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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 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.