The ATLAS 5.5 GHz survey of the extended Chandra Deep Field South: catalogue, source counts and spectral indices

Star forming galaxies are thought to dominate the sub-mJy radio population, but recent work has shown that low luminosity AGN can still make a significant contribution to the faint radio source population. Spectral indices are an important tool for understanding the emission mechanism of the faint radio sources. We have observed the extended Chandra Deep Field South at 5.5 GHz using a mosaic of 42 pointings with the Australia Telescope Compact Array (ATCA). Our image reaches an almost uniform sensitivity of ∼12 µJy rms over 0.25 deg2 with a restoring beam of 4.9 × 2.0 arcsec, making it one of the deepest 6cm surveys to date. We present the 5.5 GHz catalogue and source counts from this field. We take advantage of the large amounts of ancillary data in this field to study the 1.4 to 5.5 GHz spectral indices of the sub-mJy population. For the full 5.5 GHz selected sample we find a flat median spectral index, αmed = −0.40, which is consistent with previous results. However, the spectral index appears to steepen at the faintest flux density levels (S5.5GHz < 0.1 mJy), where αmed = −0.68. We performed stacking analysis of the faint 1.4 GH