Probing Cosmic Star Formation Using Long Gamma‐Ray Bursts: New Constraints from theSpitzer Space Telescope

We report on IRAC-4.5mic, IRAC-8.0mic and MIPS-24mic deep observations of 16 Gamma-Ray Burst (GRBs) host galaxies performed with the Spitzer Space Telescope, and we investigate in the thermal infrared the presence of evolved stellar populations and dust-enshrouded star-forming activity associated with these objects. Our sample is derived from GRBs that were identified with sub-arcsec localization between 1997 and 2001, and only a very small fraction (~20%) of the targeted sources is detected down to f_4.5mic ~3.5microJy and f_24mic ~85microJy (3sigma). This likely argues against a population dominated by massive and strongly-starbursting (i.e., SFR > ~100 Msol/yr) galaxies as it has been recently suggested from submillimeter/radio and optical studies of similarly-selected GRB hosts. Furthermore we find evidence that some GRBs do not occur in the most infrared-luminous regions -- hence the most actively star-forming environments -- of their host galaxies. Should the GRB hosts be representative of all star-forming galaxies at high redshift, models of infrared galaxy evolution indicate that > ~50% of GRB hosts should have f_24mic > ~100microJy. Unless the identification of GRBs prior to 2001 was prone to strong selection effects biasing our sample against dusty galaxies, we infer in this context that the GRBs identified with the current techniques can not be directly used as unbiased probes of the global and integrated star formation history of the Universe.
ApJ in press, 23 pages, 8 figures (scheduled for the ApJ 10 May 2006, v642 2 issue). Full resolution available at http://perceval.as.arizona.edu/~elefloch/Publis/ms_grb_spitzer.pdf