Аннотация
We report the discovery of a candidate z~8 galaxy lensed by a foreground
z~1.7 group, found in the pure-parallel HST images taken as part of the BoRG
Survey. The system consists of a Y-dropout galaxy with m=25.92 +/-0.14 which
appears to be strongly lensed by a foreground group with a photometric z~1.7,
which would be the highest-z deflector discovered to date. Several pieces of
circumstantial evidence point toward the lensing hypothesis: 1) The z~8 dropout
is close in projection to the foreground group, 2) it is slightly elongated
tangentially in the direction of the putative critical curve, 3) a possible
counter image is marginally detected and 4) the Y-dropout is not clustered,
implying that it is intrinsically fainter that it appears, i.e. it has been
magnified. We show that the configuration can be accurately reproduced by an
SIE deflector and a source with a Sersic surface brightness profile. The best
fit model has a deflector Einstein Radius of 1.49"+0.03"/-0.01", and the
unlensed z~8 source has a Sersic index n=0.92 +0.24/-0.16 and an effective
radius of r_e=0.08" +/- 0.01" with mAB =27.4 +/-0.1 which has been magnified by
a factor of 3.7 +/- 0.2. A non-parametric reconstruction of the source with
LENSVIEW corroborates the derived source parameters. The lens model yields a
group mass of 1.08 +0.10/-0.15*10^12 M_sun and a mass-to-light ratio of
M/L_B=30.7 +3.7/-12.6 M_sun/L_sun within the Einstein Radius. Assuming a
standard dark matter fraction of 1/3, the main deflector galaxy has a stellar
mass-to-light ratio of M/L_B=2.2 +0.4/-1.1 M_sun/L_sun. If confirmed, the
system provides a unique opportunity to obtain spectroscopy of a typical z~8
galaxy and an absolute measure of mass for a galaxy at z~1.7. The discovery of
this candidate is qualitatively consistent with the theoretical expectation
that 10% of z~8 galaxies with luminosity L>2L* are significantly magnified.
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