ERO R1 in the field of Cl 0939+4713: Evidence for an S0-like galaxy at z ∼ 1.5

We present further observations of the extremely red object ERO J094258+4659.2, identified earlier by lye et al. as ERO R1 in their deep images of the cluster A851. We estimate its redshift independently by eight-band photometric redshift determination and cross-correlation of a new H-band spectrum with the optical spectra of local E/S0 galaxies and conclude that it lies at z ∼ 1.5. Although its colors are consistent with both an elliptical galaxy and an S0 galaxy at that redshift, its elongated shape and exponential luminosity profile suggest the presence of an evolved stellar disk component. We rule out the possibility that these properties are strongly influenced by gravitational lensing by the foreground cluster and therefore conclude that this object is more likely to be an S0-like galaxy rather than a lensed elliptical. The H-band spectrum does not show strong Hα emission, and the star formation rate therefore appears to be very modest. The presence of such a galaxy with an apparently relaxed disk of stars at this high redshift provides a new and strong constraint on theoretical models that aim to explain the formation and evolution of galaxies.