HST Confirms Sub-5 kpc Dual Quasar Pairs at Cosmic Noon

TL;DR

This study leverages a novel HST program that couples high-resolution WFC3/IR imaging with slitless spectroscopy to resolve and spectroscopically confirm two sub-arcsecond dual quasars at $z>1$, probing the sub-5 kpc regime during cosmic noon. From a pool of 59,025 SDSS quasars in the HSC footprint, 11 high-priority candidates were followed up, resulting in two robust detections: SDSS J1625+4309 at $z=1.647$ with a $4.7$ kpc separation and SDSS J0229$-$0514 at $z=3.174$ with a $3.2$ kpc separation. Detailed 2D image decomposition reveals distinct host morphologies (a disturbed, extended host for J1625+4309 and a compact host for J0229$-$0514), indicating different merger stages. Black hole masses are estimated via virial methods, yielding $M_{ m BH} \sim 10^{8.6-8.7}$ M$_\odot$ and $10^{8.1-8.3}$ M$_\odot$ for the two systems, with LOS velocity offsets around $0.7-1.0\times10^{3}$ km s$^{-1}$. These results expand the census of close dual quasars at $1<z<3$, providing empirical constraints on SMBH merger statistics and enabling future JWST/NIRSpec IFU studies of merger-induced gas dynamics.

Abstract

During cosmic noon ($z\sim1-3$), when both star formation and black hole growth peaked, galaxy mergers are predicted to trigger dual active galactic nuclei (AGN) that eventually coalesce as supermassive black hole (SMBH) binaries. However, observations of dual quasars with sub-5 kpc separations-the critical phase preceding final coalescence-have remained challenging due to angular resolution limitations. We present the discovery and confirmation of two sub-arcsecond dual quasars at $z>1$, selected from 59,025 SDSS quasars, which fall within the footprint of the Hyper Suprime-Cam Survey. Using high-resolution Hubble Space Telescope (HST) imaging and slitless spectroscopy, we confirmed SDSS J1625+4309 ($z=1.647$, separation 0.55"/4.7 kpc) and SDSS J0229$-$0514 ($z=3.174$, separation 0.42"/3.2 kpc), probing the sub-5 kpc separation regime. Through novel combination of WFC3/IR direct imaging (F140W) and grism spectroscopy (G141), we resolve both components morphologically and spectroscopically confirm their dual nature via detection of H$β$+[OIII] and MgII emission lines in each nucleus. Two-dimensional image decomposition reveals distinct host galaxy morphologies: J1625+4309 shows an extended, disturbed structure ($R_e$=4.7 kpc) indicative of an ongoing major merger, while J0229$-$0514 exhibits a compact host ($R_e$=1.4 kpc) suggesting an advanced coalescence stage. Black hole mass estimates based on virial relations yield M$_{\mathrm{BH}} \sim 10^{8.1}-10^{8.7} M_\odot$ with line-of-sight velocity offsets of $(0.7\pm0.1)\times10^{3}$ km s$^{-1}$ and $(1.0\pm0.2)\times10^{3}$ km s$^{-1}$, respectively. These confirmations directly constrain the frequency and properties of close dual quasars, opening new avenues for studying SMBH mergers at cosmic noon.

Figures (7)

• Figure 1: The pseudo-color HSC images for all 11 candidates and redshift information with their names, redshifts, and combined HSC bands indicated. The two confirmed dual quasar systems are marked with red boxes (i.e., SDSS J0229$-$0514 at $z=3.174$ and SDSS J1625$+$4309 at $z=1.647$).
• Figure 2: HST observations of the two confirmed dual quasar systems. For each system: direct F140W imaging ( left), two-dimensional spectra ( upper right), and one-dimensional spectra ( lower right). All four components (ID1a, ID1b, ID2a, ID2b) are clearly resolved.
• Figure 3: Redshift versus projected separation for confirmed dual quasars in the literature. Our newly identified systems (red hexagons) populate the sparsely sampled parameter space at $1<z<3$ with separations $<5$ kpc, extending the sample below the $\theta=0.\!\!^{\prime\prime}7$ angular separation limit (lower dashed curve). Many early certification results are concentrated above $\theta \sim 3^{\prime\prime}$ (upper dashed curve), reflecting a limitation based on credibility selection. Symbol types denote different discovery programs: Junkkarinen_2001hennawi_2006hennawi_2010kayo_2012shields_2012more_2016Schechter_2017eftekharzadeh_2017derosa_2018goulding_2019Silverman_2020tang_2021Mannucci_2022benitez_2022Ciurlo_2023Chen_2023Perna_2023matsuoka_2024Scialpi_2024zamora_2024, and this work. J0229$-$0514 at $z=3.174$ (lower right hexagon) represents the highest-redshift confirmed sub-5 kpc dual quasar to date.
• Figure 4: Multi-component decomposition of J1625+4309 and J0229-0514 in the F140W band. For each subgraph: Left to right: (i) Original HST/WFC3 direct image; (ii) Best-fit quasar point-source model from Galight; (iii) Residual map after quasar subtraction; (iv) Normalized residuals ($\mathrm{Data - Model}$)/$\sigma$; (v) Demonstrates the radial surface brightness curve from the center of the image of each cut in 1D. The scale is already marked in the figure.
• Figure 5: Spectral fitting results for J1625+4309. For each panel: Left: Calibrated 1D spectrum with identified emission lines within the observed wavelength range, with emission lines position indicated. Upper right: The results of the spectral fitting focused on the H$\beta$+[O iii] region for the dual quasar systems. The multi-component decomposition including: power-law continuum (blue), Fe II template (where detected, green), broad emission lines (orange), and narrow lines (magenta). Lower right: Residuals of the spectral fit.