J/A+A/712/A67 VLBI images of UGC 2369S (Xu+, 2026)
Direct VLBI evidence for a buried active galactic nucleus in the triple-merger
luminous infrared galaxy UGC 2369S.
Xu W., Frey S., Cui L., Gabanyi K.E.
<Astron. Astrophys. 712, A67 (2026)>
=2026A&A...712A..67X 2026A&A...712A..67X (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; Radio sources ; VLBI
Keywords: techniques: high angular resolution - galaxies: active -
galaxies: individual: UGC 2369S - galaxies: jets - galaxies: nuclei -
radio continuum: galaxies
Abstract:
UGC 2369S is a luminous infrared galaxy (LIRG) undergoing a late-stage
merger in a triple system, where the heavily obscured northern core is
suspected to host an active galactic nucleus (AGN). However, severe
dust and gas obscuration makes definitive confirmation challenging.
We aim to provide direct observational evidence for the buried AGN
through high-resolution radio imaging, while investigating the AGN
accretion and feedback properties within this merger-driven gas-rich
environment.
We analyzed archival European VLBI Network (1.6GHz) and Very Long
Baseline Array (1.7 and 5GHz) data of UGC 2369S. Through
high-resolution imaging and visibility-domain Gaussian modeling, we
characterized the morphology and intensity of its milliarcsecond-scale
radio emission.
A compact radio component is detected in the northern core, exhibiting
high brightness temperatures (Tb>107K) and a flat radio spectrum
(alpha=-0.45), which confirms the presence of an obscured AGN. The
sub-Eddington accretion rate (λEdd∼2.7x10-4) indicates
that it falls within the radiatively inefficient accretion flow state.
We provide direct imaging evidence for an AGN in the northern core of
UGC 2369S, revealing a deeply buried, jet-emitting low-luminosity AGN
enshrouded by a Compton-thick gas cocoon. This demonstrates that VLBI
is a uniquely effective tool for disentangling nuclear accretion and
feedback processes within the heavily obscured environments of
multiple-merger systems.
Description:
UGC 2369S is a luminous infrared galaxy undergoing a late-stage triple
merger. Using archival EVN (1.6GHz) and VLBA (1.7 and 5GHz) data, we
detect a compact, flat-spectrum, high-brightness-temperature radio
core in the northern component, providing direct evidence for a deeply
buried, jet-emitting low-luminosity AGN enshrouded by a Compton-thick
gas cocoon.
Naturally weighted VLBI FITS images of the northern core of UGC 2369S
shown in Figure 1 of the paper, for three epochs/frequencies: EVN
1.6GHz (2003-05-27), VLBA 1.7GHz (1996-01-17), and VLBA 5.0GHz
(2003-07-17). The EVN 1.6GHz and VLBA 1.7GHz maps were imaged in
Stokes I; the VLBA 5.0GHz archival data contained only the LL
polarization, and the corresponding map represents LL. Pixel values
are in Jy/beam. World coordinates (J2000, RA---SIN/DEC--SIN) and all
relevant parameters are stored in each FITS header.
Key parameters of each image are as follows.
EVN 1.6 GHz - beam 32.8x27.2 mas, PA -2.5 deg,
rms 0.172 mJy/beam, pixel 4.73 mas, Stokes I.
VLBA 1.7 GHz - beam 11.8x5.77 mas, PA 21.4 deg,
rms 0.162 mJy/beam, pixel 1.69 mas, Stokes I.
VLBA 5.0 GHz - beam 3.75x1.52 mas, PA -8.0 deg,
rms 0.088 mJy/beam, pixel 0.348 mas, LL pol.
Objects:
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RA (2000) DE Designation(s)
------------------------------------------------------
02 54 01.80 +14 58 14.6 UGC 2369S = MCG+02-08-029
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 99 3 List of VLBI FITS images shown in Fig. 1
fits/* . 3 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 37 A10 "date" Obs.date Observation date
39- 45 F7.5 GHz Freq Observed frequency
47- 49 I3 Kibyte size Size of FITS file
51- 62 A12 --- FileName Name of FITS file, in subdirectory fits
64- 99 A36 --- Title Title of the FITS file
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Acknowledgements:
Wancheng Xu, xuwancheng(at)xao.ac.cn
License: CC-BY-4.0
(End) Wancheng Xu [Konkoly Obs., Hungary], Patricia Vannier [CDS] 01-Jul-2026