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

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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 in the field of the z = 3.79 quasar PC 1643 + 4631A. The infrared images were obtained in 0''.5 seeing and show that the object (denoted HR 10) 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 HR 10 shows a single, possibly broad emission feature at 1.60 μm, which we identify as Hα + [N II] at z = 1.44. The luminosity and width of this emission line indicates either intense star formation (~20 h−2 M☉ yr−1) or the presence of an active nucleus. Based on the rest frame UV-optical spectral energy distribution, the luminosity of HR 10 is estimated to be 3–8 L*. The colors of HR 10 are unusually red for a galaxy (at z = 1.44 the age of HR 10 is at most 2–8 Gyr depending on cosmology) and indicate that HR 10 is dusty. HR 10 is detected weakly at radio wavelengths; this is consistent with either the starburst or active galactic nucleus hypothesis. If HR 10 is a typical representative of its class, EROs are numerous and represent a significant component of the luminous objects in the universe at z ≈ 1.5.
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