  We present new observations of the GOODS-N field obtained at 5.5 GHz with the
Karl G. Jansky Very Large Array (VLA). The central region of the field was
imaged to a median r.m.s. of 3 microJy/beam with a resolution of 0.5 arcsec.
From a 14-arcmin diameter region we extracted a sample of 94 radio sources with
signal-to-noise ratio greater than 5. Near-IR identifications are available for
about 88 percent of the radio sources. We used different multi-band diagnostics
to separate active galactic nuclei (AGN), both radiatively efficient and
inefficient, from star-forming galaxies. From our analysis, we find that about
80 percent of our radio-selected sample is AGN-dominated, with the fraction
raising to 92 percent when considering only the radio sources with redshift
>1.5. This large fraction of AGN-dominated radio sources at very low flux
densities (the median flux density at 5.5 GHz is 42 microJy), where
star-forming galaxies are expected to dominate, is somewhat surprising and at
odds with other results. Our interpretation is that both the frequency and
angular resolution of our radio observations strongly select against radio
sources whose brightness distribution is diffuse on scale of several kpc.
Indeed, we find that the median angular sizes of the AGN-dominated sources is
around 0.2-0.3 arcsec against 0.8 arcsec for star-forming galaxies. This
highlights the key role that high frequency radio observations can play in
pinpointing AGN-driven radio emission at microJy levels. This work is part of
the eMERGE legacy project.
