Where Do Quasars Live? DESI DR1 Constraints from PAC Measurements

Quasar environments connect the growth of supermassive black holes, active galactic nucleus feedback, and galaxy evolution. Small-scale quasar clustering probes the one-halo regime and can test whether quasar activity depends on central--satellite status. We seek to obtain precise small-scale measurements of quasar environments at $0.8<z<1.0$ and use them to constrain the quasar--halo connection, particularly the relative probability for satellite subhalos and central halos of the same halo accretion mass to host a quasar. We apply the Photometric Objects Around Cosmic Webs (PAC) method to DESI Data Release~1 quasars and photometric galaxies from the DESI Legacy Imaging Surveys DR9, measuring the excess projected surface density of neighbouring galaxies around quasars, $\bar{n}_{2}w_{\rm p}$, down to a stellar mass of $M_{\ast}=10^{10.80}M_{\odot}$ over $0.1<r_{\rm p}/(h^{-1}\,\mathrm{Mpc})<15$. we jointly model the excess surface-density measurements, the quasar and luminous red galaxy (LRG) autocorrelation functions, and the quasar--LRG cross-correlation. We interpret these measurements using an N-body simulation together with a stellar-to-halo mass relation, an explicit stellar-mass-incompleteness model, and a Gaussian quasar occupation as a function of halo accretion mass. The Gaussian quasar occupation peaks at $\log_{10}(M_{\rm acc}/h^{-1}M_{\odot})=12.88^{+0.02}_{-0.02}$ with width $\sigma_{\rm q}=0.51^{+0.02}_{-0.01}$, and the relative satellite-hosting parameter, $B$, defined as the quasar-hosting probability of a satellite subhalo relative to that of a central halo at fixed halo accretion mass, is $B=1.01^{+0.03}_{-0.03}$. Within the adopted model framework, quasars are consistent with being equally likely to reside in central halos and satellite subhalos at fixed halo accretion mass. PAC has strong potential to deliver precise small-scale measurements of quasar environments.