2025/07/17 by E. Shablovinskaia, Cláudio Ricci, Shablovinskaia, E. +21 · 1 citation
Engineering · Physics and Astronomy · #Astronomical Observations and Instrumentation #Astrophysics and Cosmic Phenomena #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #Radio Astronomy Observations and Technology
paper · doi:10.48550/arxiv.2507.13014
openalex publication_date 2025/07/17 · openalex created_date 2025/10/18 · openalex updated_date 2026/07/28
We report the discovery of a highly polarized millimeter (mm) continuum source in the central region of NGC 4945, identified through ALMA Band 3 observations. This starburst Seyfert 2 galaxy contains numerous compact mm sources, yet only one - located approximately 3.4" (~60 pc) from the galactic center and unresolved with ~0.1" resolution - exhibits an unusually high polarization degree of 50% ± 14%, likely originating from non-thermal synchrotron radiation. The source is faint, yet clearly detected in two separate epochs of observation taken 14 days apart, with flux of 0.104 ± 0.018 and 0.125 ± 0.016 mJy, as well as in earlier ALMA observations, showing no variability at any timescale. The spectral index remains stable within large uncertainties, -1.8 ± 2.5 and -1.3 ± 2.5. The source, which we further refer to as Punctum due to its compactness, revealed no clear counterparts in existing X-ray or radio observations. Assuming association with the central region of NGC 4945, we estimate upper limits for its luminosity of ~1 × 1037 erg s-1 in the 3-6 keV X-ray band (from archival Chandra data) and ~5 × 1035 erg s-1 at 23 GHz (from archival ATCA data). A comparison of the radio, mm (including polarization), and X-ray properties with known astrophysical sources emitting synchrotron radiation, such as accreting neutron stars, supernova remnants, and non-thermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined.