The VLA Frontier Field Survey: a comparison of the radio and UV/optical size of 0.3 z 3 star-forming galaxies

To investigate the growth history of galaxies, we measure the rest-frame radio, ultraviolet (UV), and optical sizes of 98 radio-selected, star-forming galaxies (SFGs) distributed over 0.3 ~ z ~ 3 and median stellar mass of log(M_*/M_{Sun}) ~ 10.4. We compare the size of galaxy stellar disks, traced by rest-frame optical emission, relative to the overall extent of star formation activity that is traced by radio continuum emission. Galaxies in our sample are identified in three Hubble Frontier Fields: MACS J0416.1-2403, MACS J0717.5+3745, and MACS J1149.5+2223. Radio continuum sizes are derived from 3 GHz and 6 GHz radio images (~ 0.6 resolution, ~ 0.9 microJy beam^{-1} noise level) from the Karl G. Jansky Very Large Array. Rest-frame UV and optical sizes are derived using observations from the Hubble Space Telescope and the ACS and WFC3 instruments. Around 13% of galaxies in our sample are moderately lensed with a magnification factor magnification 2, while the rest of the galaxies have a median magnification ~ 1.1. We find no clear dependence between the 3GHz radio size and stellar mass of SFGs, which contrasts with the positive correlation between the UV/optical size and stellar mass of galaxies. Focusing on SFGs with log(M_*/M_{Sun}) 10, we find that the radio/UV/optical emission tends to be more compact in galaxies with high star-formation rates (SFR ~ 100 M_{Sun} yr^{-1}), suggesting that the production of stars in the most active SFGs preferentially occurs within a central, compact starburst. We also find that the physical radio/UV/optical size of SFGs with log(M_*/M_{Sun}) 10 increases by a factor of 1.5--2 from z ~ 3 to z ~0.3, yet the radio emission remains two-to-three times more compact than that from the UV/optical. These findings indicate that these massive SFGs at 0.3 ~ z ~ 3 tend to harbor centrally enhanced star formation activity relative to their outer-disks.