ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945
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| Autores principales: | , , , , , , , , , , , |
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| Formato: | Preprint |
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2025
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| author | Shablovinskaia, E. Ricci, C. Chang, C-S. Paladino, R. Diaz, Y. Belfiori, D. Aalto, S. Koss, M. Kawamuro, T. Lopez-Rodriguez, E. Mushotzky, R. Privon, G. C. |
| author_facet | Shablovinskaia, E. Ricci, C. Chang, C-S. Paladino, R. Diaz, Y. Belfiori, D. Aalto, S. Koss, M. Kawamuro, T. Lopez-Rodriguez, E. Mushotzky, R. Privon, G. C. |
| contents | 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 $\times$ 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 non-thermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_13014 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945 Shablovinskaia, E. Ricci, C. Chang, C-S. Paladino, R. Diaz, Y. Belfiori, D. Aalto, S. Koss, M. Kawamuro, T. Lopez-Rodriguez, E. Mushotzky, R. Privon, G. C. High Energy Astrophysical Phenomena Astrophysics of Galaxies 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 $\times$ 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 non-thermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined. |
| title | ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945 |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2507.13014 |