ALMA OBSERVATIONS OF THE HOST GALAXY OF GRB 090423 AT z=8.23: DEEP LIMITS ON OBSCURED STAR FORMATION 630 MILLION YEARS AFTER THE BIG BANG

We present rest-frame far-infrared (FIR) and optical observations of the host galaxy of GRB 090423 at z = 8.23 from the Atacama Large Millimeter Array (ALMA) and the Spitzer Space Telescope, respectively. The host remains undetected to 3 sigma limits of F-nu(222 GHz) less than or similar to 33 mu Jy and F-nu(3.6 mu m) less than or similar to 81 nJy. The FIR limit is about 20 times fainter than the luminosity of the local ULIRG Arp 220 and comparable to the local starburst M 82. Comparing this with model spectral energy distributions, we place a limit on the infrared (IR) luminosity of LIR(8-1000 mu m) less than or similar to 3x10(10) L-circle dot, corresponding to a limit on the obscured star formation rate of SFRIR less than or similar to 5 M-circle dot yr(-1). For comparison, the limit on the unobscured star formation rate from Hubble Space Telescope rest-frame ultraviolet (UV) observations is SFRUV less than or similar to 1 M-circle dot yr(-1). We also place a (for a stellar population age of 100 Myr and constant star formation rate). Finally, we compare our millimeter observations to those of field galaxies at z greater than or similar to 4 (Lyman break galaxies, Ly alpha emitters, and submillimeter galaxies) and find that our limit on the FIR luminosity is the most constraining to date, although the field galaxies have much larger rest-frame UV/optical luminosities than the host of GRB 090423 by virtue of their selection techniques. We conclude that GRB host galaxies at z greater than or similar to 4, especially those with measured interstellar medium metallicities from afterglow spectroscopy, are an attractive sample for future ALMA studies of high redshift obscured star formation.