On The Scaling Relations Between Black Holes And The Galaxy/Bulge Mass And Their Connections With The Black Hole Occupation Fraction

Abstract

We investigate the black hole (BH)--host galaxy scaling relations predicted by the semianalytic model \texttt{FEGA25}, focusing on their dependence on BH mass threshold, galaxy morphology, and redshift, and on their connection with the BH occupation fraction (BHOF). Using the YS200 cosmological simulation as our reference volume, we study the $M_{\rm BH}$--$M_{\star}$ and $M_{\rm BH}$--$M_{\rm bulge}$ relations for different BH mass selections and compare their redshift evolution with the hydrodynamical zoom-in simulation \texttt{NewCluster}. We find that BH mass cuts primarily affect the low-mass regime, while the selected relations progressively converge toward the intrinsic no-cut relations at higher host masses. The $M_{\rm BH}$--$M_{\star}$ relation also depends systematically on galaxy morphology, with increasingly bulge-dominated systems preferentially occupying higher stellar masses. Over $z=0$--$3$, FEGA25 shows no clear systematic evolution of either scaling relation, consistent with a broadly preserved statistical connection between BH and host-galaxy growth. NewCluster shows no clear monotonic evolution either, although it exhibits larger scatter, particularly toward high host masses. We further show that BHOF and BH--host scaling relations encode complementary information. A low BHOF generally corresponds to a stronger selection-induced offset, but the mapping is not unique: substantially different occupation fractions can yield similar median scaling relations, and at fixed BHOF the bias depends on the host property and underlying BH mass distribution. At $z=0$, \texttt{FEGA25} is consistent with available observational constraints for both stellar- and bulge-mass relations, while predicting increased scatter and bending toward lower host masses.