Linking galaxy structural properties and star formation activity to
  black hole activity with IllustrisTNG
  We study the connection between active galactic nuclei (AGN) and their host
galaxies through cosmic time in the large-scale cosmological IllustrisTNG
simulations. We first compare BH properties, i.e. the hard X-ray BH luminosity
function, AGN galaxy occupation fraction, and distribution of Eddington ratios,
to available observational constraints. The simulations produce a population of
BHs in good agreement with observations, but we note an excess of faint AGN in
hard X-ray (L_x ~ 10^{43-44} erg/s), and a lower number of bright AGN
(L_x>10^{44} erg/s), a conclusion that varies quantitatively but not
qualitatively with BH luminosity estimation method. The lower Eddington ratios
of the 10^{9} Msun BHs compared to observations suggest that AGN feedback may
be too efficient in this regime. We study galaxy star formation activity and
structural properties, and design sample-dependent criteria to identify
different galaxy types (star-forming/quiescent, extended/compact) that we apply
both to the simulations and observations from the candels fields. We analyze
how the simulated and observed galaxies populate the specific star formation
rate - stellar mass surface density diagram. A large fraction of the z=0
M_{star}>10^{11} Msun quiescent galaxies first experienced a compaction phase
(i.e. reduction of galaxy size) while still forming stars, and then a quenching
event. We measure the dependence of AGN fraction on galaxies' locations in this
diagram. After correcting the simulations with a redshift and AGN
luminosity-dependent model for AGN obscuration, we find good qualitative and
quantitative agreement with observations. The AGN fraction is the highest among
compact star-forming galaxies (16-20% at z~1.5-2), and the lowest among compact
quiescent galaxies (6-10% at z~1.5-2).
