Discovery of Merging Twin Quasars at z=6.05

Discovery of Merging Twin Quasars at z=6.05
Yoshiki Matsuoka1
, Takuma Izumi2
, Masafusa Onoue3,4,5
, Michael A. Strauss6
, Kazushi Iwasawa7
,
Nobunari Kashikawa8
, Masayuki Akiyama9
, Kentaro Aoki10
, Junya Arita8
, Masatoshi Imanishi2,11
,
Rikako Ishimoto8
, Toshihiro Kawaguchi12
, Kotaro Kohno13,14
, Chien-Hsiu Lee15
, Tohru Nagao1
,
John D. Silverman3
, and Yoshiki Toba1,2,16
1
Research Center for Space and Cosmic Evolution, Ehime University, Matsuyama, Ehime 790-8577, Japan; yk.matsuoka@cosmos.ehime-u.ac.jp
2
National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
3
Kavli Institute for the Physics and Mathematics of the Universe, WPI, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
4
Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
5
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, Peopleʼs Republic of China
6
Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA
7
ICREA and Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028 Barcelona, Spain
8
Department of Astronomy, School of Science, The University of Tokyo, Tokyo 113-0033, Japan
9
Astronomical Institute, Tohoku University, Aoba, Sendai 980-8578, Japan
10
Subaru Telescope, National Astronomical Observatory of Japan, Hilo, HI 96720, USA
11
Department of Astronomical Science, Graduate University for Advanced Studies (SOKENDAI), Mitaka, Tokyo 181-8588, Japan
12
Department of Economics, Management and Information Science, Onomichi City University, Onomichi, Hiroshima 722-8506, Japan
13
Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 181-0015, Japan
14
Research Center for the Early Universe, University of Tokyo, Tokyo 113-0033, Japan
15
W. M. Keck Observatory, Kamuela, HI 96743, USA
16
Academia Sinica Institute of Astronomy and Astrophysics, Taipei 10617, Taiwan
Received 2024 February 29; revised 2024 March 20; accepted 2024 March 20; published 2024 April 5
Abstract
We report the discovery of two quasars at a redshift of z = 6.05 in the process of merging. They were
serendipitously discovered from the deep multiband imaging data collected by the Hyper Suprime-Cam (HSC)
Subaru Strategic Program survey. The quasars, HSC J121503.42−014858.7 (C1) and HSC J121503.55−014859.3
(C2), both have luminous (>1043
erg s−1
) Lyα emission with a clear broad component (full width at half
maximum >1000 km s−1
). The rest-frame ultraviolet (UV) absolute magnitudes are M1450 = − 23.106 ± 0.017
(C1) and −22.662 ± 0.024 (C2). Our crude estimates of the black hole masses provide M M
log 8.1 0.3
BH 
( ) = 
in both sources. The two quasars are separated by 12 kpc in projected proper distance, bridged by a structure in the
rest-UV light suggesting that they are undergoing a merger. This pair is one of the most distant merging quasars
reported to date, providing crucial insight into galaxy and black hole build-up in the hierarchical structure
formation scenario. A companion paper will present the gas and dust properties captured by Atacama Large
Millimeter/submillimeter Array observations, which provide additional evidence for and detailed measurements of
the merger, and also demonstrate that the two sources are not gravitationally lensed images of a single quasar.
Unified Astronomy Thesaurus concepts: Double quasars (406); Quasars (1319); Reionization (1383); High-redshift
galaxies (734); Active galactic nuclei (16); Galaxy mergers (608); Supermassive black holes (1663)
1. Introduction
Quasars at high redshifts are an important and unique probe of
the epoch of reionization (EoR; referring to z 6 in this Letter),
a critical epoch for understanding the seeding and initial growth
of supermassive black holes (SMBHs), the evolution of the host
galaxies at an early stage of hierarchical structure formation, and
the spatial and temporal progress of the reionization. A
significant number of EoR quasars have been discovered in
the past few decades, exploiting wide-field (>100 deg2
class)
optical and near-infrared (IR) imaging surveys (e.g., Fan
et al. 2023, and references therein). The ultraviolet (UV) quasar
luminosity function (QLF) has now been established at z = 6
and 7 (e.g., Matsuoka et al. 2018c, 2023; Schindler et al. 2023),
demonstrating that UV emission from quasars makes only a
minor contribution to cosmic reionization if the flat QLF slope
below the characteristic luminosity continues to the unobserved
faint end.
In the meantime, the James Webb Space Telescope (JWST)
has produced groundbreaking results in the past two years. Its
extremely high IR sensitivity has revealed a weak broad
component in Balmer emission lines of many high-z galaxies,
signaling the presence of low-luminosity active galactic nuclei
(AGNs) out to z ∼ 11 (e.g., Greene et al. 2024; Maiolino
et al. 2023). The number density of such AGNs exceeds the
extrapolation of the classical QLF by several orders of
magnitude (e.g., Harikane et al. 2023; Matthee et al. 2024),
changing the paradigm of SMBH activity happening in the EoR.
On the other hand, there are still missing pieces in the AGN
demographics in the EoR, one of which is pairs of quasars or
AGNs in mergers. Hierarchical structure formation within the
Lambda Cold Dark Matter model suggests that galaxies grow
via frequent mergers. If a significant fraction of those galaxies
contain an SMBH at the center, as implied from the
measurements in the local Universe (e.g., Kormendy &
Ho 2013), then one would naturally expect SMBH pairs in
the merging galaxies. If the merger induces gas inflow toward
The Astrophysical Journal Letters, 965:L4 (8pp), 2024 April 10 https://doi.org/10.3847/2041-8213/ad35c7
© 2024. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title
of the work, journal citation and DOI.
1

the SMBHs (e.g., Hopkins et al. 2006), then such systems
would be observed as pairs of quasars or AGNs. The observed
frequency of such pairs constrains many key factors, such as
the relative importance of mergers for galaxy and SMBH
evolution, the timescales associated with SMBH interaction
and coalescence, and the number density of possible gravita-
tional wave sources. Quasar pairs can also serve as a signpost
of galaxy overdense regions (e.g., Onoue et al. 2018) and as a
probe of the small-scale distribution of the foreground
intergalactic medium (Rorai et al. 2017).
Searches for pairs of quasars or AGNs have used various
techniques, typically based on wide-field surveys (e.g., De
Rosa et al. 2019, and references therein). The most recent
efforts include those reported by Silverman et al. (2020) and
Tang et al. (2021), who used Subaru Hyper Suprime-Cam
(HSC; see below) high-resolution images and identified pairs at
z „ 3 among the known Sloan Digital Sky Survey (SDSS; York
et al. 2000) quasars. Shen et al. (2021) exploited astrometry
information from the Gaia satellite mission (Gaia Collaboration
et al. 2016) to find two pairs among the SDSS quasars at
z = 2–3. On the other hand, no pairs were found by Sandoval
et al. (2023) in their search from a large X-ray catalog at z ∼ 3.
There are also projects to search for quasar pairs motivated by
investigation of gravitational lensing (e.g., Richards et al. 2006;
Inada et al. 2012; Yue et al. 2023). From such a project based
on the Dark Energy Survey (Abbott et al. 2018), Yue et al.
(2021) found a candidate of quasar pair at z = 5.66, with a
separation of 7.3 kpc. If confirmed, this would be the first
quasar pair reported in the EoR. In addition, quasars with
merging galaxy companions have been reported in the EoR
(e.g., Decarli et al. 2017, 2019), in which the companion
galaxies are frequently invisible in the rest-UV and are only
identified by submillimeter observations. Most recently, JWST
observations are finding signatures of dual AGNs in individual
EoR galaxies, via double components or off-nucleus emission
of broad Balmer lines (Übler et al. 2023; Maiolino et al. 2023).
This Letter presents the discovery of a pair of merging
quasars at z = 6.05, HSC J121503.42−014858.7 and HSC
J121503.55−014859.3 (C1 and C2, hereafter). The two quasars
are separated by 12 kpc, forming one of the most distant pairs
of quasars or AGNs reported to date. We describe the target
selection and spectroscopic observations in Section 2. The
nature of the two sources is discussed in Section 3, based on
their imaging and spectroscopic properties. A summary appears
in Section 4. We adopt the cosmological parameters H0 =
70 km s−1
Mpc−1
, ΩM = 0.3, and ΩΛ = 0.7. All magnitudes
refer to CModel magnitudes from the HSC data reduction
pipeline, which are measured by fitting galaxy models
convolved with the point-spread function (PSF) to the observed
source profile (Bosch et al. 2018). The magnitudes have been
corrected for Galactic extinction (Schlegel et al. 1998), and are
reported in the AB system (Oke & Gunn 1983). A companion
paper (T. Izumi et al. 2024, in preparation) will present the gas
and dust properties of these quasars captured by Atacama Large
Millimeter/submillimeter Array (ALMA) observations, as well
as their kinematic modeling.
2. Observations
Figure 1 presents a three-color (HSC r-, i-, and z-band)
composite image around the two quasars, C1 (west) and C2
(east). Their observed properties are summarized in Table 1.
Here, μz/y represents the second-order moment of the source on
the z-/y-band image, normalized to those of field stars as a
model of PSF (i.e., an unresolved source has μz/y = 1). C1 was
originally selected from the HSC Subaru Strategic Program
(SSP; Aihara et al. 2018) imaging survey. Its red i − z and
relatively blue z − y colors as well as the fact that it is not (or
only marginally) spatially resolved made it an EoR quasar
candidate in our “Subaru High-z Exploration of Low-
Luminosity Quasars (SHELLQs)” project (Matsuoka
et al. 2016, 2018a, 2018b, 2018c, 2019, 2019, 2022, 2023).17
The initial follow-up spectroscopy was carried out with Subaru
Telescope on 2018 April 24, as a part of the Subaru intensive
program S16B-011I. We used the Faint Object Camera and
Spectrograph (FOCAS; Kashikawa et al. 2002) in the multi-
object spectroscopy mode. The combination of the VPH900
grism, SO58 order-sorting filter, and 1 0 slitlets yielded
spectral coverage from 0.75 to 1.05 μm with resolution
R ∼ 1200. The slit angle18
was set to 90°. We took seven
10 minute exposures under the clear sky, with the seeing
conditions of 0 8–1 0. The data reduction was performed with
the Image Reduction and Analysis Facility (IRAF) using the
dedicated FOCASRED package in a standard manner. The
wavelength scale was calibrated with reference to sky emission
lines, and the flux calibration was tied to Feige 34, a white
dwarf standard star, observed on the same night. Slit losses
were corrected for by scaling the spectrum to match the HSC z-
band magnitude.
The initial spectroscopy revealed strong and asymmetric
Lyα emission at the observed wavelength of λobs = 8576 Å,
indicating that C1 exists at zLyα = 6.053. Soon after the
spectroscopic identification, we noticed that C1 is accompanied
by a fuzzy source with similar i − z and z − y colors (see
Figure 1). This fuzz, named C2, is separated by 2 0 from C1
toward the east. We carried out another set of spectroscopy
with FOCAS on 2019 April 25 and 26, and May 10, as a part of
the Subaru intensive program S18B-071I. This time we
oriented the slit angle to 106° so that C1 and C2 were
observed simultaneously. The total exposure time in this run
was 270 minutes. The sky condition was mostly clear, with the
seeing of 0 4–0 7. All the other instrument configurations and
data reduction were identical to those in the initial spectrosc-
opy. We further obtained additional exposures totaling
100 minutes on 2021 March 2 using the same observational
settings as in the 2019 run. The sky condition was clear with
the seeing of 0 6.
We also acquired near-IR spectra of the two sources with the
Fast Turnaround program (ID: GN-2020A-FT-106) at the
Gemini North telescope. We used the Gemini Near-InfraRed
Spectrograph (GNIRS; Elias et al. 2006) in the cross-dispersed
mode, with the 32 l/mm grating and the central wavelength set
to 1.65 μm. The slit width was 1 0, giving spectral coverage
from 0.85 to 2.5 μm and resolution R ∼ 500. We oriented the
slit angle to 106° and took 63 × 5 minute exposures in total,
spread over a month (2020 June 3, 4, and 14, and July 6 and 7).
The observations were carried out in the queue mode, with the
requested sky conditions of 50 percentile cloud coverage and
70 percentile image quality. The data reduction was performed
with IRAF using the Gemini GNIRS package in a standard
manner. The wavelength scale was calibrated with reference to
17
We clarify that the present two quasars were not included in the previous
SHELLQs publications, and are reported here for the first time.
18
Slit angle is measured from north to east, such that 90° refers to a slit
aligned to the east–west direction.
2
The Astrophysical Journal Letters, 965:L4 (8pp), 2024 April 10 Matsuoka et al.

Argon lamp spectra. The flux calibration and telluric absorption
correction were tied to standard stars HIP 54849 and HIP
61637, observed immediately before or after the target
observations at similar airmass. We scaled the GNIRS
spectrum to match the FOCAS spectrum where they overlap
in wavelength.
3. Results and Discussion
3.1. Nature of the Two Sources
Figure 1 presents the two-dimensional FOCAS spectra of C1
(west) and C2 (east), coadded across all exposures. The
extracted one-dimensional spectra are shown in Figure 2 (upper
panels). We also detected a strong emission line from C2, whose
peak wavelength is consistent with that measured in C1. The
asymmetric profiles and the presence of the Gunn & Peterson
(1965) trough at the shorter wavelengths confirm the
identification of the line as Lyα, redshifted to zLyα = 6.053.
Since Lyα redshifts of EoR objects are relatively uncertain (see
also below), we report a formal redshift of z = 6.05 in this Letter.
The full width at half maximum (FWHM) of Lyα, uncorrected
for intergalactic medium (IGM) absorption, are vFWHM = 320 ±
20 km s−1
and 810 ± 180 km s−1
for C1 and C2, respectively.
We also detected flat continuum emission redwards of the line.
The rest-UV absolute magnitudes of the two sources are
M1450 = − 23.106 ± 0.017 (C1) and −22.662 ± 0.024 (C2) at
the rest-frame wavelength λrest = 1450 Å. These values were
obtained by extrapolating the continuum flux density at
λobs = 9000–9300 Å, where the sky emission is relatively weak,
with a power-law model with a slope α = − 1.5 (Fλ ∝ λ−1.5
;
e.g., Vanden Berk et al. 2001). Assuming a quasar bolometric
correction of BC1350 = 3.81 (Shen et al. 2011), we get the
bolometric luminosity of Lbol = (6.2 ± 0.1) × 1045
erg s−1
and
(4.1 ± 0.1) × 1045
erg s−1
for C1 and C2, respectively.
Figure 1. Top: three-color (HSC r-, i-, and z-band) composite image around C1 and C2, the two reddest sources at the center. North is up and east to the left, and the
image size is approximately 90″ × 25″. The limiting magnitude for point sources is ∼26. The inset shows an expanded view of C1 and C2, with the thin dotted lines
representing a 1 0 slitlet used for FOCAS spectroscopy. Bottom: two-dimensional FOCAS spectrum of C1 (upper trace of light) and C2 (lower trace), created by
stacking all available data.
Table 1
Imaging and Spectroscopic Measurements
Object R.A. Decl. gAB rAB iAB zAB yAB
C1 12:15:03.42 −01:48:58.7 26.25 ± 0.28 <26.09 25.73 ± 0.22 23.78 ± 0.11 23.14 ± 0.12
C2 12:15:03.55 −01:48:59.3 <26.75 <26.32 <26.50 24.40 ± 0.15 23.75 ± 0.18
L μz (HSC) μy (HSC) μz (FOCAS) μy (FOCAS)
C1 1.35 ± 0.16 1.27 ± 0.15 1.29 ± 0.16 1.34 ± 0.15
C2 1.60 ± 0.20 0.99 ± 0.23 1.92 ± 0.25 1.63 ± 0.21
L zLyα M1450 Lbol (erg s−1
) vFWHM (km s−1
) EWrest (Å) Lline (erg s−1
) Comment
C1 6.053 −23.106 ± 0.017 (6.2 ± 0.1) × 1045
L L L L
L L L L 1450 ± 170 15 ± 2 (1.34 ± 0.13) × 1043
Lyα (broad)
L L L L 360 ± 30 12 ± 2 (1.02 ± 0.12) × 1043
Lyα (narrow)
C2 6.053 −22.662 ± 0.024 (4.1 ± 0.1) × 1045
L L L
L L L L 1290 ± 60 27 ± 2 (1.84 ± 0.07) × 1043
Lyα (broad)
Note. The magnitude lower limits are given at 3σ confidence level. The line FWHMs (vFWHM) have been corrected for line broadening due to the finite instrumental
resolution. The equivalent widths (EWrest) are reported in the rest frame.
3

It is clear from Figure 2 (upper panels) that the Lyα profile
has a relatively broad component in both sources, with a narrow
core component seen only in C1. We fit two Gaussians and one
Gaussian to the C1 and C2 spectra redward of the line peak,
respectively, as displayed in Figure 2 (lower panels). The local
continuum emission was estimated at λobs = 8694–8738 Å,
where strong lines from the targets and the sky are absent, and
was subtracted before the model fitting. We found that the broad
Lyα components of the two sources have similar widths,
vFWHM = 1450 ± 170 km s−1
(C1) and 1290 ± 60 km s−1
(C2).
Luminosity and other line properties from the best-fit models are
reported in Table 1.
While the redshift was fixed to zLyα = 6.053 during the
model fitting, adopting alternative values does not change our
Figure 2. Upper panels: FOCAS spectra of C1 (top) and C2 (middle) created by stacking all available data, along with a sky spectrum as a guide to the expected noise
(bottom). The dotted lines represent the expected positions of Lyα and N V λ1240 emission lines, as well as interstellar absorption lines of Si II λ1260, Si II λ1304,
and C II λ1335, given the redshift of zLyα = 6.053. An unidentified line at λobs = 9082 Å in C1 (see the main text) is marked by an arrow. Lower panels: continuum
subtracted spectra of C1 (left) and C2 (right) around Lyα. The thick red lines represent the best-fit models, while the thin red lines (only in C1) represent their broad
and narrow components. The spectral window used for the fitting (λobs = 8576–8615 Å) is shown by the vertical dashed lines. The gray shaded area marks the
wavelength range affected by strong sky emission. The spectra in both upper and lower panels were smoothed using inverse-variance-weighted means over 3 pixels,
for display purposes.
4

conclusion that a broad-line component is present. Due to
severe absorption from the IGM, the intrinsic peak of Lyα is
often located blueward of the observed peak, which would
indicate an intrinsically broader line width than estimated
above. This is likely the case for C2, whose ALMA
observations of the [C II] 158 μm line indicate z[C II] = 6.044
(T. Izumi et al. 2024, in preparation). On the other hand, C1 has
z[C II] = 6.057, i.e., the observed Lyα peak is blueshifted
relative to [C II]. When we fix the Lyα redshift of the broad
component to z[C II] = 6.057, we get its width of vFWHM =
1100 ± 100 km s−1
in C1 and 1100 ± 60 km s−1
in C2. We
also note that the FWHM estimate remains almost unchanged
when we fit only the nuclear part of the spatially resolved C2
spectrum.
The spectral properties mentioned above suggest the
presence of quasars in both sources. The widths of the Lyα
broad components exceed the common threshold of quasar
classification, vFWHM = 500–1000 km s−1
(e.g., Schneider
et al. 2010; Pâris et al. 2012), and are similar to values found in
faint AGNs revealed by JWST spectroscopy of EoR galaxies
(e.g., Greene et al. 2024; Harikane et al. 2023; Maiolino
et al. 2023). These values are found at the lower end of the
FWHM distribution of low-z Seyfert 1 galaxies in SDSS (e.g.,
Hao et al. 2005) and of high-z low-luminosity quasars found in
SHELLQs. On the other hand, star-forming galaxies cannot
produce line components that are significantly broader than
∼500 km s−1
, even with outflows (e.g., Newman et al. 2012;
Swinbank et al. 2019). We note that Lyα is spatially resolved
in C2, and the line component extending to >1000 km s−1
belongs to the nuclear part of the two-dimensional spectrum
(see also below). The observed Lyα luminosities of >1043
erg s−1
are also very high for non-AGN galaxies, and overlap
with the lower end of the distribution of other SHELLQs
quasars (e.g., Onoue et al. 2021). At lower redshifts (z ∼ 2–3),
Lyα emitters with such high luminosities almost always harbor
AGNs, identified via characteristic X-ray, UV, radio continuum
emission, and/or high-ionization optical lines (e.g., Konno
et al. 2016; Sobral et al. 2018; Spinoso et al. 2020). The
continuum luminosities of C1 and C2 (M1450 ∼ − 23 mag) are
roughly 10 times higher than the characteristic luminosity of
the galaxy luminosity function at z = 6 (Harikane et al. 2022),
and it would be unexpected (though not impossible) if a close
pair of such luminous high-z galaxies were found.
Other than Lyα, no strong emission lines are detected from
C1 or C2. We found a small spectral bump at the expected
wavelength of N V λ1240 in both C1 and C2 (see Figure 2), but
the adjacent bright sky emission hampers robust identification
of this feature. The 3σ upper limit of the N V/Lyα (broad) ratio
is ∼0.2 in both sources, which is consistent with the ratio
measured in low-z SDSS quasars (∼0.02; Vanden Berk
et al. 2001). The GNIRS spectra of the two targets are very
noisy (see Figure 3) even with >5 hr on-source exposure, only
allowing us to identify continuum emission from C1. There is a
spectral bump at the expected position of C IV λ1549 in C1, but
the detection is marginal at most. On the other hand, the optical
spectrum of C1 (Figure 2) exhibits a weak but clear emission
line at λobs = 9082 Å, which is also apparent in the two-
dimensional spectrum in Figure 1. This line corresponds to
λrest = 1288 Å at zLyα = 6.053, where no emission line is
known. It could be due to an overlapping foreground source,
whose faint blue emission extends northward of C1 (see the
HSC image of Figure 1), but the present data cannot provide
any robust identification.
It is well known that quasar emission line properties, in
particular those of C IV λ1549, Mg II λ2800, and Hβ, are
sensitive to SMBH masses (MBH). Correlation in the form of
M v L
BH FWHM
2
line line
µ º
g
M is observed for the above three
lines, where Lline is the line luminosity and γ is a constant close
to 0.5 (e.g., Vestergaard & Peterson 2006). Here we obtain
crude mass estimates of the two quasars via the broad Lyα
component, which is also sensitive to MBH (e.g., Takahashi
et al. 2024). As is clear from Table 1, C1 and C2 have similar
Lyα properties in the broad components, suggesting similar
MBH. We looked into the spectroscopic properties of SDSS
quasars measured by Rakshit et al. (2020) and found 678/579
quasars whose Lya
M (γ = 0.5 is assumed) values lie
within ± 0.1 dex of the Lyα broad component of C1/C2.
Both of these matched samples have median masses
M M
log 8.1
BH 
( ) = , with a relatively small scatter of
0.3 dex. We thus estimate that both C1 and C2 have
M M
log 8.1 0.3
BH 
( ) =  . The corresponding Eddington
ratios are ∼0.4 and ∼0.3 for C1 and C2, respectively. These
estimates are approximate at most and need to be updated with
future measurements of, e.g., Balmer lines in the rest-frame
optical with JWST.
Similar objects with luminous (>1043
erg s−1
) and relatively
narrow (total FWHM of <500 km s−1
, uncorrected for IGM
absorption) Lyα have been identified at z 6 in our SHELLQs
project (e.g., Matsuoka et al. 2022). Though most of them do
not exhibit a broad Lyα component, we consider them to be
candidate (possibly obscured) quasars, based on their high Lyα
luminosities. We have carried out follow-up IR spectroscopy of
seven such objects with LLyα = 1043.3
–1044.3
erg s−1
and found
positive evidence for the presence of AGNs overall. JWST
observations revealed broad components in Hβ and Hα from
two objects, providing clear signatures of AGNs. Two other
objects observed with Keck/MOSFIRE show strong high-
ionization lines, C IV λλ 1548, 1550 in one object (Onoue
Figure 3. GNIRS spectra of C1 (top) and C2 (middle) created by stacking all
available data, along with the error spectrum dominated by the sky background
(bottom). The vertical lines represent the expected positions of Lyα, N V
λ1240, C IV λ1549, C III] λ1906, and Mg II λ2800, given the redshift of
zLyα = 6.053. The spectra were smoothed using inverse-variance-weighted
means over 9 pixels, for display purposes.
5

et al. 2021) and N IV λλ 1483, 1487 in the other (M. Onoue
et al. 2024, in preparation).19
Their large rest-frame equivalent
widths are difficult to explain with star-forming activity alone,
pointing to the presence of hard AGN radiation. The remaining
three objects were observed with X-Shooter on the Very Large
Telescope, but no emission lines other than Lyα were detected.
However, the sensitivity of the observations is somewhat lower
than those mentioned above. Overall, accumulating evidence
suggests that at least some of the SHELLQs objects with
luminous and narrow Lyα host AGNs.
3.2. Extended Emission and Merging Signatures
The two objects are likely in physical association with each
other, as indicated by their close separation in both the
transverse and line-of-sight directions. The angular separation
of 2 0 corresponds to a projected distance of 12 kpc (proper) or
82 kpc (comoving). Moreover, we see extended Lyα emission
bridging the two objects, as is clear from Figure 4 (left). This
emission component is detected with the signal-to-noise ratio
(S/N) of ∼8, when the signal and noise are measured in the
“bridge” and “blank” boxes indicated in the figure, respec-
tively. More spectacular bridging, tails, and other extended
structures have been identified around C1 and C2 in the [C II]
158μm line emission with our ALMA observations, whose
analysis will be presented in a companion paper (T. Izumi et al.
2024, in preparation).
The rest-UV emission connecting the two sources is also
visible in our deep FOCAS images presented in Figure 5. The
bridging emission was detected only in the z band containing
Lyα, with S/N ∼5 when the signal and noise are measured in
the “bridge” and “blank” boxes indicated in the figure. The z-
and y-band images were taken on 2019 May 10–11, under the
seeing condition of 0 4–0 6. The total exposure time is
30 minutes in each filter.
Similar extended rest-UV emission is seen around other
high-z quasars (Farina et al. 2019), in some cases accompanied
by a merging galaxy (Decarli et al. 2019). While such emission
is sometimes observed around an isolated quasar, the fact that it
is visible only in the area connecting C1 and C2 in the present
case provides a strong indication of a merger in progress. If the
nearly 1:1 ratio of black hole masses indicates similar stellar
masses in the merging two quasars, then it agrees with the
results from hydrodynamical simulations, which predict that
dual SMBH activity appears most frequently in merging
galaxies with a mass ratio close to one, in close separations
(<10 kpc; e.g., Capelo et al. 2015, 2017).
The above FOCAS images also confirmed the spatial
extendedness of C2, which has the normalized second-order
moments of μz = 1.92 ± 0.25 and μy = 1.63 ± 0.21. The large
μz is most likely due to the spatially resolved Lyα of C2 (see
Figure 4), while the measurement of μy > 1 may suggest a
contribution from the host galaxy to the continuum emission.
C1 has a more compact shape, with μz = 1.29 ± 0.16 and
μy = 1.34 ± 0.15. On the other hand, the FOCAS spectra in
Figure 2 show no evidence of interstellar absorption lines, such
as Si II λ1260, Si II λ1304, and C II λ1335, indicating that the
host galaxy makes a subdominant contribution at most to the
continuum spectrum, in either C1 or C2. We note that our
SMBH mass estimates are not affected by host galaxy
contamination, since they are derived from the luminosity
and width of the broad Lyα component only. The bolometric
luminosity and Eddington ratios reported above would be
upper limits if there were significant host galaxy contamination.
We have ruled out the possibility that these two sources are
gravitationally lensed images of a single quasar. The ALMA
observations mentioned above revealed significantly brighter
(>5 times) far-IR continuum emission from C1 than from C2,
in contrast to their similar brightness in the rest-UV (∼0.6 mag
difference; see Table 1). A continuous velocity gradient is
observed throughout C1, C2, and the surrounding extended
[C II] 158 μm emission (blueshift to redshift from east to west,
overall). In addition, only C2 has a spatially resolved Lyα
profile, in which the west and east edges have a relative
velocity offset of ∼300 km s−1
and the nucleus has a velocity
component extending to ∼1000 km s−1
. We will present a
Figure 4. Close-up view of the two-dimensional Lyα spectra of C1 (upper trace, at ∼22 kpc) and C2 (lower trace, at ∼10 kpc). The left panel displays the stacked data
of all available exposures, totaling 370 minutes (i.e., an enlarged view of Figure 1). The white boxes represent the regions used to calculate the S/N of the bridging
emission (signal in the “bridge” box and noise in the “blank” boxes). The right panel displays a single 20 minute exposure taken under the best-seeing condition (∼0″
4), which reveals a spatially resolved Lyα profile in C2. The angular distance of 1″ corresponds to 5.7 kpc at the source redshift.
19
The ionization potentials of C IV and N IV are 64 and 77 eV, respectively.
6

detailed analysis of gas kinematics, combining both the Lyα
and [C II] measurements, in the companion paper.
4. Summary
This Letter is the twentieth in a series of publications from the
SHELLQs project, a high-z quasar survey based on HSC-SSP
imaging. We report the serendipitous discovery of two merging
quasars at z = 6.05, one of the most distant pairs of quasars or
AGNs known to date. The quasars, HSC J121503.42−014858.7
(C1) and HSC J121503.55−014859.3 (C2), have similar rest-
UV properties overall, with M1450 = − 23.106 ± 0.017 (C1) and
−22.662 ± 0.024 (C2). Both sources have a broad Lyα
component with FWHM > 1000 km s−1
, giving crude estimates
of SMBH masses of M M
log 8.1 0.3
BH 
( ) =  . The close
separation (2 0, corresponding to a projected proper distance of
12 kpc) and the bridging emission structure indicate that the two
objects are undergoing a merger, which may have caused the
observed quasar activity. Indeed, ALMA observations have
revealed a spectacular extended structure surrounding the two
quasars, whose detailed analysis will be presented in our
companion paper (T. Izumi et al. 2024, in preparation).
Acknowledgments
This research is based on data collected at the Subaru
Telescope, which is operated by the National Astronomical
Observatory of Japan. We are honored and grateful for the
opportunity to observe the Universe from Maunakea, which has
cultural, historical, and natural signiﬁcance in Hawaii. We
appreciate the staff members of the telescope for their support
during our FOCAS observations.
This research is based, in part, on data obtained at the
international Gemini Observatory, a program of NSF’s
NOIRLab, via the time exchange program between Gemini
and the Subaru Telescope. The international Gemini Observa-
tory at NOIRLab is managed by the Association of Universities
for Research in Astronomy (AURA) under a cooperative
agreement with the National Science Foundation on behalf of
the Gemini partnership: the National Science Foundation
(United States), the National Research Council (Canada),
Agencia Nacional de Investigación y Desarrollo (Chile),
Ministerio de Ciencia, Tecnología e Innovación (Argentina),
Ministério da Ciência, Tecnologia, Inovações e Comunicações
(Brazil), and Korea Astronomy and Space Science Institute
(Republic of Korea).
Y.M. was supported by the Japan Society for the Promotion
of Science (JSPS) KAKENHI grant No. 21H04494. K.I.
acknowledges the support under the grant PID2022-
136827NB-C44 provided by MCIN/AEI/10.13039/
501100011033/FEDER, EU.
The HSC collaboration includes the astronomical commu-
nities of Japan and Taiwan, and Princeton University. The HSC
instrumentation and software were developed by the National
Astronomical Observatory of Japan (NAOJ), the Kavli Institute
for the Physics and Mathematics of the Universe (Kavli
IPMU), the University of Tokyo, the High Energy Accelerator
Research Organization (KEK), the Academia Sinica Institute
for Astronomy and Astrophysics in Taiwan (ASIAA), and
Princeton University. Funding was contributed by the FIRST
program from the Japanese Cabinet Ofﬁce, the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
the Japan Society for the Promotion of Science (JSPS), Japan
Science and Technology Agency (JST), the Toray Science
Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton
University.
This Letter is based on data collected at the Subaru
Telescope and retrieved from the HSC data archive system,
which is operated by the Subaru Telescope and Astronomy
Data Center (ADC) at NAOJ. Data analysis was, in part,
carried out with the cooperation of the Center for Computa-
tional Astrophysics (CfCA) at NAOJ.
This Letter makes use of software developed for the Vera C.
Rubin Observatory. We thank the Rubin Observatory for
Figure 5. FOCAS z-band (left) and y-band (right) images around the two quasars, obtained under seeing conditions of 0 4–0 6. North is up and east to the left. The
scale bars represent 1″. The boxes represent the regions used to calculate the S/N of the bridging emission (signal in the “bridge” box and noise in the “blank” boxes).
7

making their code available as free software at http://pipelines.
lsst.io/.
The Pan-STARRS1 Surveys (PS1) and the PS1 public
science archive have been made possible through contributions
by the Institute for Astronomy, the University of Hawaii, the
Pan-STARRS Project Ofﬁce, the Max Planck Society and its
participating institutes, the Max Planck Institute for Astron-
omy, Heidelberg, and the Max Planck Institute for Extra-
terrestrial Physics, Garching, The Johns Hopkins University,
Durham University, the University of Edinburgh, the Queen’s
University Belfast, the Harvard-Smithsonian Center for Astro-
physics, the Las Cumbres Observatory Global Telescope
Network Incorporated, the National Central University of
Taiwan, the Space Telescope Science Institute, the National
Aeronautics and Space Administration under grant No.
NNX08AR22G issued through the Planetary Science Division
of the NASA Science Mission Directorate, the National
Science Foundation grant No. AST-1238877, the University
of Maryland, Eotvos Lorand University (ELTE), the Los
Alamos National Laboratory, and the Gordon and Betty Moore
Foundation.
ORCID iDs
Yoshiki Matsuoka https:/
/orcid.org/0000-0001-5063-0340
Takuma Izumi https:/
/orcid.org/0000-0001-9452-0813
Masafusa Onoue https:/
/orcid.org/0000-0003-2984-6803
Michael A. Strauss https:/
/orcid.org/0000-0002-0106-7755
Kazushi Iwasawa https:/
/orcid.org/0000-0002-4923-3281
Nobunari Kashikawa https:/
/orcid.org/0000-0003-
3954-4219
Masayuki Akiyama https:/
/orcid.org/0000-0002-2651-1701
Kentaro Aoki https:/
/orcid.org/0000-0003-4569-1098
Junya Arita https:/
/orcid.org/0009-0007-0864-7094
Masatoshi Imanishi https:/
/orcid.org/0000-0001-6186-8792
Rikako Ishimoto https:/
/orcid.org/0000-0002-2134-2902
Toshihiro Kawaguchi https:/
/orcid.org/0000-0002-
3866-9645
Kotaro Kohno https:/
/orcid.org/0000-0002-4052-2394
Chien-Hsiu Lee https:/
/orcid.org/0000-0003-1700-5740
Tohru Nagao https:/
/orcid.org/0000-0002-7402-5441
John D. Silverman https:/
/orcid.org/0000-0002-0000-6977
Yoshiki Toba https:/
/orcid.org/0000-0002-3531-7863
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