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

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.

arXiv commentAccepted to A&A

References in corpus (51)

• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
• BAT AGN Spectroscopic Survey - V. X-ray properties of the Swift/BAT 70-month AGN catalog
• Powerful jets from black hole X-ray binaries in Low/Hard X-ray states
• First M87 Event Horizon Telescope Results. VII. Polarization of the Ring
• Defining the intrinsic AGN infrared spectral energy distribution and measuring its contribution to the infrared output of composite galaxies
• Compton-thick Accretion in the local Universe
• Pulsar spectra of radio emission
• On the origin of radio emission in radio quiet quasars
• Rotation in [CII]-emitting gas in two galaxies at a redshift of 6.8
• The Hubble flow around the CenA / M83 galaxy complex
• High-redshift star formation in the ALMA era
• The Distribution of H2O Maser Emission in the Nucleus of NGC 4945
• The ALMA Spectroscopic Survey in the HUDF: CO luminosity functions and the molecular gas content of galaxies through cosmic history
• BASS XXII: The BASS DR2 AGN Catalog and Data
• The 1.28 GHz MeerKAT Galactic Center Mosaic
• High--frequency predictions for number counts and spectral properties of extragalactic radio sources. New evidences of a break at mm wavelengths in spectra of bright blazar sources
• The distance to the Vela pulsar gauged with HST parallax oservations
• Polarised radio filaments outside the Galactic plane
• The variable radio-to-X-ray spectrum of the magnetar XTE J1810-197
• The ALMA Interferometric Pipeline Heuristics
• Global spectral energy distribution of the Crab Nebula in the prospect of the Planck satellite polarisation calibration
• Simultaneous multifrequency radio observations of the Galactic Centre magnetar SGR J1745-2900
• Polarization measurements analysis I. Impact of the full covariance matrix on polarization fraction and angle measurements
• ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C
• BASS XXXII: Studying the Nuclear Mm-wave Continuum Emission of AGNs with ALMA at Scales $\lesssim$ 100-200 pc
• The mm-wave compact component of AGN
• First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B
• A Tight Correlation Between Millimeter and X-ray Emission in Accreting Massive Black Holes from <100 Milliarcsecond-resolution ALMA Observations
• Detection of the magnetar SGR J1745-2900 up to 291 GHz with evidence of polarized millimetre emission
• The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies. II. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC 4945 Starburst
• Super Star Clusters in the Central Starburst of NGC 4945
• ALMA hints at the presence of turbulent disk galaxies at z > 5
• Polarized thermal emission from dust in a galaxy at redshift 2.6
• Molecular line emission in NGC 4945, imaged with ALMA
• Neutral Hydrogen Absorption Toward XTE J1810-197: the Distance to a Radio-Emitting Magnetar
• An 86-GHz search for Pulsars in the Galactic Center with the Atacama Large Millimeter/submillimeter Array
• Matter ejections behind the highs and lows of the transitional millisecond pulsar PSR J1023+0038
• Detection of 183 GHz water megamaser emission towards NGC 4945
• The remnant of Supernova 1987A resolved at 3 mm wavelength
• Statistics of the fractional polarisation of extragalactic dusty sources in Planck HFI maps
• The first detection of a pulsar with ALMA
• X-Ray Luminosity and Size Relationship of Supernova Remnants in the LMC
• Joint ALMA/X-ray monitoring of the radio-quiet type 1 AGN IC 4329A
• A kiloparsec-scale ordered magnetic field in a galaxy at z=5.6
• The radio jets of SS 433 at millimetre wavelengths
• Millimeter emission from supermassive black hole coronae
• ALMA polarimetry of radio-quiet AGNs
• Detection of millimeter-wave coronal emission in a quasar at cosmological distance using microlensing
• Rotation Measure synthesis study and polarized properties of PSR J1745-2900 at 7 mm

Cited by in corpus (1)

• ALMA polarimetry of radio-quiet AGNs