The Redshift of an Extremely Red Object and the Nature of the Very Red Galaxy Population

Infrared surveys have discovered a significant population of bright (K<19)extremely red (R-K>6) objects. Little is known about the properties of theseobjects on account of their optical faintness (R>24). Here, we report deepinfrared imaging and spectroscopy of one of the extremely red objects (EROs)discovered by Hu \& Ridgway (1994) in the field of the z=3.79 quasarPC1643+4631A. The infrared images were obtained in 0.5" seeing, and show thatthe object (denoted HR10) is not a dynamically relaxed elliptical galaxydominated by an old stellar population as was previously suspected, but insteadhas an asymmetric morphology suggestive of either a disk or an interactingsystem. The infrared spectrum of HR10 shows a single, possibly broad emissionfeature at 1.60um which we identify as Halpha+[NII] at z=1.44. The luminosityand width of this emission line indicates either intense star formation (about20h^{-2} solar masses/yr) or the presence of an active nucleus. Based on therest frame UV-optical spectral energy distribution, the luminosity of HR10 isestimated to be 3 to 8 L*. The colors of HR10 are unusually red for a galaxy(at z=1.44 the age of HR10 is at most 2 to 8 Gyr depending on cosmology), andindicate that HR10 is dusty. HR10 is detected weakly at radio wavelengths; thisis consistent with either the starburst or AGN hypothesis. If HR10 is a typicalrepresentative of its class, EROs are numerous and represent a significantcomponent of the luminous objects in the Universe at z approx 1.5.Comment: Uu-encoded compressed gzip tarfile containing postscript files of 15-page paper and 5 figures. Accepted for publication in Ap