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

We report on IRAC 4.5 mu m, IRAC 8.0 mu m, and MIPS 24 mu m deep observations of 16 gamma-ray burst (GRB) 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 subarcsecond localization between 1997 and 2001, and only a very small fraction(similar to 20%) of the targeted sources are detected down to f(4: 5) mu m similar to 3: 5 mu Jy and f(24) mu m similar to 85 mu Jy (3 sigma). This likely argues against a population dominated by massive and strongly starbursting ( i.e., SFR greater than or similar to 100 M-circle dot yr(-1)) galaxies as 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 greater than or similar to 50% of GRB hosts should have f(24) mu m greater than or similar to 100 mu Jy. 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 cannot be directly used as unbiased probes of the global and integrated star formation history of the universe.