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 \gtrsim 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 2 L ^ { * } 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 .