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

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

来源:Arxiv_logoArxiv
英文摘要

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.

E. Shablovinskaia、C. Ricci、C-S. Chang、R. Paladino、Y. Diaz、D. Belfiori、S. Aalto、M. Koss、T. Kawamuro、E. Lopez-Rodriguez、R. Mushotzky、G. C. Privon

天文学

E. Shablovinskaia,C. Ricci,C-S. Chang,R. Paladino,Y. Diaz,D. Belfiori,S. Aalto,M. Koss,T. Kawamuro,E. Lopez-Rodriguez,R. Mushotzky,G. C. Privon.ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945[EB/OL].(2025-07-17)[2025-08-16].https://arxiv.org/abs/2507.13014.点此复制

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