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.