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

TL;DR

The authors report the discovery of Punctum, a faint yet highly polarized ($PD \approx 50\% \pm 14\%$) compact mm source in the central region of the starburst/Seyfert 2 galaxy NGC 4945, detected with ALMA Band 3 polarimetry at about $3.4''$ ($\sim 60\ \mathrm{pc}$) from the nucleus. The source is unresolved ($\lesssim 2$ pc) and shows no clear X-ray or radio counterparts, with an inferred mm luminosity of $L_{\rm mm} \approx 2\times 10^{35}\ \mathrm{erg\,s^{-1}}$; archival X-ray and radio limits place upper bounds of $L_X \lesssim 1\times 10^{37}$ and $L_{\rm radio} \lesssim 5\times 10^{35}\ \mathrm{erg\,s^{-1}}$. The polarization and steep, poorly constrained spectrum strongly favor synchrotron emission from an ordered magnetic field, but none of the standard source classes (magnetars, SNRs, non-thermal filaments) provide a satisfactory match, suggesting Punctum may represent a rare or new type of compact synchrotron emitter in galactic nuclei. The study demonstrates ALMA's capability to uncover and characterize such unusual polarized mm sources, motivating deeper multiwavelength polarimetric follow-up to determine its nature and prevalence.

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% $\pm$ 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.

Figures (3)

• Figure 1: ALMA observations of NGC 4945 on 2023$-$10$-$11 (S1, left column) and on 2023$-$10$-$25 (S2, right column). Upper panel: total intensity with the contours corresponding to the $0.07, 0.19, 0.31, 0.54$ mJy/beam levels in the box $\sim$10$\times$10$"$. The location of the AGN is marked with a red cross. Middle panel: polarized intensity with the overplotted total intensity contours. Bottom panel: polarization degree in per cent in the box of $\sim$0.5$\times$0.5$"$. The orientation of the polarization vector is given with black ticks.
• Figure 2: ALMA observations of the Punctum in different epochs. Due to the low resolution in 2021--07--11 epoch, only upper limits are given. The 3$\sigma \approx 0.05$ mJy level for S1 and S2 epochs is given in a black dotted line.
• Figure 3: ALMA observations of NGC 4945 on 2023--10--11 (S1, left column) and on 2023--10--25 (S2, right column). Upper panel: polarized intensity with the overplotted total intensity contours corresponding to the $0.07, 0.19, 0.31, 0.54$ mJy/beam levels in the box $\sim$5$\times$5$"$. Bottom panel: polarization degree in per cent in the box of $\sim$0.6$\times$0.6$"$. The orientation of the polarization vector is given with gray ticks.