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

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
³ Joint ALMA Observatory, Avenida Alonso de Cordova 3107, Vitacura, Santiago 7630355, Chile
⁴ INAF, Istituto di Radioastronomia di Bologna, via Piero Gobetti 101, 40129 Bologna, Italy
⁵ Department of Space, Earth and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
⁶ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁸ Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, 560-0043 Osaka, Japan
⁹ Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA
¹⁰ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹¹ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
¹² National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
¹³ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
¹⁴ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA

^⋆ Corresponding author: elena.shablovinskaia@mail.udp.cl

Received: 26 February 2025
Accepted: 12 July 2025

Abstract

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_.^″4 (∼60 pc) from the galactic center and unresolved with ∼0_.^″1 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 × 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 nonthermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined.