The environment of high-redshift AGN

Abstract: In order to understand the formation and evolution of local massive galaxies and to reveal the processes that engineered the tight correlations found between their supermassive black hole (SMBH) mass and bulge mass or velocity dispersion, the study of powerful, high-redshift active galactic nuclei (AGN) and their environment is crucial. Observations of high-z quasars and radio galaxies suggest that these AGN live in overdense environment surrounding by star-forming Ly&alpha-emitters and submillimeter galaxies. At smaller scales several high-z quasars and radio galaxies have gas-rich, star-forming companion galaxies, such as the subject of the appended paper, the system of SMM J04135+10277. This system consists of a SCUBA discovered type-1 quasar and a gas-rich companion galaxy at ~5" (~40 kpc) distance from the AGN position. The case of SMM J04135+10277 is particularly interesting, since the companion galaxy is associated with one of the most massive molecular gas reservoirs found in the high-z Universe (M_H2~10^(11) M_Sun), while the quasar seems to be gas-poor. To determine the properties of the companion galaxy, we construct and model its spectral energy distribution (SED). We find that the companion galaxy is a heavily dust-obscured (A_V &ge 2.8), star-forming galaxy, with a star formation rate of ~1000 M_Sun/yr. To test the uniqueness of this system and investigate the expected frequency of such quasar-star-forming galaxy pairs, we use the cosmological, semi-analytic model, GALFORM. We take the outputs of the simulation at z=2.8 to find quasar-companion pairs at different separations. We find that 22% of our simulated quasar sample have at least one companion galaxy in their <100 kpc environment and 0.3% have bright, star-forming companions (SFR>100 M_Sun/yr). To make a direct comparison between the model predictions and the case of SMM J04135+10277, we focus on systems with an integrated SFR of >500 M_Sun/yr inside the aperture and compute their relative gas content and SFR. We find that in 67% of these systems the molecular gas masses of the host galaxies of the quasars are lower compared to the integrated value of their companions at a distance of <50 kpc and the star formation is dominated by the companion galaxies and not the quasars' hosts. This means that the case of SMM J04135+10277 is representative of highly star-forming quasar-companion galaxy systems at z=2.8.