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 less than or similar to 19) extremely red (R-K greater than or simila r to 6) objects. Little is known about the properties of these objects on account of their optical faintness (R greater than or similar to 2 4). Here we report deep infrared imaging and spectroscopy of one of th e extremely red objects (EROs) discovered by Hu & Ridgway in the held of the z = 3.79 quasar PC 1643+4631A. The infrared images were obtaine d in 0 ''.5 seeing and show that the object (denoted HR 10) is not a d ynamically relaxed elliptical galaxy dominated by an old stellar popul ation as was previously suspected, but instead has an asymmetric morph ology suggestive of either a disk or an interacting system. The infrar ed spectrum of HR 10 shows a single, possibly broad emission feature a t 1.60 mu m, which we identify as H alpha+[N II] at z = 1.44. The lumi nosity and width of this emission line indicates either intense star f ormation (similar to 20 h(-2) M. yr(-1)) or the presence of an active nucleus. Based on the rest frame UV-optical spectral energy distributi on, the luminosity of HR 10 is estimated to be 3-8 L. The colors of H R 10 are unusually red for a galaxy (at z = 1.44 the age of HR 10 is a t most 2-8 Gyr depending on cosmology) and indicate that HR 10 is dust y. HR 10 is detected weakly at radio wavelengths; this is consistent w ith either the starburst or active galactic nucleus hypothesis. If HR 10 is a typical representative of its class, EROs are numerous and rep resent a significant component of the luminous objects in the universe at z approximate to 1.5.