Misc,
Discovery of an old galaxy at z=6.027, multiply imaged by the massive cluster Abell 383
(2011)cite arxiv:1102.5092 Comment: 5 pages, 3 figures, submitted to MNRAS letters.
Abstract
We report the discovery of an old z=6.027 galaxy, multiply imaged by the
cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3
imaging, as well as in Warm Spitzer observations. This galaxy was selected as a
pair of i-dropouts; its suspected high redshift was confirmed by the
measurement of a strong Lyman-alpha line in both images using Keck/DEIMOS.
Combining Hubble and Spitzer photometry after correcting for contamination by
line emission (estimated to be a small effect), we identify a strong Balmer
break of 1.5 magnitudes, suggesting the presence of old stars from an early
episode of star formation. Taking into account the magnification factor of
11.4+/-1.9 (2.65+/-0.17 mag) for the brightest image, the unlensed AB magnitude
for the source is 27.2+/-0.05 in the H band and 25.7+/-0.08 at 3.6 um,
corresponding to a 0.4 L* galaxy. The UV slope is consistent with beta~2.0, and
from the rest-frame UV continuum we measure a current star formation rate of
2.4+/-1.1 Msol/yr. The unlensed half-light radius is measured to be 300 pc,
from which we deduce a star-forming surface density of ~10 Msol/yr/kpc2. The
Lyman-alpha emission is found to be extended over ~3 arcsec along the slit,
corresponding to ~5 kpc in the source plane. This can be explained by the
presence of a much larger envelope of neutral hydrogen around the star-forming
region. Finally, fitting the spectral energy distribution using 7 photometric
data points, we derive the following properties: an intrinsic stellar mass of
M*=6.3+2.8 -1.2 10^9 Msol, an age of 640-940 Myrs (corresponding to a redshift
of formation of 18+/-4, and very little reddening. The star-formation rate of
this object was much stronger in the past than at the time of observation,
suggesting that we may be missing a fraction of galaxies at z~6 which have
already faded in rest-frame UV wavelengths.
