Beyond the Clouds: S3 as the most distant extended Milky Way stream, not of LMC origin

Context . While the influence of the LMC on Milky Way (MW) stellar streams has been extensively studied, streams associated with the Clouds have received far less attention. Beyond the Magellanic Stream, only four stream candidates (S1–S4) have been reported. Aims . We focus on the S3 stream, a long (~30°) and narrow (~1.2°) structure at 60–80 kpc that is nearly aligned with the LMC. Our goals are: (1) to validate the stream through a kinematic analysis of S3 candidates with Gaia DR3 data; (2) to enlarge the sample of potential members with machine-learning methods; and (3) to model the stream in order to test its association with either the MW or the LMC. Methods . We selected new S3 candidates with a supervised neural network classifier trained on Gaia DR3 astrometry and photometry, and further reduced contamination through a polygon cut in the proper-motion space. To investigate the origin of S3, we evolved stream models within time-dependent, deforming MW and LMC halos, thereby accounting for possible effects of the MW–LMC interaction. Results . We identify 1542 high-confidence new S3 stream candidates and find that the stream’s apparent width has grown from ~1.2° to ~3–4° compared to previous studies. We also present a list of 440 potential S3 red clump stars, which are valuable targets for spectroscopic follow-up thanks to their well-defined luminosities and ability to yield precise distances. Both modelling and a comparison of S3 stars’ closest approach distance and velocity with the LMC’s escape velocity indicate that S3 likely does not originate from the LMC and instead represents a distant (~75 kpc) MW stream. Conclusions . S3 is the most distant (~75 kpc) extended (~30° long, ~3–4° thick) MW stream known, offering a unique probe of the outer halo and the LMC’s recent influence. Its angular width corresponds to a physical thickness of ~4–5 kpc, making S3 among the thickest streams discovered.