A Disk Galaxy of Old Stars at z ~ 2.5.pdf

a r X i v : a s t r o - p h / 0 3 1 2 5 5 0 v 1 1 9 D e c 2 0 0 3 Draft version February 2, 2008 Preprint typeset using LATEX style emulateapj v. 9/08/03 A DISK GALAXY OF OLD STARS AT Z～ 2.51 Alan Stockton Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 Gabriela Canalizo Institute of Geophysics and Planetary Physics and Department of Earth Sciences, University of California, Riverside, CA 95521 Toshinori Maihara Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Draft version February 2, 2008 ABSTRACT We describe observations of a galaxy in the field of the z = 2.483 radio galaxy 4C23.56, photometri- cally 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 4C23.56, 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 r1/4-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 Se?rsic 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 b