Abstract
We report the detection of a z ~ 7 galaxy strongly lensed by the massive galaxy cluster A2218 (z = 0.175) at 3.6 and 4.5 μm using the Spitzer Space Telescope and at 1.1 μm using the Hubble Space Telescope. The new data indicate a refined photometric redshift in the range of 6.6-6.8 depending on the presence of Lyα emission. The spectral energy distribution is consistent with having a significant Balmer break, suggesting that the galaxy is in the poststarburst stage with an age of at least ~50 Myr and quite possibly a few hundred million years. This suggests the possibility that a mature stellar population is already in place at such a high redshift. Compared with typical Lyman break galaxies at z ~ 3-4, the stellar mass is an order of magnitude smaller (~109 M☉), but the specific star formation rate (star formation rate/Mstar) is similarly large (>10-9 yr-1), indicating equally vigorous star-forming activity.
Footnotes
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This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Support for this work was provided by NASA through contract 960785 issued by JPL/Caltech.
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This work is also based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 9452.