15 GRB Host Galaxies – sub-L* galaxies Berger et al. 2004 (in prep) Selection bias? Dusty hosts will hide the optical afterglow  no localizations.

16. 16 Swift Launch: October 2004

17. 17 Palomar 60-inch: Now a robotic telescope

18. 18 Solving the GRB Mystery: An Experimental Approach

19. 19

20. 20 GRB 021004: Host+OT Spectrum (Fox, Price, Barth, et al.)

21. 21 S. Kulkarni, E. Berger & Caltech GRB group Gamma-Ray Burst Host Galaxies: A different diagnostic of high redshift star formation

22. 22 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 An ULIRG undergoing a nuclear starburst:

23. 23 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.

24. 24 QSO Mg II (metallic line) Absorbers (Steidel) GRB hosts: a few kpc?

25. 25 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

26. 26 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

27. 27 Location, location, location … Bloom et al. 2001

28. 28 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.

29. 29 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 T e > 37000 K, low metallicities

30. 30 Distribution of Mg II 2796 Equivalent Widths QSO Absorbers (Steidel & Sargent) GRBs 000926 010222 990712 970508 990510 990123