Statistics of Galactic-scale Quasar Pairs at Cosmic Noon

Abstract

The statistics of galactic-scale quasar pairs can elucidate our understanding of the dynamical evolution of supermassive black hole (SMBH) pairs, the duty cycles of quasar activity in mergers, or even the nature of dark matter, but they have been challenging to measure at cosmic noon, the prime epoch of massive galaxy and SMBH formation. Here we measure a double quasar fraction of ∼6.2 ± 0.5 × 10−4 integrated over ∼0.″3–3″ separations (projected physical separations of ∼3–30 kpc at z ∼ 2) in luminous (L bol > 1045.8 erg s−1) unobscured quasars at 1.5 < z < 3.5 using Gaia EDR3-resolved pairs around SDSS DR16 quasars. The measurement was based on a sample of 60 Gaia-resolved double quasars (out of 487 Gaia pairs dominated by quasar+star superpositions) at these separations, corrected for pair completeness in Gaia, which we quantify as functions of pair separation, magnitude of the primary, and magnitude contrast. The double quasar fraction increases toward smaller separations by a factor of ∼5 over these scales. The division between physical quasar pairs and lensed quasars in our sample is currently unknown, requiring dedicated follow-up observations (in particular, deep, subarcsecond-resolution IR imaging for the closest pairs). Intriguingly, at this point, the observed pair statistics are in rough agreement with theoretical predictions both for the lensed quasar population in mock catalogs and for dual quasars in cosmological hydrodynamic simulations. Upcoming wide-field imaging/spectroscopic space missions such as Euclid, CSST, and Roman, combined with targeted follow-up observations, will conclusively measure the abundances and host galaxy properties of galactic-scale quasar pairs, offset AGNs, and subarcsecond lensed quasars across cosmic time.

A Candidate Dual QSO at Cosmic Noon

“…However, only 17 red QSOs have been imaged with HST at z ∼ 2 (11 F2M quasars reported in Glikman et al 2015, and6 W2M QSOs, which include W2M 1220). This fraction of 0.06 (1/17) would be 1 order of magnitude higher than that found for the luminous, unobscured QSOs investigated in Shen et al (2023). Given that red QSOs are known to be hosted by major mergers, this population may be the most likely for finding dual QSOs, although with only a single system we cannot draw broad conclusions.…”

“…The serendipitous discovery of a QSO pair in HST imaging of a red QSO raises the question of their frequency compared to unreddened QSOs. Shen et al (2023) investigate statistically the incidence of QSO pairs using Gaia detections of known SDSS QSOs with L bol > 10 45.8 erg s −1 at 1.5 < z < 3.5 with separations of 0 4-3″ and find an integrated pair fraction of ∼6 × 10 −4 . Assuming this fraction is constant at the 0 26 separation of W2M 1220 and is a factor of ∼10 higher given its lower luminosity (L bol ∼ 10 44.8 erg s −1 ), the pair fraction is estimated to be ∼10 −3 Shen et al (2023).…”

“…Shen et al (2023) investigate statistically the incidence of QSO pairs using Gaia detections of known SDSS QSOs with L bol > 10 45.8 erg s −1 at 1.5 < z < 3.5 with separations of 0 4-3″ and find an integrated pair fraction of ∼6 × 10 −4 . Assuming this fraction is constant at the 0 26 separation of W2M 1220 and is a factor of ∼10 higher given its lower luminosity (L bol ∼ 10 44.8 erg s −1 ), the pair fraction is estimated to be ∼10 −3 Shen et al (2023). However, only 17 red QSOs have been imaged with HST at z ∼ 2 (11 F2M quasars reported in Glikman et al 2015, and6 W2M QSOs, which include W2M 1220).…”

A Survey for High-redshift Gravitationally Lensed Quasars and Close Quasar Pairs. I. The Discoveries of an Intermediately Lensed Quasar and a Kiloparsec-scale Quasar Pair at z ∼ 5

“…The best-fit image models and the residuals provided by Tractor are useful in identifying objects with close companions. Specifically, many lensed quasars and kiloparsec-scale quasar pairs have separations of Δθ  1″ (e.g., Shen et al 2023;Yue et al 2022a) and are blended in ground-based images. When fitted as a PSF or Sérsic profile, these objects will exhibit large image-fitting residuals.…”

“…Comparison between detailed observations of close quasar pairs (e.g., Green et al 2010) and zoom-in hydrodynamical simulations of galaxy mergers (e.g., Capelo et al 2017) helps us to understand how galaxy mergers trigger quasar activities. The statistics of quasar pairs (e.g., the fraction of close pairs among all quasars) encode information about the coevolution of SMBHs, their host galaxies, and their close environments (e.g., Steinborn et al 2016;Volonteri et al 2016;Shen et al 2023).…”