Near-Infrared Observations of the Extremely Red Object Cl 0939+4713B: An Old Galaxy at z ∼ 1.58?

Near-infrared imaging and spectroscopic observations of the extremely red object Cl 0939+4713B (R-K ∼ 7 mag) have been obtained with the Near Infrared Camera on the Keck I Telescope of the W. M. Keck Observatory. The imaging shows a slightly elongated structure, while the spectroscopy shows a continuum break that allows us to determine a redshift of z = 1.58 for this system. The fits of a range of models to the infrared spectrum suggest that it is predominantly an old (>109 yr) stellar system that suffers little extinction, while the measured R and I magnitudes suggest an age of ∼3 × 108 yr. The limit on the equivalent width of any emission line in the infrared spectrum suggests that Cl 0939+4713B is not an actively star-forming galaxy. This system, though similar in R-K color to the object HR 10 (also known as J1645+46), is much different in morphology and emission-line strengths, demonstrating the heterogeneity of extremely red extragalactic objects selected on the basis of large R-K values.