1999AJ....118.2065S

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^+0.01_–0.03 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 ∼3x10⁸ 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.