Relations Between Environments of Quasars and Galaxy Formation

TL;DR

This paper models the environments of quasars within a hierarchical galaxy formation framework, predicting their distribution, bias, and surrounding galaxy counts across redshifts, and compares these with observations to refine quasar formation theories.

Contribution

It introduces a semi-analytic model linking galaxy and quasar formation, reproducing key observed relations and predicting quasar environments and distributions over cosmic time.

Findings

Quasars are mostly found in galaxy groups at low redshift.

Quasars occupy a range of environments from small groups to clusters at higher redshifts.

The model's predictions can be tested against future observations to constrain quasar formation scenarios.

Abstract

We investigate the environments of quasars such as number distribution of galaxies using a semi-analytic model which includes both galaxy and quasar formations based on the hierarchical clustering scenario. We assume that a supermassive black hole is fueled by accretion of cold gas and that it is a source of quasar activity during a major merger of the quasar host galaxy with another galaxy. This major merger causes spheroid formation of the host galaxy. Our model can reproduce not only general form of the galaxy luminosity functions in the local Universe but also the observed relation between a supermassive black hole mass and a spheroid luminosity, the present black hole mass function and the quasar luminosity functions at different redshifts. Using this model, we predict the mean number of quasars per halo, bias parameter of quasars and the probability distribution of the number of galaxies around quasars. In our model, analysis of the mean number of quasars per halo shows that the spatial distribution of galaxies is different from that of quasars. Furthermore, we found from calculation of the probability distribution of galaxy numbers that at $0.2 \lesssim z \lesssim 0.5$, most quasars are likely to reside in galaxy groups. On the other hand, at $1 \lesssim z \lesssim 2$ most quasars seem to reside in more varied environments than at a lower redshift; quasars reside in environments ranging from small groups of galaxies to clusters of galaxies. Comparing these predictions with observations in future will enable us to constrain our quasar formation model.