The Ellipticity Distribution of Ambiguously Blended Objects
  Using overlapping fields with space-based Hubble Space Telescope (HST) and
ground-based Subaru Telescope imaging we identify a population of blended
galaxies that are blended to such a large degree that they are detected as
single objects in the ground-based monochromatic imaging, which we label as
'ambiguous blends'. For deep imaging data, such as the depth targeted with the
Large Synoptic Survey Telescope (LSST), the ambiguous blend population is both
large ($\sim 14$%) and has a distribution of ellipticities that is different
from that of unblended objects in a way that will likely be important for the
weak lensing measurements. Most notably, for a limiting magnitude of $i \sim
27$ we find that ambiguous blending results in a ~14% increase in shear noise
(or ~12% decrease in the effective projected number density of lensed galaxies;
neff) due to 1) larger intrinsic ellipticity dispersion, 2) a scaling with the
galaxy number density $N_{gal}$ that is shallower than 1/$\sqrt{N_{gal}}$. For
the LSST Gold Sample ($i < 25.3$) there is a ~7% increase in shear noise (or
~7% decrease in $n_{eff}$). More importantly than these increases in the shear
noise, we find that the ellipticity distribution of ambiguous blends has an RMS
13% larger than that of non-blended galaxies. Given the need of future weak
lensing surveys to constrain the ellipticity distribution of galaxies to better
than a percent in order to mitigate cosmic shear multiplicative biases, the
different ellipticity distribution of ambiguous blends could be a dominant
systematic if unaccounted for.
