Relations between Galaxy Formation and the Environments of Quasars

We investigate the environments of quasars such as number distribution ofgalaxies using a semi-analytic model which includes both galaxy and quasarformations based on the hierarchical clustering scenario. We assume that asupermassive black hole is fueled by accretion of cold gas and that it is asource of quasar activity during a major merger of the quasar host galaxy withanother galaxy. This major merger causes spheroid formation of the host galaxy.Our model can reproduce not only general form of the galaxy luminosityfunctions in the local Universe but also the observed relation between asupermassive black hole mass and a spheroid luminosity, the present black holemass function and the quasar luminosity functions at different redshifts. Usingthis model, we predict the mean number of quasars per halo, bias parameter ofquasars and the probability distribution of the number of galaxies aroundquasars. In our model, analysis of the mean number of quasars per halo showsthat the spatial distribution of galaxies is different from that of quasars.Furthermore, we found from calculation of the probability distribution ofgalaxy numbers that at $0.2 \lesssim z \lesssim 0.5$, most quasars are likelyto 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 lowerredshift; quasars reside in environments ranging from small groups of galaxiesto clusters of galaxies. Comparing these predictions with observations infuture will enable us to constrain our quasar formation model.Comment: Revised version accepted by PASJ; 20 pages with 8 figure