Download to read offline
![identified within a new sample of spatially resolved mid-IR dual
AGN selected using the Wide-field Infrared Survey Explorer
(WISE; E. L. Wright et al. 2010). Using follow-up high-reso-
lution Karl G. Jansky Very Large Array (VLA) and Very Long
Baseline Array (VLBA) radio observations, we demonstrate that
J1218/J1219+1035 not only constitutes a bona fide triple AGN,
but also the first known triple radio AGN. The mid-IR selection
strategy is described in Section 2, and the target is introduced in
Section 3. Radio observations and results are presented in
Sections 4 and 5. All quoted physical separations are projected
separations (rp). A flat ΛCDM cosmology is adopted, with ΩΛ
= 0.69, Ωm = 0.31, and H0 = 67.7 km s−1
Mpc−1
(Planck
Collaboration et al. 2020). The physical scale at the redshift of
the target (z ∼ 0.08) is 1.56 kpc arcsec–1
.
2. Mid-IR Selection
R. Pfeifle et al. (2025, in preparation) assembled a new
sample of 133 spatially resolved mid-IR dual-AGN candidates
using the AllWISE point source catalog (E. L. Wright et al.
2019). Each AGN in a candidate pair satisfied the WISE two-
band mid-IR color cut of W1[3.4 μm] − W2[4.6 μm] � 0.8
(95% reliability; D. Stern et al. 2012, with magnitude cut
W2 � 15.05), and was required to have A-quality photometry
in the first two WISE bands (w1snr�10, w2snr � 10) and
clean contamination and confusion flags (cc_flags = 0). The
pairs were further limited to angular separations of �60″ (a
threshold adopted during the selection process to avoid wide-
separation contaminants; at z = 0.1, this separation corre-
sponds to ∼110 kpc, the approximate physical separation
cutoff suggest by R. Pfeifle 2025). This range of separations
samples a substantial portion of the merger sequence and
specifically probes dual AGN in earlier stage mergers (pre-
dicted to make up approximately 50% or more of all dual
AGN; N. Chen et al. 2023). This separation range is also
aligned with previous works that searched for dual AGN
regardless of merger stage (e.g., X. Liu et al. 2011b; M. Koss
et al. 2012; R. S. Barrows et al. 2023). These pairs were
morphologically classified via visual inspection using Dark
Energy Camera Legacy Survey (DeCaLs; A. Dey et al. 2019)
grz imaging. The sample was limited to pairs with close red-
shift proximity and/or likely hosted in galaxy mergers iden-
tified by interaction-induced disturbances. For more details on
the parent sample, see Pfeifle et al. (in preparation). As part of
the multiwavelength examination, radio survey observations
were retrieved where available. During the initial visual
examination of the radio images, one unique system stood out:
J1218+1035 NW, J1218+1035 SE, and J1219+1035.
3. J1218/J1219+1035
J1218+1035 NW and SE comprise an intermediate-stage
merger where the galaxies are beginning to coalesce and the
nuclei exhibit an angular separation of 13.9
= (rp = 22.6 kpc).
They are offset in velocity by |Δv| = 292.47 ± 12.44 km s−1
(spectroscopic redshifts z1, NW = 0.08707 ± 0.00004 and z2, SE =
0.08601 ± 0.00002). Both exhibit mid-IR colors indicative
of mid-IR AGN (NW: W1 − W2 = 1.338; SE: W1 − W2 =
0.894 ± 0.012, respectively), satisfying the two-band WISE
AGN cut defined by D. Stern et al. (2012; W1 − W2 � 0.8)
and the three-band WISE color cut defined by T. H. Jarrett
et al. (2011).
J1218+1035 NW and SE is actually a multimerger:
J1219+1035 is a third galaxy residing rp = 97 kpc (θ = 60.2)
from J1218+1035 SE, exhibiting a spectroscopic redshift con-
sistent with J1218+1035 NW and SE (z3 = 0.08614 ± 0.00001),
with a velocity offset of |Δv| = 35.88 ± 12.45 km s−1
from
J1218+1035 SE. Redshift proximity and the large tidal tail
extending to the SE from J1219+1035 in the DeCaLS optical
image (see Figure 1) both suggest that J1219+1035 is actively
interacting with J1218+1035 NW and SE. Though 97 kpc is a
significant separation, R. Pfeifle (2025) recently demonstrated
that this falls within the expected distribution of projected pair
separations of simulated interacting galaxy pairs (from the Illu-
stris-TNG100 cosmological simulation; D. R. Patton et al. 2024)
after undergoing a close pericenter passage <10 kpc. A separa-
tion of 97 kpc is below the fiducial upper limit for dual-AGN
separations (∼110 kpc) proposed by R. Pfeifle (2025). Panel A of
Figure 1 provides the optical grz DeCaLS image of the system.
The mid-IR colors of J1219+1035 (W1 − W2 = 0.463 ± 0.005,
W2 − W3 = 3.892 ± 0.011) indicate that it is not a mid-IR AGN.
J1218+1035 SE and J1219+1035 were spectroscopically
classified as a narrow-line Seyfert 2 and a composite (AGN/
star-forming) galaxy, respectively, by X. Liu et al. (2011b)
based on Sloan Digital Sky Survey optical spectra. X. Liu et al.
(2011b) flagged the system as an optical dual-AGN candidate.
While composite galaxies can host AGN (e.g., see E. C. Moran
et al. 2002; A. D. Goulding & D. M. Alexander 2009; M. Koss
et al. 2012), such optical spectroscopic emission line ratios can
also arise from post-asymptotic giant branch (AGB) stars (e.g.,
R. Singh et al. 2013) or shocks (e.g., M. G. Allen et al. 2008;
J. A. Rich et al. 2015), making the origin of the optical
emission in J1219+1035 ambiguous. Our follow-up optical
spectra of J1218+1035 NW and SE, obtained with the Low
Resolution Imaging Spectrometer (LRIS) on Keck (see
Appendix B, Figure 3), showed that J1218+1035 NW resides
at the same redshift as J1218+1035 SE, but does not confirm
optical Baldwin, Phillips & Telervich (BPT) classification of
J1218+1035 NW. Thus, in the mid-IR, evidence exists for
AGN in both J1218+1035 NW and SE, and clear evidence of
an AGN in J1218+1035 SE exists in the optical regime.
Neither the optical spectroscopy nor the mid-IR photometry
offered definitive evidence for an AGN in J1219+1035.
The existing radio survey observations revealed that there
were possible compact radio cores associated with each optical
nucleus. The brightest of these was coincident with J1219
+1035, providing the first evidence of a triple AGN. However,
sensitivity and resolution limitations required pointed radio
observations to confirm the presence of a radio core coincident
with all three optical host galaxies, and to fully characterize the
radio emission.
4. Radio Observations and Analysis
High-sensitivity radio observations of the targets were made
with the VLA at the S band (2–4 GHz, central frequency 3
GHz), X band (8–12 GHz, central frequency 10 GHz), and Ku
band (12–18 GHz, central frequency 15 GHz), at A config-
uration. This provides resolutions of ∼0.70 at the S band,
∼0.22 at the X band, and ∼0.16 at the Ku band (project code
24B-367, PI: Schwartzman).
VLBA observations of the central radio core (J1218+1035 SE)
were made at C band (4.6–5.1 GHz, central frequency 4.9 GHz)
under project code BS347 (PI: Schwartzman). The observations
provide a theoretical resolution of ∼2 mas.
2
The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.](https://image.slidesharecdn.com/schwartzman2025apjl995l581-251230161446-2a2d8075/75/TheFirstTripleRadioActiveGalacticNucleus-inanOngoingGalaxyMerger-2-2048.jpg)
![Following the VLA Observing Guide, a flux density scale
calibration accuracy of 5% was assumed for the S band, 5% for
the X band, and 3% for the Ku band. Table 1 presents the flux
densities of all three radio cores, errors, and calculated
luminosities. At 3, 10, and 15 GHz, the image rms values are
15.5, 7.61, and 7.06 μJy bm−1
, respectively. In the 4.9 GHz
VLBA observations, the final image rms value is 21 μJy bm−1
.
Figure 1 presents the VLA radio observations. Three cores
are visible at all VLA bands, coincident with the optical host
galaxies. At the highest resolution (Ku band), the source
positions are J1218+1035 NW: 12:18:57.41, +10:35:51.29;
J1218+1035 SE: 12:18:58.34, +10:35:47.62; and
J1219+1035: 12:19:01.76, +10:35:15.22.
We note that none of the three radio cores qualify as “radio-
loud.” There are several definitions of radio-loudness, including
the threshold of log10(L1.4GHz) � 24[W Hz−1
], presented in
K. I. Kellermann et al. (2016). Reference thresholds vary
slightly; for example, P. N. Best et al. (2005) use a cut of
log10(L1.4GHz) > 24 to indicate a powerful radio source,
J. J. Condon et al. (1991) set the limit at log10(L1.4GHz) > 23, and
V. Smolčić et al. (2009) use a value of log10(L1.4GHz) > 22.5.
Given that none of the three radio cores meet these thresholds,
they can be identified as “radio-quiet.” We distinguish them from
those sources that exhibit no radio emission (e.g., “radio-silent”
sources).
5.2. Compactness and Brightness Temperature
For radio sources observed at small scales, compactness and
brightness temperature can be used to disambiguate AGN
emission from other types (R. D. Baldi et al. 2018). Here, we
apply these parameters to the central source, J1218+1035 SE.
Compactness (C) indicates what fraction of the total emis-
sion from a source is effectively within the peak. It is typically
calculated as Sintegrated/Ipeak, the ratio of the flux density
integrated over the beam to its peak value. A C of ∼1 would
indicate a compact core, while extended emission such as a jet
might exhibit a C > 5. The highest-resolution detection
achieved of J1218+1035 SE is with the VLA at 15 GHz,
where C is 1.18, within the expected range for a radio AGN
(e.g., T. W. Shimwell et al. 2022; V. Smolčić et al. 2017).
Brightness temperature requires the milliarcsecond-scale
spatial resolution provided by the VLBA observations. The
limit separating nonthermal AGN from star formation emis-
sion is Tb = 105
K (J. J. Condon et al. 1991). For J1218+1035
SE, the upper-limit flux from the VLBA observations was used
to calculate an upper-limit brightness temperature of
1.73 × 105
K. This value is consistent with AGN emission, but
does not eliminate the possibility that the emission at VLBA
scales is much fainter than the current limit, meaning the
brightness temperature could fall below Tb = 105
K.
We have also calculated brightness temperature using the
VLA fluxes of J1218+1035 SE, assuming the theoretical
resolution of the VLBA at each frequency. These calculations
assume that the source is fully compact on VLBA scales, and
are thus also upper-limit estimations. For J1218+1035 SE, we
calculate a brightness temperature of 1.23 × 106
, 6.55 × 105
,
and 6.48 × 105
K at 3, 10, and 15 GHz, respectively.
5.3. Spectral Analysis
Spectral index can be used to confirm radio-AGN emission,
and to reveal information about the quasar environment,
physical conditions, and evolutionary stage (G. V. Bicknell
et al. 1998; C. P. O’Dea 1998; M. Orienti & D. Dallacasa
2014; K. Nyland et al. 2020; C. P. O’Dea & D. J. Saikia 2021).
Two-band quasi-instantaneous spectral indices were calculated
for all three radio cores, and are presented in Table 2.
A radio spectrum was also built for each core, as shown in
Figure 2. Each spectrum was fit with a standard power law,
and the spectral indices are listed in the upper-left corner
(P. Patil et al. 2021, 2022; P. Duffy & K. M. Blundell 2012;
C. P. O’Dea & D. J. Saikia 2021).
Two of the radio cores, J1218+1035 NW and SE, have
spectral indices of ∼−0.7, consistent with optically thin syn-
chrotron emission from an AGN. The third core, J1219+1035,
has a slightly steeper spectral index of −1.28, which could
indicate AGN-driven jet activity. Though there is no indication
of extension in the radio morphology, jet activity could still
exist on scales below the resolution of the observations. Given
the Ku-band resolution, any extended emission would have a
projected physical size of ≲250 pc.
While the spectral indices presented in Figure 2 are derived
from a standard power-law fit to all three flux measurements,
the spectral indices presented in Table 2 are two-band, quasi-
instantaneous measurements. Column 4 presents the spectral
indices between 3 and 15 GHz, which are consistent (within
1σ) with the measurements shown in Figure 2. Additionally,
both two-band spectral indices presented in Table 2 are con-
sistent (within 2σ) with each other. Thus, there is no statisti-
cally significant evidence of curvature in any of the radio cores
Table 1
Radio Flux Densities and Luminosities
Source S3 GHz log L3 GHz
( ) S6 GHz
V LBA
log L6 GHz
V LBA
( ) S10 GHz log L10 GHz
( ) S15 GHz log L15 GHz
( )
(mJy b−1
) (W Hz−1
) (mJy b−1
) (W Hz−1
) (mJy b−1
) (W Hz−1
) (mJy b−1
) (W Hz−1
)
J1219+1035 0.44 ± 0.02 21.95 ⋯ ⋯ 0.11 ± 0.01 21.35 0.056 ± 0.005 21.05
J1218+1035 SE 0.068 ± 0.01 21.14 <0.021 <20.63 0.027 ± 0.007 20.74 0.022 ± 0.004 20.65
J1218+1035 NW 0.47 ± 0.01 21.98 ⋯ ⋯ 0.21 ± 0.007 21.63 0.14 ± 0.005 21.45
Note. Column (1): source name. Column (2): VLA 3 GHz peak flux measured in mJy beam–1
. Column (3): VLA 3 GHz peak luminosity measured in W Hz–1
.
Column (4): VLBA 6 GHz upper-limit flux of any undetected sources, equivalent to image rms. Column (5): VLBA 6 GHz upper-limit luminosity. Columns (6)–(7):
VLA 10 GHz peak flux and luminosity. Columns (8)–(9): VLA 15 GHz peak flux and luminosity.
Table 2
Radio Spectral Indices
Source 3 GHz
10 GHz
10 GHz
15 GHz
3 GHz
15 GHz
J1219+1035 −1.15 ± 0.17 −1.69 ± 0.69 −1.28 ± 0.14
J1218+1035 SE −0.77 ± 0.60 −0.49 ± 0.86 −0.69 ± 0.37
J1218+1035 NW −0.67 ± 0.08 −1.10 ± 0.29 −0.78 ± 0.06
Note. Column (1): source name. Columns (2)–(4): quasi-instantaneous spec-
tral indices between 3 and 10 GHz, 10 and 15 GHz, and 3 and 15 GHz.
4
The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.](https://image.slidesharecdn.com/schwartzman2025apjl995l581-251230161446-2a2d8075/75/TheFirstTripleRadioActiveGalacticNucleus-inanOngoingGalaxyMerger-4-2048.jpg)
![ionospheric total electron content corrections, and requantizer
gains. Calibration was performed using antenna delay, band-
pass, and complex gain solutions. The flux densities for the
primary calibrators were taken from the R. A. Perley &
B. J. Butler (2017) extension to the J. W. M. Baars et al. (1977)
scale. The gain solutions were then transferred to the target
sources.
Once pipeline calibration was complete, radio frequency
interference (RFI) was removed using a combination of the
CASA tasks RFLAG and TFCROP. Self-calibration was not
possible due to the target’s low signal-to-noise. All imaging
was completed with CASA, and was performed with a multi-
term multifrequency synthesis deconvolver (U. Rau &
T. J. Cornwell 2011) and natural weighting, in order to max-
imize point source sensitivity. Two Taylor terms were used to
model the frequency dependence of the sky, and to account for
the VLA’s wideband receivers. Clean masks were employed at
all stages and were drawn manually.
A.3. VLBA Observations
VLBA observations were made of the central source, J1218
+1035 SE. The focus on J1218+1035 SE was chosen due to
the relative radio faintness of the source. This VLBA fre-
quency was chosen to highlight the compact emission in a
regime where the source was known to emit in the radio, while
avoiding RFI issues at lower frequencies.
All VLBA data were correlated using the DiFX software
correlator (A. T. Deller et al. 2011). Phase referencing with a
switching angle of 2° was used to account for the expected
faintness of the source. Two minute scans on the phase, rate,
and delay calibrator preceded and followed three minute scans
of the target. A coherence-check calibrator was also observed.
The VLBA observations were taken on 2025 February 6,
with a total observation time of 5.5 hr. A second observation
was take on 2025 February 12, with a total observation time of
5.5 hr, but was found to be unusable due to RFI issues. The
amplitude check calibrator was J1229+0203, the complex gain
calibrator was J1218+1105, and the coherence calibrator was
J1230+1223. The final synthesized beam is 3.78 mas,
1.63 mas, −0.
°
1 (Bmaj, Bmin, PA), with a 1σ sensitivity of
21 μJy beam−1
.
A.4. VLBA Calibration
The VLBA observations were calibrated manually using
CASA, and following standard VLBA procedures for phase-
referenced observations (I. M. van Bemmel et al. 2022;
J. Linford 2022). The process is described in detail in
E. Schwartzman et al. (2025), and includes RFI flagging,
single and multiband delay fitting, and bandpass and amplitude
correction.
Standard VLBA phase referencing was performed
(J. M. Wrobel 2000; K. Nyland et al. 2013; M. J. Reid &
M. Honma 2014), and the phase calibrator was verified to be
compact in nature. All target imaging was completed with AIPS,
and was performed with a Clark deconvolver (B. G. Clark 1980).
Various weighting schemes were attempted to produce the best
possible noise pattern, and the final image was made with a
Briggs weighting (D. S. Briggs 1995) with a robust value of 0.
Appendix B
Keck Spectroscopy
Here (Figure 3) we present the Keck LRIS long-slit obser-
vations of J1218+1035 NW and SE. While these observations
are sufficient for redshift determination, the objects unfortu-
nately fell on a bad pixel on the detector, which affected the
[O III]λ5007 and Hα emission line measurements. While we
were able to determine redshifts and obtain a lower limit on the
log([O III]/Hβ) emission line ratio for J1218+1035 NW (log
([O III]/Hβ) � 0.47), too much of the Hα emission line profile
was lost to derive an approximate (and reliable) flux limit.
Additional follow-up observations are required to confirm the
optical BPT classification of J1218+1035 NW. J1218+1035
SE is a known narrow-line Seyfert 2 as identified by X. Liu
et al. (2011b).
ORCID iDs
Emma Schwartzmanaa https:/
/orcid.org/0000-0002-6454-861X
Ryan W. Pfeifleaa https:/
/orcid.org/0000-0001-8640-8522
Tracy E. Clarkeaa https:/
/orcid.org/0000-0001-6812-7938
Kimberly A. Weaveraa https:/
/orcid.org/0009-0008-4232-486X
Nathan J. Secrestaa https:/
/orcid.org/0000-0002-4902-8077
Figure 3. Keck LRIS long-slit spectra for J1218+1035 NW and SE.
7
The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.](https://image.slidesharecdn.com/schwartzman2025apjl995l581-251230161446-2a2d8075/75/TheFirstTripleRadioActiveGalacticNucleus-inanOngoingGalaxyMerger-7-2048.jpg)
Uploaded bySérgio Sacani
TheFirstTripleRadioActiveGalacticNucleus inanOngoingGalaxyMerger
Galaxymergersarepredictedtobeacriticalstageofmerger-drivenSMBHgrowthandgalaxyevolution.Systems exhibitingmultipleactivegalacticnuclei (AGN) arean important observational tool for constraining theprevalenceandpropertiesofmergingSMBHs.Mostmulti-AGNselectionmethods focusonthe identificationof AGNpairs,whilefewfocusonthemoreelusivetripleAGN,whichareapredictionofhierarchicalgrowthand naturallaboratoriesforstudyingSMBHdynamics.Thesesystemsarerare;onlytwoconfirmedcasesexistlocally (z<0.1).Here,wepresentanew,confirmedtripleAGNsystem,WISEJ121857.42+103551.2/WISEJ121901.77 +103515.0,withnuclearseparationsof22.6and97kpc, identifiedwithinasampleofspatiallyresolvedmid-IR dualAGN.Thesystemexhibitsmultiwavelengthevidencefor threedistinctAGNhostedinanongoinggalaxy merger.Toconfirmthenatureof thissystem,wepresentnewradioobservationswiththeKarlG. JanskyVery LargeArrayinA-configurationat3,10, and15GHz,providingsubarcsecondresolutions, andtheVeryLong BaselineArrayat4.9GHz, providingmilliarcsecondresolutions.Theseobservationsconfirmthissystemasa tripleAGN,andthefirst tripleAGNinwhichall threenucleihost radioAGN.
More Related Content
Similar to TheFirstTripleRadioActiveGalacticNucleus inanOngoingGalaxyMerger
TheFirstTripleRadioActiveGalacticNucleus inanOngoingGalaxyMerger
- 1. The First TripleRadio Active Galactic Nucleus in an Ongoing Galaxy Merger Emma Schwartzman1 aa, Ryan W. Pfeifle2,3,10 aa, Tracy E. Clarke1 aa, Kimberly A. Weaver2 aa, Nathan J. Secrest4 aa, Barry Rothberg4,5 aa, Miranda McCarthy2,6,7 aa, Daniel Stern8,9 aa, Peter G. Boorman8 aa, and Joanna Piotrowska8 aa 1 US Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC 20375, USA 2 X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA 3 Oak Ridge Associated Universities, NASA NPP Program, Oak Ridge, TN 37831, USA 4 US Naval Observatory, 3450 Massachusetts Ave. NW, Washington, DC 20392, USA 5 Department of Physics and Astronomy, George Mason University, 4400 University Dr., MSN 3F3, Fairfax, VA 22030, USA 6 Southeastern Universities Research Association, Washington, DC 20005, USA 7 Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA 8 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA, USA 9 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Received 2025 October 7; revised 2025 November 14; accepted 2025 November 15; published 2025 December 15 Abstract Galaxy mergers are predicted to be a critical stage of merger-driven SMBH growth and galaxy evolution. Systems exhibiting multiple active galactic nuclei (AGN) are an important observational tool for constraining the pre- valence and properties of merging SMBHs. Most multi-AGN selection methods focus on the identification of AGN pairs, while few focus on the more elusive triple AGN, which are a prediction of hierarchical growth and natural laboratories for studying SMBH dynamics. These systems are rare; only two confirmed cases exist locally (z < 0.1). Here, we present a new, confirmed triple AGN system, WISEJ121857.42+103551.2/WISEJ121901.77 +103515.0, with nuclear separations of 22.6 and 97 kpc, identified within a sample of spatially resolved mid-IR dual AGN. The system exhibits multiwavelength evidence for three distinct AGN hosted in an ongoing galaxy merger. To confirm the nature of this system, we present new radio observations with the Karl G. Jansky Very Large Array in A-configuration at 3, 10, and 15 GHz, providing subarcsecond resolutions, and the Very Long Baseline Array at 4.9 GHz, providing milliarcsecond resolutions. These observations confirm this system as a triple AGN, and the first triple AGN in which all three nuclei host radio AGN. Unified Astronomy Thesaurus concepts: Radio active galactic nuclei (2134); Radio astronomy (1338) 1. Introduction The hierarchical model of galaxy evolution is that massive galaxies assemble via mergers of their smaller counterparts (F. Schweizer 1982, 1996; A. Toomre & J. Toomre 1972; B. Rothberg & R. D. Joseph 2004). Most massive galaxies host central supermassive black holes (SMBHs) of 106−10 M⊙ (J. Kormendy & D. Richstone 1995). During mergers, the SMBHs sink via dynamical friction and coalesce (J. E. Barnes & L. Hernquist 1992; P. F. Hopkins et al. 2008; M. Volonteri et al. 2016). Host–SMBH coevolution is evident from scaling relations between SMBH mass and galaxy properties (K. Gebhardt et al. 2000; L. Ferrarese & D. Merritt 2000; T. M. Heckman & P. N. Best 2014). Simulations show that gravitational and hydrodynamical torques in mergers funnel gas to the nuclei, triggering SMBH accretion, which ignite as active galactic nuclei (AGN; P. F. Hopkins et al. 2008; J. E. Barnes & L. Hernquist 1996; J. C. Mihos & L. Hernquist 1996; P. F. Hopkins & E. Quataert 2010; P. R. Capelo & M. Dotti 2017; L. Blecha et al. 2018; K. A. Blumenthal & J. E. Barnes 2018). These processes can yield pairs of AGN and triple AGN in gravitationally interacting systems of galaxies (R. W. Pfeifle et al. 2019a; R. Pfeifle 2025). At high redshift, there is at least one case of a quasar triplet (S. G. Djorgovski et al. 2007) as well as a quasar quartet (J. F. Hennawi et al. 2015), both confirmed via optical spectroscopy. Triple AGN candidates in the local Universe have been found both serendipitously and systematically, typically via optical, hard X-ray, or mid-IR preselection (X. Liu et al. 2011a, 2011b; M. Koss et al. 2012; X. Liu et al. 2019; R. W. Pfeifle et al. 2019a). For the majority of triple AGN candidates reported locally (A. J. Barth et al. 2008; X. Liu et al. 2011a, 2011b; E. Kalfountzou et al. 2017; A. Foord et al. 2021) and at higher redshift (K. Schawinski et al. 2011; E. P. Farina et al. 2013; R. J. Assef et al. 2018), evidence for three active SMBHs is often circumstantial and/ or the reported AGN triplets exhibit separations larger than those expected for interacting galaxies. Only two triple AGN in ongoing mergers are confirmed locally: HCG 16, an inter- mediate/early-stage system with AGN separations of 15 and 87 kpc (M. J. L. Turner et al. 2001; M. Koss et al. 2012), and J0849+1114, a late-stage merger with all active nuclei within 10 kpc (X. Liu et al. 2019; R. W. Pfeifle et al. 2019a). Radio interferometry is a key tool for discovering and confirming multi-AGN: high-resolution radio imaging robustly identifies AGN, though only ∼10% of single AGN are radio- loud (D. E. Osterbrock 1993). In the radio, J0849+1114 hosts two AGN driving double-sided jets (S. Peng et al. 2022); no radio-AGN fraction has been measured for HCG 16. To date, no interacting/merging system hosting three radio-emitting AGN has been reported. Given the scarcity of confirmed triple AGN, expanding the sample is critical for understanding SMBH growth and evolution. In this work, we present multiwavelength evidence for a new triple AGN in the local Universe, WISEJ121857.42+103551.2/ WISEJ121901.77+103515.0 (hereafter J1218/J1219+1035), The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 https://doi.org/10.3847/2041-8213/ae2002 © 2025. The Author(s). Published by the American Astronomical Society. aaaaaaa 10 NASA Postdoctoral Program Fellow 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
- 2. identified within anew sample of spatially resolved mid-IR dual AGN selected using the Wide-field Infrared Survey Explorer (WISE; E. L. Wright et al. 2010). Using follow-up high-reso- lution Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) radio observations, we demonstrate that J1218/J1219+1035 not only constitutes a bona fide triple AGN, but also the first known triple radio AGN. The mid-IR selection strategy is described in Section 2, and the target is introduced in Section 3. Radio observations and results are presented in Sections 4 and 5. All quoted physical separations are projected separations (rp). A flat ΛCDM cosmology is adopted, with ΩΛ = 0.69, Ωm = 0.31, and H0 = 67.7 km s−1 Mpc−1 (Planck Collaboration et al. 2020). The physical scale at the redshift of the target (z ∼ 0.08) is 1.56 kpc arcsec–1 . 2. Mid-IR Selection R. Pfeifle et al. (2025, in preparation) assembled a new sample of 133 spatially resolved mid-IR dual-AGN candidates using the AllWISE point source catalog (E. L. Wright et al. 2019). Each AGN in a candidate pair satisfied the WISE two- band mid-IR color cut of W1[3.4 μm] − W2[4.6 μm] � 0.8 (95% reliability; D. Stern et al. 2012, with magnitude cut W2 � 15.05), and was required to have A-quality photometry in the first two WISE bands (w1snr�10, w2snr � 10) and clean contamination and confusion flags (cc_flags = 0). The pairs were further limited to angular separations of �60″ (a threshold adopted during the selection process to avoid wide- separation contaminants; at z = 0.1, this separation corre- sponds to ∼110 kpc, the approximate physical separation cutoff suggest by R. Pfeifle 2025). This range of separations samples a substantial portion of the merger sequence and specifically probes dual AGN in earlier stage mergers (pre- dicted to make up approximately 50% or more of all dual AGN; N. Chen et al. 2023). This separation range is also aligned with previous works that searched for dual AGN regardless of merger stage (e.g., X. Liu et al. 2011b; M. Koss et al. 2012; R. S. Barrows et al. 2023). These pairs were morphologically classified via visual inspection using Dark Energy Camera Legacy Survey (DeCaLs; A. Dey et al. 2019) grz imaging. The sample was limited to pairs with close red- shift proximity and/or likely hosted in galaxy mergers iden- tified by interaction-induced disturbances. For more details on the parent sample, see Pfeifle et al. (in preparation). As part of the multiwavelength examination, radio survey observations were retrieved where available. During the initial visual examination of the radio images, one unique system stood out: J1218+1035 NW, J1218+1035 SE, and J1219+1035. 3. J1218/J1219+1035 J1218+1035 NW and SE comprise an intermediate-stage merger where the galaxies are beginning to coalesce and the nuclei exhibit an angular separation of 13.9 = (rp = 22.6 kpc). They are offset in velocity by |Δv| = 292.47 ± 12.44 km s−1 (spectroscopic redshifts z1, NW = 0.08707 ± 0.00004 and z2, SE = 0.08601 ± 0.00002). Both exhibit mid-IR colors indicative of mid-IR AGN (NW: W1 − W2 = 1.338; SE: W1 − W2 = 0.894 ± 0.012, respectively), satisfying the two-band WISE AGN cut defined by D. Stern et al. (2012; W1 − W2 � 0.8) and the three-band WISE color cut defined by T. H. Jarrett et al. (2011). J1218+1035 NW and SE is actually a multimerger: J1219+1035 is a third galaxy residing rp = 97 kpc (θ = 60.2) from J1218+1035 SE, exhibiting a spectroscopic redshift con- sistent with J1218+1035 NW and SE (z3 = 0.08614 ± 0.00001), with a velocity offset of |Δv| = 35.88 ± 12.45 km s−1 from J1218+1035 SE. Redshift proximity and the large tidal tail extending to the SE from J1219+1035 in the DeCaLS optical image (see Figure 1) both suggest that J1219+1035 is actively interacting with J1218+1035 NW and SE. Though 97 kpc is a significant separation, R. Pfeifle (2025) recently demonstrated that this falls within the expected distribution of projected pair separations of simulated interacting galaxy pairs (from the Illu- stris-TNG100 cosmological simulation; D. R. Patton et al. 2024) after undergoing a close pericenter passage <10 kpc. A separa- tion of 97 kpc is below the fiducial upper limit for dual-AGN separations (∼110 kpc) proposed by R. Pfeifle (2025). Panel A of Figure 1 provides the optical grz DeCaLS image of the system. The mid-IR colors of J1219+1035 (W1 − W2 = 0.463 ± 0.005, W2 − W3 = 3.892 ± 0.011) indicate that it is not a mid-IR AGN. J1218+1035 SE and J1219+1035 were spectroscopically classified as a narrow-line Seyfert 2 and a composite (AGN/ star-forming) galaxy, respectively, by X. Liu et al. (2011b) based on Sloan Digital Sky Survey optical spectra. X. Liu et al. (2011b) flagged the system as an optical dual-AGN candidate. While composite galaxies can host AGN (e.g., see E. C. Moran et al. 2002; A. D. Goulding & D. M. Alexander 2009; M. Koss et al. 2012), such optical spectroscopic emission line ratios can also arise from post-asymptotic giant branch (AGB) stars (e.g., R. Singh et al. 2013) or shocks (e.g., M. G. Allen et al. 2008; J. A. Rich et al. 2015), making the origin of the optical emission in J1219+1035 ambiguous. Our follow-up optical spectra of J1218+1035 NW and SE, obtained with the Low Resolution Imaging Spectrometer (LRIS) on Keck (see Appendix B, Figure 3), showed that J1218+1035 NW resides at the same redshift as J1218+1035 SE, but does not confirm optical Baldwin, Phillips & Telervich (BPT) classification of J1218+1035 NW. Thus, in the mid-IR, evidence exists for AGN in both J1218+1035 NW and SE, and clear evidence of an AGN in J1218+1035 SE exists in the optical regime. Neither the optical spectroscopy nor the mid-IR photometry offered definitive evidence for an AGN in J1219+1035. The existing radio survey observations revealed that there were possible compact radio cores associated with each optical nucleus. The brightest of these was coincident with J1219 +1035, providing the first evidence of a triple AGN. However, sensitivity and resolution limitations required pointed radio observations to confirm the presence of a radio core coincident with all three optical host galaxies, and to fully characterize the radio emission. 4. Radio Observations and Analysis High-sensitivity radio observations of the targets were made with the VLA at the S band (2–4 GHz, central frequency 3 GHz), X band (8–12 GHz, central frequency 10 GHz), and Ku band (12–18 GHz, central frequency 15 GHz), at A config- uration. This provides resolutions of ∼0.70 at the S band, ∼0.22 at the X band, and ∼0.16 at the Ku band (project code 24B-367, PI: Schwartzman). VLBA observations of the central radio core (J1218+1035 SE) were made at C band (4.6–5.1 GHz, central frequency 4.9 GHz) under project code BS347 (PI: Schwartzman). The observations provide a theoretical resolution of ∼2 mas. 2 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 3. Both VLA andVLBA observations were reduced and calibrated with the Common Astronomy Software Applica- tion (CASA; CASA Team et al. 2022). VLA calibration made use of VLA Pipeline 1.4.2, using CASA version 5.3.0. VLBA calibration followed standard procedures for phase- referenced observations (J. Linford 2022; I. M. van Bemmel et al. 2022), and was imaged with the Astronomical Imaging Processing System (AIPS; E. W. Greisen 2003). All observational and calibration details can be found in Appendix A. 5. Radio Results 5.1. Morphology and Flux Densities All three radio cores are visible at the VLA S, X, and Ku bands as compact, unresolved point sources. J1218+1035 SE was not detected in the targeted VLBA observations. Flux densities for every detection were measured with the Python Blob Detection and Source Finder (PyBDSF; N. Mohan & D. Rafferty 2015), which fits one or more elliptical Gaussian components on a defined image region. Figure 1. Optical DeCaLS grz images and radio VLA images of J1218/J1219+1035. (A): DeCaLS grz image of the interacting system J1218+1035 NW/SE and J1219+1035. The two other galaxies to the north of J1218+1035 NW and to the northeast of J1218+1035 SE reside at higher redshifts and are not associated with this system. (B), (C), and (D): VLA 3, 10, and 15 GHz images, respectively, of the interacting system. In each main panel, J1218+1035 NW/SE and J1219+1035are marked with dashed, dashed–dotted, and dotted white circles, the band is given in the top-left corner, and a 30″ scale bar is given in the bottom-left corner. In each of the VLA panels, the thumbnails in the bottom-right corner show the 10″ × 10″ field of view centered on the individual AGN and better exhibit the radio emission associated with those nuclei. 3 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 4. Following the VLAObserving Guide, a flux density scale calibration accuracy of 5% was assumed for the S band, 5% for the X band, and 3% for the Ku band. Table 1 presents the flux densities of all three radio cores, errors, and calculated luminosities. At 3, 10, and 15 GHz, the image rms values are 15.5, 7.61, and 7.06 μJy bm−1 , respectively. In the 4.9 GHz VLBA observations, the final image rms value is 21 μJy bm−1 . Figure 1 presents the VLA radio observations. Three cores are visible at all VLA bands, coincident with the optical host galaxies. At the highest resolution (Ku band), the source positions are J1218+1035 NW: 12:18:57.41, +10:35:51.29; J1218+1035 SE: 12:18:58.34, +10:35:47.62; and J1219+1035: 12:19:01.76, +10:35:15.22. We note that none of the three radio cores qualify as “radio- loud.” There are several definitions of radio-loudness, including the threshold of log10(L1.4GHz) � 24[W Hz−1 ], presented in K. I. Kellermann et al. (2016). Reference thresholds vary slightly; for example, P. N. Best et al. (2005) use a cut of log10(L1.4GHz) > 24 to indicate a powerful radio source, J. J. Condon et al. (1991) set the limit at log10(L1.4GHz) > 23, and V. Smolčić et al. (2009) use a value of log10(L1.4GHz) > 22.5. Given that none of the three radio cores meet these thresholds, they can be identified as “radio-quiet.” We distinguish them from those sources that exhibit no radio emission (e.g., “radio-silent” sources). 5.2. Compactness and Brightness Temperature For radio sources observed at small scales, compactness and brightness temperature can be used to disambiguate AGN emission from other types (R. D. Baldi et al. 2018). Here, we apply these parameters to the central source, J1218+1035 SE. Compactness (C) indicates what fraction of the total emis- sion from a source is effectively within the peak. It is typically calculated as Sintegrated/Ipeak, the ratio of the flux density integrated over the beam to its peak value. A C of ∼1 would indicate a compact core, while extended emission such as a jet might exhibit a C > 5. The highest-resolution detection achieved of J1218+1035 SE is with the VLA at 15 GHz, where C is 1.18, within the expected range for a radio AGN (e.g., T. W. Shimwell et al. 2022; V. Smolčić et al. 2017). Brightness temperature requires the milliarcsecond-scale spatial resolution provided by the VLBA observations. The limit separating nonthermal AGN from star formation emis- sion is Tb = 105 K (J. J. Condon et al. 1991). For J1218+1035 SE, the upper-limit flux from the VLBA observations was used to calculate an upper-limit brightness temperature of 1.73 × 105 K. This value is consistent with AGN emission, but does not eliminate the possibility that the emission at VLBA scales is much fainter than the current limit, meaning the brightness temperature could fall below Tb = 105 K. We have also calculated brightness temperature using the VLA fluxes of J1218+1035 SE, assuming the theoretical resolution of the VLBA at each frequency. These calculations assume that the source is fully compact on VLBA scales, and are thus also upper-limit estimations. For J1218+1035 SE, we calculate a brightness temperature of 1.23 × 106 , 6.55 × 105 , and 6.48 × 105 K at 3, 10, and 15 GHz, respectively. 5.3. Spectral Analysis Spectral index can be used to confirm radio-AGN emission, and to reveal information about the quasar environment, physical conditions, and evolutionary stage (G. V. Bicknell et al. 1998; C. P. O’Dea 1998; M. Orienti & D. Dallacasa 2014; K. Nyland et al. 2020; C. P. O’Dea & D. J. Saikia 2021). Two-band quasi-instantaneous spectral indices were calculated for all three radio cores, and are presented in Table 2. A radio spectrum was also built for each core, as shown in Figure 2. Each spectrum was fit with a standard power law, and the spectral indices are listed in the upper-left corner (P. Patil et al. 2021, 2022; P. Duffy & K. M. Blundell 2012; C. P. O’Dea & D. J. Saikia 2021). Two of the radio cores, J1218+1035 NW and SE, have spectral indices of ∼−0.7, consistent with optically thin syn- chrotron emission from an AGN. The third core, J1219+1035, has a slightly steeper spectral index of −1.28, which could indicate AGN-driven jet activity. Though there is no indication of extension in the radio morphology, jet activity could still exist on scales below the resolution of the observations. Given the Ku-band resolution, any extended emission would have a projected physical size of ≲250 pc. While the spectral indices presented in Figure 2 are derived from a standard power-law fit to all three flux measurements, the spectral indices presented in Table 2 are two-band, quasi- instantaneous measurements. Column 4 presents the spectral indices between 3 and 15 GHz, which are consistent (within 1σ) with the measurements shown in Figure 2. Additionally, both two-band spectral indices presented in Table 2 are con- sistent (within 2σ) with each other. Thus, there is no statisti- cally significant evidence of curvature in any of the radio cores Table 1 Radio Flux Densities and Luminosities Source S3 GHz log L3 GHz ( ) S6 GHz V LBA log L6 GHz V LBA ( ) S10 GHz log L10 GHz ( ) S15 GHz log L15 GHz ( ) (mJy b−1 ) (W Hz−1 ) (mJy b−1 ) (W Hz−1 ) (mJy b−1 ) (W Hz−1 ) (mJy b−1 ) (W Hz−1 ) J1219+1035 0.44 ± 0.02 21.95 ⋯ ⋯ 0.11 ± 0.01 21.35 0.056 ± 0.005 21.05 J1218+1035 SE 0.068 ± 0.01 21.14 <0.021 <20.63 0.027 ± 0.007 20.74 0.022 ± 0.004 20.65 J1218+1035 NW 0.47 ± 0.01 21.98 ⋯ ⋯ 0.21 ± 0.007 21.63 0.14 ± 0.005 21.45 Note. Column (1): source name. Column (2): VLA 3 GHz peak flux measured in mJy beam–1 . Column (3): VLA 3 GHz peak luminosity measured in W Hz–1 . Column (4): VLBA 6 GHz upper-limit flux of any undetected sources, equivalent to image rms. Column (5): VLBA 6 GHz upper-limit luminosity. Columns (6)–(7): VLA 10 GHz peak flux and luminosity. Columns (8)–(9): VLA 15 GHz peak flux and luminosity. Table 2 Radio Spectral Indices Source 3 GHz 10 GHz 10 GHz 15 GHz 3 GHz 15 GHz J1219+1035 −1.15 ± 0.17 −1.69 ± 0.69 −1.28 ± 0.14 J1218+1035 SE −0.77 ± 0.60 −0.49 ± 0.86 −0.69 ± 0.37 J1218+1035 NW −0.67 ± 0.08 −1.10 ± 0.29 −0.78 ± 0.06 Note. Column (1): source name. Columns (2)–(4): quasi-instantaneous spec- tral indices between 3 and 10 GHz, 10 and 15 GHz, and 3 and 15 GHz. 4 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 5. at the 2σlevel. Deeper, higher signal-to-noise observations, or additional frequency observations, would be necessary to further constrain any spectral curvature. 6. Discussion 6.1. The First Triple Radio AGN J1218/1219+1035 constitutes a triple galaxy merger, where J1218+1035 NW and SE are separated by 22.6 kpc and J1219 +1035 is separated from J1218+1035 SE by 97 kpc. All three nuclei have consistent redshifts with velocity offsets of |Δv| < 400 km s−1 . J1218+1035 NW and SE comprise a spatially resolved dual mid-IR AGN, and J1218+1035 SE hosts an optical AGN (additional optical spectroscopic follow- up is needed for J1218+1035 NW). J1219+1035 appears to be a composite (AGN/star-forming) galaxy (X. Liu et al. 2011b). All three nuclei exhibit compact radio emission in existing radio survey observations. The new radio observations provide evidence that all three nuclei host radio AGN. In Figure 1, J1218+1035 NW and SE and J1219+1035 are clearly visible at all three VLA fre- quencies. All detections are compact and coincident with their optical host galaxies. Both J1218+1035 NW and SE display a spectral index consistent with optically thin synchrotron emission, while the spectral index of J1219+1035 is slightly steeper, possibly indicating AGN-driven jet activity. Though no source was detected in the VLBA image, the image rms was used to place an upper limit on the flux of any undetected source, and to calculate the corresponding bright- ness temperature. At 1.73 × 105 K, this is in excess of the standard limit for star formation, 105 K, supporting the AGN- driven radio emission scenario. Overall, using a combination of multiwavelength results and new radio parameters, we conclude that J1218+1035/J1219 +1035 is the third confirmed triple AGN in the local Universe, and the first confirmed triple radio AGN. 6.2. Existing Triple AGN and Future Selection Strategies Only two triple AGN systems in ongoing galaxy mergers at z ≲ 0.1 have been previously confirmed. HCG 16 (M. Koss et al. 2012) is a compact group of an intermediate and early- stage merger with three AGN at separations of 15 kpc (NGC 833 and 835) and 87 kpc (NGC 835 and NGC 839). The second is J0849+1114, a late-stage triple merger in which all three nuclei reside within 10 kpc (R. W. Pfeifle et al. 2019a; X. Liu et al. 2019; X. Xu et al. 2025). J1218/J1219+1035 represents a similar merger phase as HCG 16 (M. Koss et al. 2012), but a far earlier phase than that of J0849+1114 (R. W. Pfeifle et al. 2019a). Common among these triple AGN is the diverse multiwavelength manifestation of their con- stituent AGN and the need for multiwavelength archival and follow-up observations for selection and confirmation. The three confirmed AGN in HCG 16 required significant multiwavelength evidence, including the nucleus of NGC 835 (which was detected in the ultrahard 14–19 keV passband in the Swift-BAT survey; M. Koss et al. 2012). XMM-Newton observations of a power-law spectrum consistent with AGN emission are seen in at least two of the cores (NGC 835 and 833; M. J. L. Turner et al. 2001; M. Koss et al. 2012), in addition to optical spectroscopic observations of Seyfert and broad line AGN signatures (NGC 833 and 839, respectively; R. R. de Carvalho & R. Coziol 1999; M. Koss et al. 2012). J0849+1114 was selected as a dual-AGN candidate based on optical spectroscopic emission line ratios (X. Liu et al. 2011b) and preselected as a dual-AGN candidate based on its red WISE mid-IR colors11 and disturbed morphology (R. W. Pfeifle et al. 2019b). Chandra revealed the three nuclei as X-ray point sources with fluxes in excess of that expected from star formation, while optical spectroscopic observations revealed three Seyfert AGN (X. Liu et al. 2019; R. W. Pfeifle et al. 2019b). We note that optical classifications alone would not have been unambiguous evidence for three AGN, since one (or two) AGN could photoionize all three nuclei (i.e., cross ionization or an extended narrow-line region). J1218/J1219+1035 is unique in that while J1218+1035 NW and SE exhibit multiwavelength evidence for AGN across multiple separate wave bands (NW: mid-IR/radio AGN, SE: optical/mid-IR/radio AGN), it was only in the radio that the unambiguous AGN nature of J1219+1035 was determined (optical composite (AGN/star-forming)/radio AGN). The radio regime is a powerful tool for identifying/ confirming triple AGN. As we have shown in this work, J1218/J1219+1035 is very active in the radio and unique among the three known triple AGN in that it exhibits radio AGN associated with all three optical nuclei. J0849+1114 has been well studied in the radio, including with high-resolution very long baseline interferometry (VLBI) and VLA multiband observations. One of the three nuclei (J0849+1114 SE) was detected with the European VLBI Network (Z. Paragi et al. 2015) at 1.7 GHz, with a brightness temperature of ∼3 × 107 K (K. É. Gabányi et al. 2019). All three optical nuclei were the subject of a VLA multiband study (S. Peng et al. 2022), in Figure 2. VLA radio spectrum of all cores in J1218/1219+1035. Square, circular, and triangular points represent J1219+1035, J1218+1035 NW, and J1218+1035 SE, respectively. Purple, green, and brown points represent 3, 10, and 15 GHz measurements, respectively. Each source is fit with a standard power law. The solid red line, dotted pink line, and dotted–dashed blue line are fit to J1219+1035, J1218+1035 NW, and J1218+1035 SE, respectively. Each power-law spectral index is shown in the upper left-hand corner. 11 J0849+1114 is unique among these three triple AGN in that the WISE resolution does not enable the direct determination of the number of mid-IR AGN (though at least one resides within the system). 5 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 6. which two ofthe nuclei were confirmed as radio AGN (J0849 +1114 SE and N), while one of the nuclei remains undetected (J0849+1114 SW). The AGN fraction in HCG 16 has not been well studied in the radio regime: the data are limited to 1.4 GHz detections of two of the three nuclei. These detections were taken from the NRAO VLA Sky Survey (J. J. Condon et al. 1998) and Faint Images of the Radio Sky at Twenty Centimeters (R. H. Becker et al. 1995) survey, but the resolution is not high enough to isolate the cores and eliminate source confusion. High-reso- lution, high-frequency VLA observations may yet indicate radio AGN in HCG 16. Based on growing evidence for the importance of radio imaging in the search for triple AGN, we recommend that future searches for triple AGN incorporate not only multiwavelength selection strategies, but specifically sensitive high-resolution radio imaging. Radio imaging has already been shown to be a promising avenue for dual-AGN identification (e.g., H. Fu et al. 2011, 2015). 7. Summary and Conclusions We present new VLA and VLBA observations of the first confirmed triple radio AGN, J1218/J1219+1035, identified within a parent sample of dual mid-IR AGN. Our conclusions are as follows: 1. J1218+1035 NW and SE was originally selected as a dual mid-IR AGN in an intermediate-stage galaxy mer- ger, with a nuclear separation of 22.6 kpc. A third galaxy, J1219+1035, resides 97 kpc from J1218+1035 SE. Based on the small velocity offset from J1218+1035 SE (<300 km s−1 ) and the observed tidal tail extending to the SE, J1219+1035 is likely interacting with J1218 +1035 NW/SE, making this a triple galaxy merger. 2. J1218+1035 NW and SE comprise a spatially resolved dual mid-IR AGN, and J1218+1035 SE also hosts a Seyfert 2 AGN. Survey radio images revealed radio emission coincident with all three nuclei, the brightest of which coincided with J1219+1035, suggesting this sys- tem hosts a triple AGN. 3. The new VLA observations reveal three radio cores with properties indicative of radio AGN. This includes spec- tral indices consistent with emission from AGN and/or AGN-driven jets. Though no source is detected in the VLBA observations, the image rms has been used to place upper limits on the flux of any undetected sources and the brightness temperature, which is consistent with AGN accretion. These new observations confirm that not only is J1218/J1219+1035 a new triple AGN, it is the first confirmed triple radio AGN. 4. Future work will focus on deep UKIRT near-IR images, to highlight host morphologies and tidal structure, and Chandra X-ray imaging, to provide insights into the X-ray properties of this system. These observations confirm the triple AGN nature of this system and highlight the necessity for diverse and multi- wavelength selection strategies in the continued search for these rare systems. Acknowledgments We thank the anonymous referee for the helpful suggestions that have improved the Letter. R.W.P. gratefully acknowledges support through an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by ORAU through a contract with NASA. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Insti- tute of Technology, under a contract with the National Aero- nautics and Space Administration (80NM0018D0004). This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2018), TOPCAT (M. B. Taylor 2005), the Common Astron- omy Software Application (CASA Team et al. 2022), and the Python Blob Detector and Source Finder (N. Mohan & D. Rafferty 2015). Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the US Department of Energy Office of Science, and the Partici- pating Institutions. The National Radio Astronomy Observa- tory and Green Bank Observatory are facilities of the US National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under license. Basic research in radio astronomy at the US Naval Research Laboratory is supported by 6.1 Base Funding. Facilities: VLA (NRAO), Sloan, VLBA (NRAO). Software: Astropy (Astropy Collaboration et al. 2018), CASA (CASA Team et al. 2022), PyBDSF (N. Mohan & D. Rafferty 2015), TOPCAT (M. B. Taylor 2005). Appendix A Radio Calibration A.1. VLA Observations The VLA observations were taken on 2025 January 4, with 3C286 as the primary flux density calibrator and J1239+0730 as the primary complex gain calibrator for observations at all three frequencies. At the S band, the synthesized beam is 0.75, 0.65, 7. ° 1 ( PA B , B , maj min ), with an observation integration time of 58 minutes, and a 1σ sensitivity of 15.5 μJy beam−1 , as measured near the phase center. At the X band, the synthesized beam is 0.23, 0.21, 10. ° 9, with an observation integration time of 32 minutes, and a 1σ sensitivity of 7.61 μJy beam−1 . At the Ku band, the synthesized beam is 0.17, 0.14, 18. ° 9, with an observation integration time of 60 minutes, and a 1σ sensitivity of 7.06 μJy beam−1 . X- and Ku-band targets were observed in repeated phase calibrator-target cycles, with a total on-source integration time of about 960 s at the X band and about 760 s at the Ku band. S-band targets were observed in repeated phase calibrator-target-phase calibrator cycles, with a total on-source integration time of about 2500 s. A.2. VLA Calibration For the S band, the data were recorded with 16 spectral windows, each having 64 channels of 2000 kHz width, cov- ering the 2–4 GHz band. For the X band, the data were recorded with 32 spectral windows, each having 64 channels of 2000 kHz width, covering the 8–12 GHz band. For the Ku band, the data were recorded with 48 spectral windows, each having 64 channels of 2000 kHz width, covering the 12–18 GHz band. The data were reduced and calibrated with the CASA VLA pipeline 1.4.2. The initial pipeline steps followed standard procedures, including Hanning smoothing, antenna position corrections, 6 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 7. ionospheric total electroncontent corrections, and requantizer gains. Calibration was performed using antenna delay, band- pass, and complex gain solutions. The flux densities for the primary calibrators were taken from the R. A. Perley & B. J. Butler (2017) extension to the J. W. M. Baars et al. (1977) scale. The gain solutions were then transferred to the target sources. Once pipeline calibration was complete, radio frequency interference (RFI) was removed using a combination of the CASA tasks RFLAG and TFCROP. Self-calibration was not possible due to the target’s low signal-to-noise. All imaging was completed with CASA, and was performed with a multi- term multifrequency synthesis deconvolver (U. Rau & T. J. Cornwell 2011) and natural weighting, in order to max- imize point source sensitivity. Two Taylor terms were used to model the frequency dependence of the sky, and to account for the VLA’s wideband receivers. Clean masks were employed at all stages and were drawn manually. A.3. VLBA Observations VLBA observations were made of the central source, J1218 +1035 SE. The focus on J1218+1035 SE was chosen due to the relative radio faintness of the source. This VLBA fre- quency was chosen to highlight the compact emission in a regime where the source was known to emit in the radio, while avoiding RFI issues at lower frequencies. All VLBA data were correlated using the DiFX software correlator (A. T. Deller et al. 2011). Phase referencing with a switching angle of 2° was used to account for the expected faintness of the source. Two minute scans on the phase, rate, and delay calibrator preceded and followed three minute scans of the target. A coherence-check calibrator was also observed. The VLBA observations were taken on 2025 February 6, with a total observation time of 5.5 hr. A second observation was take on 2025 February 12, with a total observation time of 5.5 hr, but was found to be unusable due to RFI issues. The amplitude check calibrator was J1229+0203, the complex gain calibrator was J1218+1105, and the coherence calibrator was J1230+1223. The final synthesized beam is 3.78 mas, 1.63 mas, −0. ° 1 (Bmaj, Bmin, PA), with a 1σ sensitivity of 21 μJy beam−1 . A.4. VLBA Calibration The VLBA observations were calibrated manually using CASA, and following standard VLBA procedures for phase- referenced observations (I. M. van Bemmel et al. 2022; J. Linford 2022). The process is described in detail in E. Schwartzman et al. (2025), and includes RFI flagging, single and multiband delay fitting, and bandpass and amplitude correction. Standard VLBA phase referencing was performed (J. M. Wrobel 2000; K. Nyland et al. 2013; M. J. Reid & M. Honma 2014), and the phase calibrator was verified to be compact in nature. All target imaging was completed with AIPS, and was performed with a Clark deconvolver (B. G. Clark 1980). Various weighting schemes were attempted to produce the best possible noise pattern, and the final image was made with a Briggs weighting (D. S. Briggs 1995) with a robust value of 0. Appendix B Keck Spectroscopy Here (Figure 3) we present the Keck LRIS long-slit obser- vations of J1218+1035 NW and SE. While these observations are sufficient for redshift determination, the objects unfortu- nately fell on a bad pixel on the detector, which affected the [O III]λ5007 and Hα emission line measurements. While we were able to determine redshifts and obtain a lower limit on the log([O III]/Hβ) emission line ratio for J1218+1035 NW (log ([O III]/Hβ) � 0.47), too much of the Hα emission line profile was lost to derive an approximate (and reliable) flux limit. Additional follow-up observations are required to confirm the optical BPT classification of J1218+1035 NW. J1218+1035 SE is a known narrow-line Seyfert 2 as identified by X. Liu et al. (2011b). ORCID iDs Emma Schwartzmanaa https:/ /orcid.org/0000-0002-6454-861X Ryan W. Pfeifleaa https:/ /orcid.org/0000-0001-8640-8522 Tracy E. Clarkeaa https:/ /orcid.org/0000-0001-6812-7938 Kimberly A. Weaveraa https:/ /orcid.org/0009-0008-4232-486X Nathan J. Secrestaa https:/ /orcid.org/0000-0002-4902-8077 Figure 3. Keck LRIS long-slit spectra for J1218+1035 NW and SE. 7 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.
- 8. Barry Rothbergaa https:/ /orcid.org/0000-0003-2283-2185 MirandaMcCarthyaa https:/ /orcid.org/0009-0005-9964-4790 Daniel Sternaa https:/ /orcid.org/0000-0003-2686-9241 Peter G. Boormanaa https:/ /orcid.org/0000-0001-9379-4716 Joanna Piotrowskaaa https:/ /orcid.org/0000-0003-1661-2338 References Allen, M. G., Groves, B. A., Dopita, M. A., Sutherland, R. S., & Kewley, L. J. 2008, ApJS, 178, 20 Assef, R. J., Stern, D., Noirot, G., et al. 2018, ApJS, 234, 23 Astropy Collaboration, Price-Whelan, A. M., Sipőcz, B. M., et al. 2018, AJ, 156, 123 Baars, J. W. M., Genzel, R., Pauliny-Toth, I. I. K., & Witzel, A. 1977, A&A, 61, 99 Baldi, R. D., Williams, D. R. A., McHardy, I. M., et al. 2018, MNRAS, 476, 3478 Barnes, J. E., & Hernquist, L. 1992, ARA&A, 30, 705 Barnes, J. E., & Hernquist, L. 1996, ApJ, 471, 115 Barrows, R. S., Comerford, J. M., Stern, D., & Assef, R. J. 2023, ApJ, 951, 92 Barth, A. J., Bentz, M. C., Greene, J. E., & Ho, L. C. 2008, ApJL, 683, L119 Becker, R. H., White, R. L., & Helfand, D. J. 1995, ApJ, 450, 559 Best, P. N., Kauffmann, G., Heckman, T. M., et al. 2005, MNRAS, 362, 25 Bicknell, G. V., Dopita, M. A., Tsvetanov, Z. I., & Sutherland, R. S. 1998, ApJ, 495, 680 Blecha, L., Snyder, G. F., Satyapal, S., & Ellison, S. L. 2018, MNRAS, 478, 3056 Blumenthal, K. A., & Barnes, J. E. 2018, MNRAS, 479, 3952 Briggs, D. S. 1995, PhD thesis, New Mexico Institute of Mining and Tech- nology https://casa.nrao.edu/Documents/Briggs-PhD.pdf Capelo, P. R., & Dotti, M. 2017, MNRAS, 465, 2643 CASA Team, Bean, B., Bhatnagar, S., et al. 2022, PASP, 134, 114501 Chen, N., Di Matteo, T., Ni, Y., et al. 2023, MNRAS, 522, 1895 Clark, B. G. 1980, A&A, 89, 377 Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693 Condon, J. J., Huang, Z. P., Yin, Q. F., & Thuan, T. X. 1991, ApJ, 378, 65 de Carvalho, R. R., & Coziol, R. 1999, AJ, 117, 1657 Deller, A. T., Brisken, W. F., Phillips, C. J., et al. 2011, PASP, 123, 275 Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168 Djorgovski, S. G., Courbin, F., Meylan, G., et al. 2007, ApJL, 662, L1 Duffy, P., & Blundell, K. M. 2012, MNRAS, 421, 108 Farina, E. P., Montuori, C., Decarli, R., & Fumagalli, M. 2013, MNRAS, 431, 1019 Ferrarese, L., & Merritt, D. 2000, ApJ, 539, L9 Foord, A., Gültekin, K., Runnoe, J. C., & Koss, M. J. 2021, ApJ, 907, 71 Fu, H., Myers, A. D., Djorgovski, S. G., et al. 2015, ApJ, 799, 72 Fu, H., Zhang, Z.-Y., Assef, R. J., et al. 2011, ApJL, 740, L44 Gabányi, K. É., Frey, S., Satyapal, S., Constantin, A., & Pfeifle, R. W. 2019, A&A, 630, L5 Gebhardt, K., Bender, R., Bower, G., et al. 2000, ApJL, 539, L13 Goulding, A. D., & Alexander, D. M. 2009, MNRAS, 398, 1165 Greisen, E. W. 2003, ASSL, 285, 109 Heckman, T. M., & Best, P. N. 2014, ARA&A, 52, 589 Hennawi, J. F., Prochaska, J. X., Cantalupo, S., & Arrigoni-Battaia, F. 2015, Sci, 348, 779 Hopkins, P. F., Hernquist, L., Cox, T. J., & Kereš, D. 2008, ApJS, 175, 356 Hopkins, P. F., & Quataert, E. 2010, MNRAS, 407, 1529 Jarrett, T. H., Cohen, M., Masci, F., et al. 2011, ApJ, 735, 112 Kalfountzou, E., Santos Lleo, M., & Trichas, M. 2017, ApJL, 851, L15 Kellermann, K. I., Condon, J. J., Kimball, A. E., Perley, R. A., & Ivezić, Ž. 2016, ApJ, 831, 168 Kormendy, J., & Richstone, D. 1995, Natur, 377, 131 Koss, M., Mushotzky, R., Treister, E., et al. 2012, ApJL, 746, L22 Linford, J. 2022, VLBA Data Calibration with CASA: Phase-Referenced Observations VLBA Scientific Memo No. 38, NRAO https://library.nrao. edu/public/memos/vlba/sci/VLBAS_38.pdf Liu, X., Hou, M., Li, Z., et al. 2019, ApJ, 887, 90 Liu, X., Shen, Y., & Strauss, M. A. 2011a, ApJL, 736, L7 Liu, X., Shen, Y., Strauss, M. A., & Hao, L. 2011b, ApJ, 737, 101 Mihos, J. C., & Hernquist, L. 1996, ApJ, 464, 641 Mohan, N., & Rafferty, D., 2015 PyBDSF: Python Blob Detection and Source Finder, Astrophysics Source Code Library, ascl:1502.007 Moran, E. C., Filippenko, A. V., & Chornock, R. 2002, ApJL, 579, L71 Nyland, K., Alatalo, K., Wrobel, J. M., et al. 2013, ApJ, 779, 173 Nyland, K., Dong, D. Z., Patil, P., et al. 2020, ApJ, 905, 74 O’Dea, C. P. 1998, PASP, 110, 493 O’Dea, C. P., & Saikia, D. J. 2021, A&ARv, 29, 3 Orienti, M., & Dallacasa, D. 2014, MNRAS, 438, 463 Osterbrock, D. E. 1993, RMxAA, 26, 65 Paragi, Z., Godfrey, L., Reynolds, C., et al. 2015, in Advancing Astrophysics with the Square Kilometre Array (AASKA14), 143 Patil, P., Whittle, M., Nyland, K., et al. 2021, AN, 342, 1166 Patil, P., Whittle, M., Nyland, K., et al. 2022, ApJ, 934, 26 Patton, D. R., Faria, L., Hani, M. H., et al. 2024, MNRAS, 529, 1493 Peng, S., Li, Z., Liu, X., et al. 2022, ApJ, 934, 89 Perley, R. A., & Butler, B. J. 2017, ApJS, 230, 7 Pfeifle, R. 2025, ApJS, 281, 25 Pfeifle, R. W., Satyapal, S., Manzano-King, C., et al. 2019a, ApJ, 883, 167 Pfeifle, R. W., Satyapal, S., Secrest, N. J., et al. 2019b, ApJ, 875, 117 Planck Collaboration, Aghanim, N., Akrami, Y., et al. 2020, A&A, 641, A6 Rau, U., & Cornwell, T. J. 2011, A&A, 532, A71 Reid, M. J., & Honma, M. 2014, ARA&A, 52, 339 Rich, J. A., Kewley, L. J., & Dopita, M. A. 2015, ApJS, 221, 28 Rothberg, B., & Joseph, R. D. 2004, AJ, 128, 2098 Schawinski, K., Urry, M., Treister, E., et al. 2011, ApJL, 743, L37 Schwartzman, E., Fudolig, P., Clarke, T. E., et al. 2025, ApJ, 987, 200 Schweizer, F. 1982, ApJ, 252, 455 Schweizer, F. 1996, AJ, 111, 109 Shimwell, T. W., Hardcastle, M. J., Tasse, C., et al. 2022, A&A, 659, A1 Singh, R., van de Ven, G., Jahnke, K., et al. 2013, A&A, 558, A43 Smolčić, V., Novak, M., Bondi, M., et al. 2017, A&A, 602, A1 Smolčić, V., Zamorani, G., Schinnerer, E., et al. 2009, ApJ, 696, 24 Stern, D., Assef, R. J., Benford, D. J., et al. 2012, ApJ, 753, 30 Taylor, M. B. 2005, ASPC, 347, 29 Toomre, A., & Toomre, J. 1972, ApJ, 178, 623 Turner, M. J. L., Reeves, J. N., Ponman, T. J., et al. 2001, A&A, 365, L110 van Bemmel, I. M., Kettenis, M., Small, D., et al. 2022, PASP, 134, 114502 Volonteri, M., Dubois, Y., Pichon, C., & Devriendt, J. 2016, MNRAS, 460, 2979 Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868 Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2019, AllWISE Source Catalog, NASA IPAC DataSet, IRSA1, doi:2019ipac.data...I1W Wrobel, J. M. 2000, ApJ, 531, 716 Xu, X., Hou, M., Li, Z., et al. 2025, ApJ, 993, 35 8 The Astrophysical Journal Letters, 995:L58 (8pp), 2025 December 20 Schwartzman et al.