Evidence for a Massive Poststarburst Galaxy at z ~ 6.5

We describe results from a search for high-redshift J-band "dropout" galaxies in the portion of the Great Observatories Origins Deep Survey (GOODS) southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF). Using observations at optical, near-infrared, and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the European Southern Observatory Very Large Telescope, we find one particularly remarkable candidate, which we designate HUDF-JD2. Its spectral energy distribution has distinctive features that are consistent with those of a galaxy at z ~ 6.5, observed several hundred million years after a powerful burst of star formation that produced a stellar mass of 6 × 1011 M☉ (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646 Å Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ≈ 2.5 are possible, but provide significantly poorer fits to the photometric data. The object is detected with Spitzer at 24 μm. We consider interpretations of the 24 μm emission as originating either from an obscured active nucleus or from star formation and find that the 24 μm detection does not help to uniquely discriminate between the z = 6.5 and 2.5 alternatives. We present optical and near-infrared spectroscopy that has, thus far, failed to detect any spectral features. This nondetection helps limit the solution in which the galaxy is a starburst or active galaxy at z ≈ 2.5, but does not rule it out. If the high-redshift interpretation is correct, HUDF-JD2 is an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in which a very large mass of stars formed within a remarkably short period of time, at very high redshift.