Small-scale double quasars discovered in the DECam Legacy Survey

TL;DR

This paper reports the discovery of four small-scale double quasars from the DECam Legacy Survey, including one new system, and analyzes their properties and distribution to inform future search strategies.

Contribution

It presents the identification and confirmation of four small-scale double quasars, including a newly discovered one, and analyzes their distribution and properties to enhance understanding of dual quasar populations.

Findings

Four double quasars identified, one newly discovered.

Distribution of dual quasars follows a power-law with index 1.45±0.48.

Steeper distribution than expected from dynamical friction.

Abstract

Dual quasars are precursors of binary supermassive black holes, which are important objects for gravitational wave study, galaxy evolution, and cosmology. I report four double quasars, one of which is newly discovered, with separations of 1$-$2 arcsec selected from the DECam Legacy Survey. The Gemini optical spectra confirm that both sources are quasars at the same redshifts. J0118$-$0104 and J0932+0722 are classified as dual quasars, whereas J0037+2058 could be either a dual quasar or a lensed quasar. I estimate the physical properties such as black hole masses and Eddington ratios for the four double quasars. The newly discovered system supplements the incomplete sample of dual quasars at $\sim$10 kpc at $z>0.5$. The results demonstrate that new high-quality imaging surveys with existing spectroscopic data could reveal additional small-scale double quasars. Combined with the dual quasars from the literature, I find a power-law index of 1.45$\pm$0.48 for the distribution of dual quasars as a function of separation between 5$-$20 kpc, steeper than that expected from dynamical friction, though it is likely due to the incomplete sample at $<$15 kpc. I also discuss the implications for future searches with new imaging surveys such as the Dark Energy Survey and the Legacy Survey of Space and Time.