First look at GRB hosts with Spit zer

1. First look at GRB hosts with Spitzer J.M. Castro Cerón, J. Hjorth, D.J. Watson & J.P.U. Fynbo DARK, Niels Bohr Institutet Danmark RTN Summer School — Σαντορίνη 2005

2. Mr.Bill.Gates@microsoft.com I should like to start by apologising for any inconvenience I might have caused you in rescheduling this presentation, originally planned for Wednesday. Should you have any complaints, you are kindly requested to direct them to: RTN Summer School — Σαντορίνη 2005

3. Why are the Spitzer observations interesting?  galaxies contain most visible light in the universe: — morphology + LF  explore stellar populations, SFR evolution.  ‘traditional’ techniques for high z: — DLAs, LBGs, SMGs, Ly-a emission selected galaxies and DRGs. — limited in: z distribution depth, completeness of mass representation. — low mass galaxies systematically underrepresented at high z.  GRBs are less affected by: dust extinction, flux limited selection effects.  MIR observations can indicate the fraction of ULIRG GRB hosts.  help to provide answers to crucial questions: — which populations of galaxies produce GRBs? in what proportion? — are GRBs truly unbiased tracers of star formation? RTN Summer School — Σαντορίνη 2005

4. What Spitzer can do!  GRB hosts sample, with measured redshifts, have mean z currently approaching 2.  at this z, an ULIRG’s continuum emission is detectable in fairly modest integration times: — IRAC at 4,5 and 8,0 µm. — MIPS at 24 µm.  to illustrate these points, we have redshifted the dusty ULIRG HR10's SED (Elbaz et al. 2002) from the GRASIL spectral synthesis code (Silva et al. 1998), to z = 1,6 and 2,7.  detection of star forming galaxies at this z is strongly favoured with MIPS at 24 µm because of the powerful PAH features that pass through the band at redshifts 1–2,8 as well as the bump caused by thermal dust emission. RTN Summer School — Σαντορίνη 2005

5. First look…  archival data for 15 GRB host galaxies.  each GRB host observed with IRAC at 4,5 and 8,0 µm and MIPS at 24,0 µm.  inhomogeneous collection of detections and non detections.  initially concentrate in hosts with z ~ 1: — at this redshift the 12 µm band is redshifted to 24 µm. — calibrated probe of the SFR. — GRB 970828  dark burst — GRB 980613  merging system — GRB 980703  radio host / large SFR — GRB 990705  grand design spiral RTN Summer School — Σαντορίνη 2005

6. Host of GRB 970828  first well localised GRB X ray afterglow with no optical counterpart: — despite prompt, deep search (Rlim~ 24,5). — Groot et al. 1998; Djorgovski et al. 2001. — dark burst, also per Jakobsson et al. 2004.  weak, short lived radio flare within the X ray error box: — allowed the identification of the host. — z  0,9578.  host has 3 components, appears to be interacting/merging: — radio flare may be located between the two optical peaks (A, B). — with the B component as the most likely radio flare host. — SFR: A [O II] 3727 ≈ 1,2 M/yr. A (UV) ≈ 1,1 M/yr. B [O II] 3727 ≈ 0,3 M/yr. RTN Summer School — Σαντορίνη 2005

7. Djorgovski et al. 2001 Djorgovski et al. 2001 GRB 970828 — 4,5 µm  5 µJy (13) GRB 970828 — 24 µm  94 µJy (4) GRB 970828 — 8,0 µm  17 µJy (4) Host of GRB 970828 Estimated SFR of about 100 M/yr RTN Summer School — Σαντορίνη 2005

8. Host of GRB 980613  afterglow was optically faint at discovery (R ≈ 23): — no rapid decay  flat spectral shape. — Hjorth et al. 2002; Djorgovski et al. 2003.  HST/STIS observations of the host: — very compact, blue V  26,1 object. — five star forming knots and/or interacting galaxy fragments. — z  1,0969 for at least two components.  component identified as host: — moderately high unobscured SFR. — high SFR per unit mass  starburst. — SFR ~ 5 M/yr. RTN Summer School — Σαντορίνη 2005

9. Djorgovski et al. 2003 Hjorth et al. 2002 GRB 980613 — 4,5 µm  39 µJy (43) Hjorth et al. 2002 Djorgovski et al. 2003 GRB 980613 — 8,0 µm  29 µJy (7) Host of GRB 980613 RTN Summer School — Σαντορίνη 2005

10. Host of GRB 980703  afterglow reported in the radio, NIR, optical and X ray bands: — Bloom et al. 1998; Holland et al. 2001.  HST/STIS observations of the host: — blue, slightly overluminous. — bright, V  23,0 R  22,6. — centre is ≈ 0,2 mag bluer than outer regions. — z  0,966.  large SFR, assuming no extinction in the host: — subtraction of elliptical isophotes fit  residuals  irregular. — afterglow location could not be connected with any special features. — typical compact star forming galaxy as found in the HDFN. — SFR  8–13 M/yr. RTN Summer School — Σαντορίνη 2005

11. GRB 980703 — 4,5 µm  9 µJy (15) Holland et al. 2001 GRB 980703 — 8,0 µm  12 µJy (3) Host of GRB 980703 RTN Summer School — Σαντορίνη 2005

12. Host of GRB 990705  afterglow reported in the NIR, optical and X ray bands: — rather steep decline of the NIR light curve. — broad band spectral analysis  high density environment. — Masetti et al. 2000; Le Floc’H et al. 2002.  HST/STIS observations of the host: — first reliable z derived from prompt emission. — z  0,8424.  nearly face on Sc spiral galaxy, typical of disk dominated systems: — MB –21,75. — with the B component as the most likely radio flare host. — SFR ≈ 5–8 M/yr. RTN Summer School — Σαντορίνη 2005

13. Le Floc’H et al. 2002 Masetti et al. 2000 GRB 990705 — 4,5 µm  14 µJy (35) GRB 990705 — 8,0 µm  12 µJy (3) Host of GRB 990705 RTN Summer School — Σαντορίνη 2005

14. Coming soon to journal near you…  measure fluxes and estimate the SFR for the rest of the galaxies.  get full SEDs for all the hosts.  statistics: — similar detection rate as SCUBA with better resolution?  for the dark bursts, infer the amount of dust present.  at z ~ 1, the 4,5 µm band corresponds to rest frame K: — calculate reddening — mass estimator RTN Summer School — Σαντορίνη 2005