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

Abstract

Infrared surveys have discovered a significant population of bright (K<19) extremely red (R-K>6) objects. Little is known about the properties of these objects on account of their optical faintness (R>24). Here, we report deep infrared imaging and spectroscopy of one of the extremely red objects (EROs) discovered by Hu \& Ridgway (1994) in the field of the z=3.79 quasar PC1643+4631A. The infrared images were obtained in 0.5&#34; seeing, and show that the object (denoted HR10) is not a dynamically relaxed elliptical galaxy dominated by an old stellar population as was previously suspected, but instead has an asymmetric morphology suggestive of either a disk or an interacting system. The infrared spectrum of HR10 shows a single, possibly broad emission feature at 1.60um which we identify as Halpha+[NII] at z=1.44. The luminosity and width of this emission line indicates either intense star formation (about 20h^{-2} solar masses/yr) or the presence of an active nucleus. Based on the rest frame UV-optical spectral energy distribution, the luminosity of HR10 is estimated 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), and indicate that HR10 is dusty. HR10 is detected weakly at radio wavelengths; this is consistent with either the starburst or AGN hypothesis. If HR10 is a typical representative of its class, EROs are numerous and represent a significant component of the luminous objects in the Universe at z approx 1.5.