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

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.

💡 Research Summary

In this paper the authors investigate the circumgalactic medium (CGM) of quasar host galaxies at intermediate redshifts (0.4 ≤ z ≤ 0.8) by exploiting strong Mg II λ2796,2803 absorption (rest‑frame equivalent width W2796 ≥ 1 Å) seen in background quasars that lie within 100 kpc projected separation of a foreground quasar. The sample is drawn from the Sloan Digital Sky Survey Data Release 16 Quasar catalog (DR16Q) and the DESI Early Data Release, yielding a total of 166 projected quasar pairs after applying strict selection criteria: (i) transverse separation < 100 kpc at the foreground redshift, (ii) foreground redshift confined to 0.4–0.8 so that Mg II remains in the optical window, (iii) a velocity offset > 6000 km s⁻¹ between the two quasars to avoid physically associated pairs, and (iv) sufficient signal‑to‑noise (S/N) to detect W2796 ≥ 1 Å at > 3σ. After these cuts, 63 % of the background spectra meet the S/N requirement.

Mg II absorption is identified by visual inspection of the background spectra within ±1500 km s⁻¹ of the foreground quasar redshift, confirmed by the expected doublet ratio and, when possible, the presence of associated Fe II or Mg I lines. The authors find 29 detections (≈ 17 % of the usable sightlines) of strong Mg II in the background quasars, of which 18 are > 3σ measurements; the remaining have 2–3σ significance. Along the line of sight to the foreground quasars themselves, only 4 detections are made out of 166 (≈ 2 %). Absorption redshifts and equivalent widths are measured with Voigt profile fitting (VoigtFit) and, where S/N is low, single‑line fits are employed.

To assess whether quasar hosts possess a CGM distinct from that of normal galaxies, the authors construct a control sample from the Mg II absorber–galaxy studies of Nielsen et al. (2013) and Huang et al. (2021), which span the same redshift interval and impact‑parameter range. They compare the distribution of W2796 versus impact parameter D, as well as the covering fraction f_c (the fraction of sightlines with W2796 ≥ 1 Å). Both the quasar‑host and control samples show a declining trend of W2796 with increasing D and similar f_c values of ≈ 0.3–0.4 at D < 100 kpc. A Kolmogorov–Smirnov (KS) test on the velocity offset distributions initially yields a significant difference (p = 0.002), but after correcting for a systematic redshift offset of the foreground quasars (μ ≈ 66 km s⁻¹), the KS statistic drops (p = 0.53), indicating no robust distinction.

The main conclusion is that, for quasars with median bolometric luminosity log L_bol ≈ 45.1 erg s⁻¹ and stellar mass log M_* ≈ 10.89 M_⊙, the CGM traced by strong Mg II is statistically indistinguishable from that of non‑active galaxies. This result contrasts with earlier works (e.g., Johnson et al. 2015; Farina et al. 2014) that reported enhanced Mg II covering fractions and stronger absorption around quasars, especially at higher luminosities.

The authors attribute the discrepancy to four factors: (1) their sample is confined to a narrow redshift slice, minimizing evolutionary effects but limiting generality; (2) they match the control galaxies in stellar mass, star‑formation rate, and redshift, thereby removing confounding correlations; (3) the analysis focuses exclusively on strong Mg II absorbers, potentially missing subtler differences that appear in weaker lines or other ions; and (4) the foreground quasars are predominantly low‑luminosity (only ~20 % exceed log L_bol = 45.5), so any luminosity‑dependent feedback on the CGM may be under‑sampled.

Limitations of the study include the relatively modest number of detections, the restriction to impact parameters > 30 kpc (the inner CGM remains poorly probed), and the exclusive reliance on Mg II, which traces cool (T ≈ 10⁴ K) gas. Systematic uncertainties in quasar redshifts also affect velocity‑offset analyses. The authors suggest several avenues for future work: expanding the sample to include more luminous quasars (L_bol > 10^45.5 erg s⁻¹), probing smaller impact parameters (< 50 kpc), obtaining deeper spectra to reach W2796 ≈ 0.3 Å (i.e., weaker absorbers), and incorporating additional ions such as C IV and O VI to map the multi‑phase CGM. Complementary IFU observations of nebular emission (e.g., Ly α,