ALMA discovery of Punctum, a highly polarized millimeter source in the nuclear starburst galaxy NGC 4945

We report the discovery of a highly polarized millimeter (mm) continuum source in the central region of NGC 4945 that was identified through ALMA Band 3 observations. This starburst Seyfert 2 galaxy contains numerous compact mm sources, but only one, located approximately 3 (∼60 pc) from the galactic center and unresolved with ∼0 resolution, exhibits an unusually high polarization degree of 50% ± 14% that likely originates from nonthermal synchrotron radiation. The source is faint, but clearly detected in two separate epochs of observation taken 14 days apart, with a flux of 0.104 ± 0.018 and 0.125 ± 0.016 mJy. It was also detected in earlier ALMA observations, where it showed no variability at any timescale. The spectral index remains stable within large uncertainties of -1.8 ± 2.5 and -1.3 ± 2.4. The source, which we further refer to as Punctum because it is so compact, revealed no clear counterparts in existing X-ray or radio observations. Assuming an association with the central region of NGC 4945, we estimated upper limits for its luminosity of ∼1 times 10^37 erg s^-1 in the 3--6 keV X-ray band (from archival Chandra data) and ∼5 times 10^35 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 nonthermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined.