ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945
E. Shablovinskaia, C. Ricci, C-S. Chang, R. Paladino, Y. Diaz, D. Belfiori, S. Aalto, M. Koss, T. Kawamuro, E. Lopez-Rodriguez, R. Mushotzky, and G. C. Privon

TL;DR
This paper reports the discovery of a highly polarized, compact millimeter source in NGC 4945 with ALMA, likely due to non-thermal synchrotron emission, but its exact nature remains unknown.
Contribution
First detection of a highly polarized mm source in NGC 4945, highlighting a new potential non-thermal emission component in starburst galaxies.
Findings
Source exhibits 50% polarization degree, indicating non-thermal synchrotron origin.
No clear counterparts in X-ray or radio data, and no variability detected.
Spectral index remains uncertain within large errors.
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% 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…
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Taxonomy
TopicsAstronomical Observations and Instrumentation · Radio Astronomy Observations and Technology · Astrophysics and Cosmic Phenomena
