Halo Occupation Distribution of Quasars: Dependence on Luminosity, Redshift, Black Hole Mass and Feedback Modes

TL;DR

This study uses cosmological simulations to analyze how quasar occupation in halos depends on redshift, luminosity, black hole mass, and feedback modes, revealing quenching effects and biases in host halo mass reconstructions.

Contribution

It provides new insights into quasar halo occupation distribution dependencies and highlights biases in observational modeling if quenching effects are ignored.

Findings

Quenching occurs at halo masses around 10^13 M_sun, affecting occupation distribution.

Significant bias in host halo mass estimates if quenching is not modeled.

Satellite quasar fraction increases from high to low redshift, reaching 20-40%.

Abstract

We use cosmological hydrodynamic simulations (IllustrisTNG and SIMBA) to explore the redshift, luminosity, and black hole mass dependence of the quasar halo occupation distribution (HOD). In both simulations, we find that the quasar activity is quenched at a characteristic halo mass ($\sim 10^{13} M_{\odot}$) scale affecting the nature of its occupation distribution function. We note that the quenching is more pronounced at low redshifts for quasars selected through a luminosity threshold. We show that a very significant bias (a factor of $\sim 10-50$ in the central occupation and $\sim 10-70\%$ in the satellite occupation fraction) is introduced in the reconstruction of quasar host halo mass distributions from the observed two-point-correlation function, if the HOD modeling does not account for the quenching effect in the central occupation function. While there is strong suppression of the occupation fraction of central quasars, the satellite occupation still follows a power-law like behavior. Our results show that the global satellite fraction of quasars increases monotonically from high to low redshifts, with $20-40 \%$ of the quasars being satellite at intermediate redshifts, consistent with previous clustering based estimates. In addition, our study reveals that while the occupation function of quasars depends on redshift, luminosity, and feedback modes, there is hardly any evolution in the supermassive black hole (SMBH; mass-selected sample) occupation. The SMBH HOD over the entire parameter space is well-modeled by a power-law and a step function similar to what has been found for galaxies and low-luminosity active galactic nuclei.