High Sensitivity Radio Surveys

Murphy et al. present initial results on a flux-limited sample of galaxies from pilot observations aimed at mapping the entire Great Observatories Origins Deep Survey-North (GOODS-N) field at 10 GHz. The GOODS-N field covers 160 arcmin² centered on the Hubble Deep Field–North and in unrivaled in terms of its ancillary data, including extremely deep Chandra, HST, Spitzer and Herschel observations, deep ground-based imaging from U through K-band, ~3500 spectroscopic redshifts acquired with 8-10m class telescopes, and deep 1.4 GHz observations.

The new VLA X-band data (8-12 GHz) presented by Murphy and his colleagues feature an r.m.s. sensitivity of 0.5 micro-Jansky and a resolution of 0.2 arcsec. This resolution is well-matched to the HST optical and infrared data from GOODS, and delivers a physical resolution of 1.9 kiloparsecs or less at any redshift. These new 10 GHz radio data provide an extinction-free view of the morphologies of dusty starburst galaxies that dominate the cosmic star formation activity between z = 1 and 3. In this redshift range, 10 GHz observations sample ~20 – 40 GHz in the source frame and provide accurate star formation rates for comparison with other diagnostics in the GOODS ancillary data, such as the far ultraviolet continuum, Ha, and [O III] 5007 Å.

These 10 GHz data provide accurate measurements of the radio sizes of high redshift star-forming galaxies. The observed median size is ~ 1.2 kiloparsecs, which corresponds to the dense, star-forming regions as also seen in dust continuum emission. The measured spectral indices of the detected galaxies are flatter than expected, implying a ~50% contribution to the rest from 20 GHz luminosity by free-free thermal emission. This important result opens a new, powerful means to obtain star formation rates for high redshift galaxies that are unbiased by dust.

Image: [Left] A source detected in the 10 GHz Jansky VLA pilot observations. [Center, Right] HST Advanced Camera System z-band and Wide Field Camera 3 F160W images, respectively, with 10 GHz VLA contours overlaid. Each panel is 5 x 5 arcsec. These data illustrate the survey’s ability to resolve the two cores in this merging galaxy pair at z = 1.2234.

Publication: Eric J. Murphy (NRAO, IPAC) et al., The GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-forming μJY Radio Sources, 2017, Astrophysical Journal, 839, 35.