Cosmic chance superpositions: Largest catalog of quasar-galaxy pairs at a projected separation of D ≲ 20 kpc

Abstract

We present the largest catalogue to date of Galaxies On Top Of Quasars (GOTOQs)---systems where the sight line to a background quasar passes directly through or very close to a foreground galaxy. Using ~1.1 million quasar spectra from the SDSS-DR16 and DESI Early Data Release, we identify 1345 unique GOTOQs over 0.0045 <= z <= 1.09, more than quadrupling the number previously known. The catalogue combines both absorption-agnostic and absorption-selected searches, enabling a robust characterisation of gas in galaxies at projected separations of D ≲ 20 kpc. The GOTOQ emission-line ratios indicate that their host galaxies are predominantly normal, star-forming disks with typical dust extinctions of AV ≃ 0.5 mag. We measure the Mg II covering fraction at D ≲ 20 kpc and find it to be remarkably high, fc=98.2-4.5+1.3 %, indicating that these systems trace gas-rich, metal-enriched regions at the disk-halo interface. The median colour excess towards the quasar line of sight of E(B - V) = 0.087 is significantly higher than that of typical Mg II absorbers, underscoring the dust-rich nature of GOTOQ sight lines. From a high signal-to-noise composite spectrum, we report the first statistical detection of the diffuse interstellar band at λ4428 (Wr = 0.055 ± 0.011 Å) and at λ5780 (Wr = 0.051 ± 0.012 Å), revealing the presence of complex organic molecules at the disk-halo interface. This catalogue provides a powerful reference sample for future multiwavelength studies of gas flows, cold gas, and dust evolution in the inner circumgalactic medium.