Does black-hole growth depend fundamentally on host-galaxy compactness?
  Possible connections between central black-hole (BH) growth and host-galaxy
compactness have been found observationally, which may provide insight into
BH-galaxy coevolution: compact galaxies might have large amounts of gas in
their centers due to their high mass-to-size ratios, and simulations predict
that high central gas density can boost BH accretion. However, it is not yet
clear if BH growth is fundamentally related to the compactness of the host
galaxy, due to observational degeneracies between compactness, stellar mass
($M_\bigstar$), and star formation rate (SFR). To break these degeneracies, we
carry out systematic partial-correlation studies to investigate the dependence
of sample-averaged BH accretion rate ($\rm \overline{BHAR}$) on the compactness
of host galaxies, represented by the surface-mass density, $\Sigma_\rm e$, or
the projected central surface-mass density within 1 kpc, $\Sigma_1$. We utilize
8842 galaxies with H < 24.5 in the five CANDELS fields at z = 0.5-3. We find
that $\rm \overline{BHAR}$ does not significantly depend on compactness when
controlling for SFR or $M_\bigstar$ among bulge-dominated galaxies and galaxies
that are not dominated by bulges, respectively. However, when testing is
confined to star-forming galaxies at z = 0.5-1.5, we find that the $\rm
\overline{BHAR}$-$\Sigma_1$ relation is not simply a secondary manifestation of
a primary $\rm \overline{BHAR}$-$M_\bigstar$ relation, which may indicate a
link between BH growth and the gas density within the central 1 kpc of
galaxies.
