Completing the Census of AGN in GOODS-S/HUDF: New Ultra-Deep Radio
  Imaging and Predictions for JWST
  A global understanding of Active Galactic Nuclei (AGN) and their host
galaxies hinges on completing a census of AGN activity without selection biases
down to the low-luminosity regime. Toward that goal, we identify AGN within
faint radio populations at cosmic noon selected from new ultra-deep, high
resolution imaging from the Karl G. Jansky Very Large Array at 6 and 3 GHz.
These radio data are spatially coincident with the ultra-deep legacy surveys in
the GOODS-S/HUDF region, particularly the unparalleled Chandra 7 Ms X-ray
imaging. Combined, these datasets provide a unique basis for a thorough census
of AGN, allowing simultaneous identification via (1) high X-ray luminosity; (2)
hard X-ray spectra; (3) excess X-ray relative to 6 GHz; (4) mid-IR colors; (5)
SED fitting; (6) radio excess via the radio-infrared relation; (7) flat radio
spectra via multi-band radio; and (8) optical spectroscopy. We uncover AGN in
fully half our faint radio sample, indicating a source density of one AGN
arcmin$^{-2}$, with a similar number of radio-undetected AGN identified via
X-ray over the same area. Our radio-detected AGN are majority radio-quiet, with
radio emission consistent with being powered predominantly by star formation.
Nevertheless, we find AGN radio signatures in our sample: $\sim12\%$ with radio
excess indicating radio-loud activity and $\sim16\%$ of radio-quiet AGN
candidates with flat or inverted radio spectra. The latter is a lower limit,
pending our upcoming deeper 3 GHz survey. Finally, despite these extensive
datasets, this work is likely still missing heavily obscured AGN. We discuss in
detail this elusive population and the prospects for completing our AGN census
with JWST/MIRI.
