Accepted for publication in The Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 6/22/04 PROBING THE COSMIC STAR FORMATION USING LONG GAMMA-RAY BURSTS: NEW CONSTRAINTS FROM THE SPITZER SPACE TELESCOPE ∗

We report on IRAC 4.5 μm, IRAC 8.0 μm, and MIPS 24 μm 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 (~20%) of the targeted sources are detected down to f4.5 μm ~ 3.5 μJy and f24 μm ~ 85 μJy (3 σ). This likely argues against a population dominated by massive and strongly starbursting (i.e., SFR 100 M☉ 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 50% of GRB hosts should have f24 μm 100 μJy. 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.