2007ApJ...671.1196M

We combine high-resolution images in four optical/infrared bands, obtained with the laser guide star adaptive optics (LGSAO) system on the Keck telescope and with the Hubble Space Telescope (HST), to study the gravitational lens system SDSS J0737+3216 (lens redshift 0.3223, source redshift 0.5812). We show that (under favorable observing conditions) ground-based images are comparable to those obtained with HST in terms of precision in the determination of the parameters of both the lens mass distribution and the background source. We also quantify the systematic errors associated with both the incomplete knowledge of the PSF and the uncertain process of lens galaxy light removal and find that similar accuracy can be achieved with Keck LGSAO as with HST. We then exploit this well-calibrated combination of optical and gravitational telescopes to perform a multiwavelength study of the source galaxy at 0.01" effective resolution. We find the Sérsic index to be indicative of a disklike object, but the measured half-light radius (r_e=0.59±0.007 stat±0.1 sys kpc) and stellar mass (M^*=2.0±1.0 stat±0.8 sysx10⁹ M_☉) place it more than 3 σ away from the local disk size-mass relation. The SDSS J0737+3216 source has the characteristics of the most compact faint blue galaxies studied and comparable size and mass to dwarf early-type galaxies in the local universe. With the aid of gravitational telescopes to measure individual objects' brightness profiles to 10% accuracy, the study of the high-redshift size-mass relation may be extended by an order of magnitude or more beyond existing surveys at the low-mass end, thus providing a new observational test of galaxy formation models.