The environments of long-duration gamma-ray bursts

1. Daniele Malesani Dark Cosmology Centre The environments oflong-duration gamma-ray bursts Nyborg - 2007 Jun 19 DFS meeting

2. Gamma-ray bursts Brief pulses of softgamma radiation GRBs are followed by afterglows at radio, optical, X-ray Short and long GRBs

3. The GRB/SN association Most long-duration GRBs are associated with core-collapse supernovae Direct spectroscopic evidence Type-Ic events Collapse of massive stars which have lost their envelope “Hypernovae” SNe with high explosion energy Hjorth et al. 2003, Stanek et al. 2003 Malesani et al. 2004

4. GRBs throughout the Universe GRBs are cosmological objects z = 2.8 Record: z = 6.295 Powerful probes of cosmic star formation Different selection biases

5. The host galaxies of long GRBs Djorgovski et al. 1998 Star-formingsystems SFR  10 M yr-1 Ages  50-100 Myr Malesani et al. 2004

6. Are GRB hosts “special”? GRB hosts are bluer than field galaxies HDF V-H color GRB Magnitude Le Floc’h et al. 2003 Fruchter et al. 1999

7. Low metallicities? 1) Low host luminosities Consistency with metallicity/luminosity relation 2) Direct analysis of nebular spectra (nearby objects) Metallicity  0.1-0.5 solar 3) GRB hosts are powerful Ly emitter small extinction  little dust (Fynbo et al. 2003) 4) Low Spitzer infrared detection rate no ULIRG  low dust contect (Le Floc’h et al. 2006)

8. Low metallicities Stanek et al. 2006 Metallicity of the 5 closest GRBs (z<0.2)

9. Intrinsic or observational bias? Most of information still comes fromoptically bright afterglows (Observational) bias against dust-poor systems? Study of host galaxies of optically-dark GRBs A few cases reveal interesting properties(moderate extinction, larger metallicity) Levan et al. 2006, Berger et al. 2007

10. Building a fair sample Large program @ VLT (PI J. Hjorth) Survey of all GRB hosts with accurate position (optical OR X-ray) R band K band GRB 050915A z=0.44

11. The theorist point of view Leading model: “collapsar” (Woosley 1993...) Relativistic jets are launched by the central accreting black hole To form the disk  large angular star angular momentum Star envelope mass loss cannot be too large low metallicity required/predicted Perhaps metallicity bias intrinsic to GRBs!

12. But... GRBs At large redshift small metallicity is expected Collapsar threshold At large z GRBs are much less biased Mean metallicity QSO DLAs Fynbo et al. 2006

13. The local environment GRB afterglows are much brighter than the host galaxies Absorption spectroscopy Gas content(Ly) Metal content Berger et al. 2006

14. The Hydrogen Large column densities: NH 1020 - 1022 cm-2 Damped-Ly systems AGN DLA GRB DLA AGN absorber observer observer absorber GRB Galaxy wings Central regions

15. Hydrogen vs metals Watson et al. 2007 Comparison of optical vsX-ray NH in principle allows measuring the metallicity No correlation GRB feedback? Ionization? The probed gas is far away

16. The dust (I) Large H columns do not correspond to large dust Small reddening in GRBafterglows Tagliaferri et al. 2007 Swift sample Kann et al. 2006

17. The dust (II) Comparison of AV and NH shows a low dust content Swift results Stratta et al. 2004

18. Summary GRBs explode in the most star-forming regions Currently-studied GRB environments are different from the bulk of star formation as it is known Possible metallicity bias – perhaps intrinsic? GRBs offer the possibility to probe the individual components of the high-redshift ISM