Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. I. Method and Candidates

Abstract

Quasar pairs, a special subclass of galaxy pairs, are valuable in the investigation of quasar interaction, clustering, co-evolution between the two quasars' host galaxies, the growth of supermassive black holes, as well as the formation and evolution of galaxies. However, quasar pairs on kpc scales are observationally rare. The scarcity of available samples hindered the deeper exploration and statistics of these objects. In this work, we apply an astrometric method to systematically search for quasar candidates within a transverse distance of 100 kpc to known quasars in the Million Quasar Catalog. These candidates are \textit{Gaia} sources with proper motions and parallaxes that are consistent with zero. Visual inspection of the sample was performed to remove the contamination of crowded stellar fields and nearby galaxies. A total of 4\,112 quasar pair candidates were isolated, with a median member separation of 8.81{\arcsec}, a median \textit{Gaia} $G$-band magnitude of 20.52, and a median redshift of 1.61. Our catalog was compared with three major candidate quasar pair catalogs and identified 3\,984 new quasar pair candidates previously uncataloged in the three catalogs. Several interesting quasar pair candidates are highlighted and discussed. We also briefly discussed our quasar selection and several techniques for improving the success rate of quasar pair selection. Extensive spectroscopic follow-up campaigns are being carried out to validate their astrophysical nature.

Figures (14)

• Figure 1: Hammer-Aitoff equatorial projection of sky region binned (top panel) and Gaia$G$-band magnitude binned (bottom panel) reference samples. The blue scatters in the top and bottom panels represent the GoodQSO_Sky and GoodQSO_Mag, respectively. The red scatters in the top and bottom panels represent the GoodSTAR_Sky and GoodSTAR_Mag, respectively.
• Figure 2: log $f_{\textsl{0pm}}$ vs. log $f_{\textsl{0plx}}$ distributions of the Good_Sky (left panel) and Good_Mag (right panel), along with the quasar selection regions of the adopted astrometric criteria. The left panel shows the distributions of the sky-unbinned GoodQSO_Sky (blue points) and GoodSTAR_Sky (red points), while the right panel displays the corresponding distributions for the magnitude-unbinned GoodQSO_Mag (blue) and GoodSTAR_Mag (red). The black dotted dashed lines represent the $f_{\textsl{0pm}}$ criteria, while the dotted ones represent the $f_{\textsl{0plx}}$ criteria. All the histograms are normalized as probability density distributions (PDFs). Due to the long-tailed distributions of stars in log $f_{\textsl{0pm}}$ and log $f_{\textsl{0plx}}$, i.e., many stars fall outside the plot range, both subplots display only small portions of the entire distributions.
• Figure 3: Same as the left panel of Figure \ref{['am_cria_skymag_unbinned']}, but for the Good_Sky at different Galactic latitude bins.
• Figure 4: log $f_{\textsl{0plx}}$ vs. Gaia$G$-band magnitude (top row) and log $f_{\textsl{0pm}}$ vs. Gaia$G$-band magnitude (bottom row) distributions of the GoodQSO_Mag (blue) and GoodSTAR_Mag (red). The color bar in each subplot represents the number density of the distribution. Note: the color map in each subplot follows its own color bar, and thus, identical colors across different subplots do not represent equivalent number densities. All the subplots display only small parts of the whole distributions to exhibit the critical data region.
• Figure 5: Similar to Figure \ref{['am_cria_skybinned']}, but for the Good_Mag at different Gaia$G$-band magnitude bins.