Circumgalactic medium of quasar host galaxies at 0.4<z<0.8 probed by strong Mg II absorption

TL;DR

This study investigates whether the circumgalactic medium (CGM) around quasar host galaxies differs from that of normal galaxies in a narrowly defined redshift range $0.4 \le z \le 0.8$ and within $D < 100$ kpc, using strong Mg II absorption as the tracer. Leveraging 166 projected quasar pairs from SDSS and DESI with Voigt-profile measurements of $W_{2796}$, the authors compare the $W_{2796}$–$D$ relation and Mg II covering fraction $f_c$ to a redshift- and $D$-matched galaxy sample, focusing on $W_{2796} \ge 1$ Å. They find no significant differences in the $W_{2796}$–$D$ and $f_c$–$D$ distributions between quasar hosts and normal galaxies, with the main caveats stemming from sensitivity limits and the predominance of low-$L_{bol}$ quasars in the sample. After accounting for instrumental sensitivity, the intrinsic scatter in the $W_{2796}$–$D$ relation for galaxies remains larger than for quasars, though the two samples remain consistent within uncertainties; line-of-sight absorptions are less common than transverse ones, hinting at anisotropy in the CGM or radiation field. Overall, the results suggest that low-luminosity quasars at $0.4 \le z \le 0.8$ do not substantially alter the inner CGM compared to typical galaxies, underlining the need for future studies to include more luminous quasars, higher-SNR data, and a broader range of impact parameters to test AGN influence more robustly.

Abstract

Using a sample of 166 projected quasar pairs we investigate the influence of active galactic nuclei on the circumgalactic medium (CGM) of the quasar host galaxies probed using strong Mg II absorption (i.e., $W_{2796}\ge 1\dot{A}$) at impact parameters ($D$) $<$100 kpc. The foreground quasars are restricted to the redshift range $0.4 \leq z \leq 0.8$ and have median bolometric luminosity and stellar mass of $10^{45.1} erg~s^{-1}$and $10^{10.89} M_\odot$ respectively. We report detections of Mg II absorption in 29 cases towards the background quasar and in 4 cases along the line of sight to the foreground quasars. We do not find any difference in the distribution of $W_{2796}$ and covering fraction ($f_c$) as a function of $D$ between quasar host galaxies and control sample of normal galaxies. These results are different from what has been reported in the literature, possibly because: (i) our sample is restricted to a narrow redshift range, (ii) comparative analysis is carried out after matching the galaxy parameters, (iii) we focus mainly on strong Mg II absorption and (iv) our sample lacks foreground quasars with high bolometric luminosity (i.e., $L_{bol}>10^{45.5}$ erg s$^{-1}$). Future studies probing luminous foreground quasars, preferably at lower impact parameters and higher equivalent width sensitivity is needed to consolidate our findings.

Figures (10)

• Figure 1: Cumulative distribution of the $3\sigma$ limit of $W_{2796}$ in the spectrum of background quasars at the location of the foreground quasars. Vertical dashed line marks the rest equivalent limit of 1Å.
• Figure 2: Comparison of the bolometric luminosity of quasars in our sample with the full compiled sample from Johnson2015MNRAS.452.2553J, which includes data from Bowen2006ApJ...645L.105B, Farina2014MNRAS.441..886F, Prochaska2014ApJ...796..140P. We label this combined dataset as "Literature" in the figure legend. Top Panel: Shows the comparison with their full sample. Bottom Panel: Focuses on quasars within the same redshift and impact parameter ranges considered in our study. The vertical dashed line marks $log(L_{bol}/ergs^{-1}) = 45.5$, the division between the luminous and low luminosity quasars used in the literature sample.
• Figure 3: Top panel: Displays the velocity offset between the redshift of the foreground quasar and the Mg ii absorption redshift, highlighting a strong correlation between the two, except mean offset quasar reading. Bottom panel: Illustrates the same velocity offset for galaxies with $D < 100$ and $0.4 \le z \le 0.8$, providing further context for the observed trends.
• Figure 4: $W_{2796}$ vs. impact parameter (D) for foreground quasars. Violet filled circles indicate detections; open circles with arrows show upper limits. Galaxy detections are shown as faded yellow circles; upper limits as open orange circles. The solid black line with violet shading shows the quasar fit; the dashed black line with yellow shading shows the galaxy fit over the same redshift range.
• Figure 5: Figure shows the distributions of $\alpha$ ( top panel) and $\beta$( bottom panel) for various realizations of control sample of quasars matching in impact parameter of galaxy data with impact parameter tolerance of 10 kpc. The results indicate that both distributions are consistent with each other, suggesting no significant discrepancies in the parameter values despite the imposed selection criteria. The vertical dashed lines represent the $\alpha$ and $\beta$ values for the galaxy sample, while the shaded region indicates the associated error.