Cosmic Chance Superpositions: Largest Catalog of Quasar-Galaxy Pairs at a Projected Separation of $D \lesssim 20$ kpc

TL;DR

This work delivers the largest catalog of quasar–galaxy pairs at $D \lesssim 20$ kpc by combining absorption-agnostic and absorption-selected searches in SDSS-DR16 and DESI-EDR, yielding 1345 GOTOQs over $0.0045 \leq z \leq 1.0866$. Emission-line analyses show that hosts are normal star-forming disks with a Balmer-decrement implying $A_V \approx 0.53$ mag and a high Mg II covering fraction of $f_c = 98.2^{+1.3}_{-4.5}$% for $W_{2796} \geq 1$ Å at $D<20$ kpc, while a high-S/N stack reveals a statistical detection of the DIB $\lambda4428$ with $W_r = 0.055 \pm 0.011$ Å. The results indicate a continuous, gas- and dust-rich inner CGM extending from the disk–halo interface to the disk itself, with the inner regions showing saturation of Mg II and enhanced dust reddening. Together, these findings provide a robust, multi-wavelength template for studying gas flows, dust evolution, and organic chemistry in the inner CGM and motivate targeted follow-up with H I 21 cm and molecular absorption surveys.

Abstract

We present the largest catalogue to date of Galaxies On Top Of Quasars (GOTOQs), systems where the sightline to a background quasar passes directly through or very close to a foreground galaxy. Using $\approx$1.1 million quasar spectra from SDSS DR16 and the DESI Early Data Release, we identify 1345 unique GOTOQs over the redshift range $0.0045 \leq z \leq 1.09$, more than quadrupling the number previously known. The catalogue combines both absorption agnostic and absorption selected searches, enabling a statistically robust characterization of gas in galaxies at projected separations $D \lesssim 20$ kpc. The GOTOQ emission line ratios indicate that their host galaxies are predominantly normal, star forming disks with typical dust extinctions of $A_V \approx 0.5$ mag. We measure the MgII covering fraction at $D \lesssim 20$ kpc and find it to be remarkably high, $f_c = 98.2^{+1.3}_{-4.5}$ per cent, 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, $E(B-V) = 0.087$, is significantly higher than that of typical MgII absorbers, underscoring the dust rich nature of GOTOQ sightlines. From a high signal to noise composite spectrum, we report the first statistical detection of the diffuse interstellar band at $λ4428$ ($W_r = 0.055 \pm 0.011$ angstrom), revealing the presence of complex organic molecules at the disk halo interface. This catalogue provides a powerful reference sample for future multi wavelength studies of gas flows, cold gas, and dust evolution in the inner circumgalactic medium.

Figures (14)

• Figure 1: Examples of absorption-agnostic GOTOQs. Each row represents a different quasar observed with SDSS, BOSS, and DESI spectrographs, respectively, from top to bottom. In each horizontal panel: the left plot displays the nebular emission lines detected in the continuum-subtracted quasar spectra with the solid blue line representing the Gaussian fits to the nebular emission lines and the right plot shows a composite $griz$ band DESI-LIS image of the quasar field, with the quasar marked by a red star and the observing fiber outlined by a blue circle. Flux scales in the left panels are in the units of $\rm{ 10^{-17}\, ergs\, s^{-1}\, cm^{-2}\, \text{\AA}^{-1}}$.
• Figure 2: Examples of absorption-selected GOTOQs. Each row represents a different quasar observed with SDSS, BOSS, and DESI spectrographs, respectively, from top to bottom. In each horizontal panel: the left plot displays the intervening Mg ii absorption in a continuum-normalized spectrum; the middle plot highlights the nebular emission lines detected in the continuum-subtracted quasar spectra, corresponding to the Mg ii absorption with the solid blue line representing the Gaussian fits to the nebular emission lines; the right plot shows a composite $griz$ band DESI-LIS image of the quasar field, with the quasar marked by a red star and the observing fiber outlined by a blue circle. Flux scales in the middle panels are in the units of $\rm{ 10^{-17}\, ergs\, s^{-1}\, cm^{-2}\, \text{\AA}^{-1}}$.
• Figure 3: The redshift distribution of the absorption-agnostic (blue histogram) and absorption-selected (green histogram) GOTOQs. The distribution for the combined sample is depicted in gray.
• Figure 4: The kernel density estimations of [O ii] line luminosities (considering only detections and excluding upper limits) for the GOTOQs in our sample is shown from DESI (blue), BOSS (orange), and SDSS (green). For comparison, the [O ii] luminosity function of field galaxies at the median redshift of our GOTOQ sample, $z = 0.54$, is shown as a dashed purple curve. The brown vertical dot-dashed line marks the characteristic [O ii] luminosity ($L^\star_{\rm [O\, II]}$) at $z = 0.54$Comparat2016.
• Figure 5: The [O ii] line luminosities of the GOTOQs captured within the fibers of DESI (blue) SDSS (green) and BOSS (orange) spectrographs are plotted against their redshift. The solid gray line corresponds to the physical area a BOSS fiber covers as a function of redshift.