English

ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945

High Energy Astrophysical Phenomena 2025-11-19 v1 Astrophysics of Galaxies

Abstract

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% ±\pm 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 ±\pm 0.018 and 0.125 ±\pm 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 ±\pm 2.5 and -1.3 ±\pm 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 ×\times 1037^{37} erg s1^{-1} in the 3-6 keV X-ray band (from archival Chandra data) and ~5 ×\times 1035^{35} erg s1^{-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.

Keywords

Cite

@article{arxiv.2507.13014,
  title  = {ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945},
  author = {E. Shablovinskaia and C. Ricci and C-S. Chang and R. Paladino and Y. Diaz and D. Belfiori and S. Aalto and M. Koss and T. Kawamuro and E. Lopez-Rodriguez and R. Mushotzky and G. C. Privon},
  journal= {arXiv preprint arXiv:2507.13014},
  year   = {2025}
}

Comments

Accepted to A&A