J/A+A/706/A253 Gaia DR3 S3 stream candidates (Jimenez-Arranz+, 2026)
Beyond the Clouds:
S3 as the most distant extended Milky Way stream, not of LMC origin.
Jimenez-Arranz O., Lilleengen S., Petersen M.S.
<Astron. Astrophys. 706, A253 (2026)>
=2026A&A...706A.253J 2026A&A...706A.253J (SIMBAD/NED BibCode)
ADC_Keywords: Surveys ; Milky Way ; Optical
Keywords: Galaxy: halo - Galaxy: kinematics and dynamics - Magellanic Clouds -
galaxies: structure
Abstract:
While the influence of the Large Magellanic Cloud (LMC) on Milky Way
stellar streams has been extensively studied, streams associated with
the Clouds themselves have received far less attention. Beyond the
Magellanic Stream, only four stellar stream candidates (S1 to S4)
associated with the Clouds have been reported.
We focus on the S3 stream, a long (about 30 degrees) and narrow (about
1.2 degrees) stellar structure located at distances of 60 to 80kpc
and nearly aligned with the LMC. Our goals are threefold: to validate
the stream through a kinematic analysis of S3 candidates using Gaia
DR3 data; to enlarge the sample of potential members using
machine-learning methods; and to model the stream in order to test its
association with either the Milky Way or the LMC.
We selected new S3 candidates using a supervised neural network
classifier trained on Gaia DR3 astrometry and photometry, and further
reduced contamination by applying a polygon selection in proper-motion
space. To investigate the origin of S3, we evolved stream models in
time-dependent and deforming Milky Way and LMC haloes, accounting for
the dynamical effects of the Milky Way-LMC interaction.
We identify 2177 S3 stream candidates with P>0.5 that satisfy the
proper-motion polygon selection, from which we define a
high-confidence subsample of 1542 new S3 stream candidates using a
probability threshold Pcut=0.8.
We also provide a catalogue of 440 potential S3 red clump stars, which
are particularly valuable targets for spectroscopic follow-up due to
their well-defined luminosities and their ability to yield precise
distance estimates.
Both the dynamical modelling and a comparison between the closest
approach distances and velocities of S3 stars relative to the LMC
escape velocity indicate that S3 is unlikely to originate from the
LMC. Instead, S3 is best interpreted as a distant Milky Way stellar
stream located at approximately 75kpc.
S3 is therefore the most distant known extended Milky Way stellar
stream, with a length of about 30 degrees and a thickness of 3 to 4
degrees, corresponding to a physical width of roughly 4 to 5kpc. This
makes S3 one of the thickest stellar streams discovered to date and a
unique probe of the outer Milky Way halo and of the recent dynamical
influence of the LMC.
Description:
This catalogue contains candidate members of the S3 stellar stream
selected from the Gaia DR3 source catalogue. Candidates were
identified using a supervised neural network classifier trained on
Gaia DR3 astrometric and photometric parameters.
Each source is assigned a probability P of belonging to the S3 stream.
In this work, we adopt Pcut=0.8 to define a high-purity subsample
used for most of the scientific analysis. However, the full catalogue
released here includes all 2177 stars with P>0.5 that also satisfy
the polygon selection in proper-motion space, in order to allow users
to adopt alternative completeness-purity trade-offs.
An additional flag identifies stars consistent with the red clump
evolutionary phase, providing a set of targets suitable for precise
distance estimation and spectroscopic follow-up.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 209 2177 S3 stream candidates
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See also:
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- GaiaDR3 Gaia DR3 unique source identifier
(source_id)
21- 39 F19.16 deg RAdeg Right ascension (ICRS) (ra)
41- 59 F19.15 deg DEdeg Declination (ICRS) (dec)
61- 79 F19.16 mas plx Parallax (parallax)
81- 91 F11.9 mas e_plx Parallax uncertainty (parallax_error)
93-110 F18.16 mas/yr pmRA Proper motion in right ascension
(includes cos(dec) factor) (pmra)
112-122 F11.9 mas/yr e_pmRA Uncertainty on pmra (pmra_error)
124-142 F19.16 mas/yr pmDE Proper motion in declination (pmdec)
144-154 F11.9 mas/yr e_pmDE Uncertainty on pmdec (pmdec_error)
156-164 F9.6 mag Gmag Mean G-band magnitude (photgmean_mag)
166-174 F9.6 mag BPmag Mean BP-band magnitude (photbpmean_mag)
176-184 F9.6 mag RPmag Mean RP-band magnitude (photrpmean_mag)
186-203 F18.16 --- Pmemb [0/1] S3 membership probability (P) (1)
205-209 A5 --- isRCcand Red clump candidate flag
(isRCcandidate) (2)
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Note (1): A probability value close to 1 indicates a high likelihood of
belonging to the S3 stellar stream.
Note (2): isRCcandidate is a boolean field. True identifies stars
consistent with the red clump locus in the Gaia colour-magnitude diagram.
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Acknowledgements:
Oscar Jimenez-Arranz, oscar.jimenez_arranz(at)fysik.lu.se,
Lund Observatory, Lund University, Sweden
(End) O. Jimenez-Arranz [Lund Univ., Sweden], P. Vannier [CDS] 07-Jan-2026