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

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

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. 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. GRB Host Galaxies: A Gallery

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

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

9. 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”

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

11. 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 ~ 0.35±0.35 mJy Fn,8.5 ~ 14±2.5mJy  SFR ~ 100 M /yr SCUBA VLA Berger, Cowie, Kulkanri, et al. 2003

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

16. Swift Launch: October 2004

17. Palomar 60-inch: Now a robotic telescope

18. Solving the GRB Mystery: An Experimental Approach

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

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

22. An ULIRG undergoing a nuclear starburst: 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

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

25. Offset & Extinction: A Mystery? • 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 VLA / VLBA Berger,, Kulkarni & Frail, 2001

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

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. 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

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