A Disk Galaxy of Old Stars at z ∼ 2.5

Abstract

We describe observations of a galaxy in the field of the z = 2.483 radio galaxy 4C 23.56, photometrically selected to have a spectral energy distribution consistent with an old stellar population at the redshift of the radio galaxy. Exploration of redshift - stellar-population - reddening constraints from the photometry indicates that the galaxy is indeed at a redshift close to that of 4C 23.56, that the age of the most recent significant star formation is roughly ≳2 Gyr, and that reddening is fairly modest, with more reddening required for the younger end of stellar age range. From analysis of a deep adaptive optics image of the galaxy, we find that an r 1/4 -law profile, common for local spheroidal galaxies, can be excluded quite strongly. On the other hand, a pure exponential profile fits remarkably well, while the best fit is given by a Sérsic profile with index n = 1.49. Reconstruction of the two-dimensional form of the galaxy from the best-fit model is consistent with a disk galaxy with neither a significant bulge component nor gross azimuthal structure. The assembly of roughly 2L* of old stars into such a configuration this early in the history of the universe is not easily explainable by any of the currently popular scenarios for galaxy formation. A galaxy with these properties would seem to require smooth but rapid infall of the large mass of gas involved, followed by a burst of extremely vigorous and efficient star formation in the resulting disk.