A Quasar Pair Sample Compiled from DESI DR1

Abstract

Interacting quasar pairs (QPs, either dual or binary) provide crucial insights into galaxy mergers, black hole growth, and large-scale structure formation. The current literature reports fewer than 200 spectroscopically confirmed QPs, highlighting the need for larger samples to enable statistically meaningful investigations. In this paper, we present a sample of 1,220 quasar pairs or candidates compiled from the DESI DR1 quasar sample. Among them, 145 systems have been previously reported. We visually classified the full sample using DESI Legacy Images and SPARCL spectra into three categories: QP (quasar pairs, N = 1020), QPC (quasar pair candidates, N = 142), LQC (lensed quasar candidates, N = 58). Within the LQC subset, we find an intriguing wide-separation ($\sim$7.15$^{\prime\prime}$) quadruply lensed quasar candidate. The redshift distribution of pair sample peaks at $z \sim 1$--$2.5$, with an overall pair fraction of $6.2^{+0.2}_{-0.2}\times10^{-4}$ (Poisson error) and a generally weak redshift dependence. 63.8\% of QPs have $|ΔV_r| < 600$\,km/s, suggesting dynamical associations. This sample offers a statistically meaningful dataset for future studies of quasar pairs, lensing events, and potentially merger-triggered or merger-induced SMBH growth across the merger sequence.

Figures (11)

• Figure 1: Flowchart of the quasar pair sample selection process.
• Figure 2: Example of a quasar pair, J0145+0024, discussed in Subsection \ref{['sec2.4.1']}.
• Figure 3: Example of a quasar-pair candidate, J0118$-$0104, discussed in Subsection \ref{['sec2.4.2']}. The image types of all panels are the same as in Figure \ref{['ex_QP']}. There is no obvious intervening lensing structure between the two quasars, and the emission lines of source A are significantly broader than those of source B. However, the components are separated by only 1.74$^{\prime\prime}$, comparable to the 1.5$^{\prime\prime}$ DESI fiber diameter, so the DESI spectra are likely affected by mutual fiber spillover 2018AA...610L...7H2023ApJ...945..167P. Given these caveats, we classify this system as QPC.
• Figure 4: Example of a lensed quasar candidate, J0941+0518, discussed in Subsection \ref{['sec2.4.3']}.
• Figure 5: Left: magnitude distributions of the full sample in the $g$-, $r$-, and $z$- bands, where the magnitudes of both components of each pair are included. The sample spans $\sim$17--24 mag, with most sources clustered around $\sim21-23$ mag. Right: angular separation distributions for the full sample and the QP sample. The excess at separations below $2^{\prime\prime}$ in the full sample mainly reflects the QPC selection at very small separations, while systems at $2-13\arcsec$ are uniformly distributed.