Environment of SDSS quasars at $z=0.4$$-$$1.0$ explored by Subaru HSC

TL;DR

This study leverages wide, deep HSC-SSP imaging to quantify the local galaxy environments of SDSS quasars at $0.4 < z < 1.0$ using a $k$-nearest neighbor density approach. By comparing 1,912 quasars to a meticulously matched galaxy sample, the authors find quasars reside in systematically lower-density environments on scales of a few hundred kiloparsecs, with no detectable correlation between local density and quasar properties such as $M_{BH}$ or $R_{Edd}$. The redshift evolution of galaxy densities drives overall trends, but the quasar–matched difference persists after accounting for redshift. The results favor merger-driven triggering as a plausible quasar fueling mechanism, while secular processes remain possible, and they find no significant environmental distinction between radio-detected and radio-undetected quasars. The work highlights the value of future spectroscopic surveys (e.g., PFS, DESI, MOONS) to refine environmental measurements at even smaller scales.

Abstract

The relationship between quasars and their galaxy environment is important for understanding the evolution of galaxies and supermassive black holes, but it is not fully understood. We perform a wide and deep exploration of the environment of quasars at $0.4 < z < 1.0$ using the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) survey. We investigate the environment of the 1,912 spectroscopically selected quasars from the Sloan Digital Sky Survey (SDSS), using photometrically selected galaxies from the HSC-SSP data, over an area of 505 deg${^{2}}$. The quasar environment is compared to the environment of matched galaxies with similar stellar mass and redshift. We employ the $k$-nearest neighbor method to define the local galaxy number density for both the quasars and the matched galaxies at a scale of a few hundred kpc. As a result, we find that the number density of galaxies around SDSS quasars is lower than that of the matched galaxies by $\sim$11--$20\%$. We also investigate possible correlations between the local galaxy number densities and the quasar properties such as black hole mass and Eddington ratio. As a result, no correlation is found between the local galaxy number densities and these properties of quasars. These results suggest that the quasar activity is not triggered by the high number density of surrounding galaxies at the scale of a few hundred kpc.

Figures (10)

• Figure 1: Distributions of the $i$-band absolute magnitudes of the parent galaxy sample with each redshifts. Black, pink, blue and skyblue histograms denote the galaxy number in the redshift bin of the $0.2 < z \leq 0.6$, $0.6 < z \leq 0.8$, $0.8 < z \leq 1.0$, and $1.0 < z \leq 1.2$, respectively. The dotted line indicates $-21.8$ mag.
• Figure 2: Distributions of redshift (left), stellar mass (center), and $i$-band absolute magnitude (right) for the parent galaxy sample (gray) and the galaxy sample for density measurement (blue).
• Figure 3: Distributions of the redshift (leftmost panel), $i$-band absolute magnitude (left-center panel), SMBH mass (right-center panel), and Eddington ratio (rightmost panel), for the parent quasar sample (dark gray, 40,664 objects) and quasar sample (light gray, 1,912 objects).
• Figure 4: Distributions of the redshift (left panel) and the stellar mass (right panel) for quasar sample (blue bar) and matched galaxy sample (thick red line).
• Figure 5: Histogram of the ratio of the photometric error and ($1 + z$) of the galaxy sample for the density measurement. The dashed line denotes to $z_{\mathrm{err}}/(1+z) = 0.1$.