The Microjansky Sky at 8.4 GHz

We present the results from two deep radio integrations at 8.4 GHz using the Very Large Array. One of the fields, at 13h, +43° (SA 13 field), has an rms noise level of 1.49 μJy and is the deepest radio image yet made. Thirty-four sources in a complete sample were detected above 7.5 μJy, and 25 are optically identified to a limit of I = 25.8, using our deep Hubble Space Telescope and ground-based images. The radio sources are usually located within 0.″5 (typically 5 kpc) of a galaxy nucleus and generally have a diameter less than 2.″5. We have also analyzed a complete flux density–limited sample at 8.4 GHz of 89 sources from five deep radio surveys, including the Hubble deep and flanking fields, as well as the two new fields. Half of all the optical counterparts are with galaxies brighter than I = 23 mag, but 20% are fainter than I = 25.5 mag. There may be a small tendency for the microjansky radio sources to prefer multigalaxy systems. The distribution of the radio spectral index between 1.4 and 8.4 GHz peaks at α ≈ -0.75 (S ∼ ν+α), with a median value of -0.6. The average spectral index becomes steeper (lower values) for sources below 35 μJy and for sources identified with optical counterparts fainter than I = 25.5 mag. This correlation suggests that there is an increasing contribution from starburst galaxies compared to AGNs at lower radio flux densities and fainter optical counterparts. The differential radio count between 7.5 and 1000 μJy has a slope of -2.11 ± 0.13 and a surface density of 0.64 sources (arcmin)-2, with a flux density greater than 7.5 μJy.