The Sloan Lens ACS Survey. XI. Beyond Hubble Resolution: Size, Luminosity, and Stellar Mass of Compact Lensed Galaxies at Intermediate Redshift

We exploit the strong lensing effect to explore the properties of intrinsically faint and compact galaxies at intermediate redshift (z _s ~= 0.4-0.8) at the highest possible resolution at optical wavelengths. Our sample consists of 46 strongly lensed emission line galaxies (ELGs) discovered by the Sloan Lens ACS Survey (SLACS). The galaxies have been imaged at high resolution with the Hubble Space Telescope (HST) in three bands (V_HST , I ₈₁₄, and H ₁₆₀), allowing us to infer their size, luminosity, and stellar mass using stellar population synthesis models. Lens modeling is performed using a new fast and robust code, KLENS, which we test extensively on real and synthetic non-lensed galaxies, and also on simulated galaxies multiply imaged by SLACS-like galaxy-scale lenses. Our tests show that our measurements of galaxy size, flux, and Sérsic index are robust and accurate, even for objects intrinsically smaller than the HST point-spread function. The median magnification is 8.8, with a long tail that extends to magnifications above 40. Modeling the SLACS sources reveals a population of galaxies with colors and Sérsic indices (median n ~ 1) consistent with the galaxies detected with HST in the Galaxy Evolution from Morphology and SEDs (GEMS) and Hubble Ultra Deep Field (HUDF) surveys, but that are (typically) ~2 mag fainter and ~5 times smaller in apparent size than GEMS and ~4 mag brighter than but similar in size to HUDF. The size-stellar-mass and size-luminosity relations for the SLACS sources are offset to smaller sizes with respect to both comparison samples. The closest analog are ultracompact ELGs identified by HST grism surveys. The lowest mass galaxies in our sample are comparable to the brightest Milky Way satellites in stellar mass (10⁷ M _sun) and have well-determined half-light radii of 0farcs05 (≈0.3 kpc).