Detection of 183 GHz water megamaser emission towards NGC 4945

Abstract

Aim: The aim of this work is to search Seyfert 2 galaxy NGC 4945, a well-known 22 GHz water megamaser galaxy, for water (mega)maser emission at 183 GHz. Method: We used APEX SEPIA Band 5 to perform the observations. Results: We detected 183 GHz water maser emission towards NGC 4945 with a peak flux density of ~3 Jy near the galactic systemic velocity. The emission spans a velocity range of several hundred km/s. We estimate an isotropic luminosity of > 1000 Lsun, classifying the emission as a megamaser. A comparison of the 183 GHz spectrum with that observed at 22 GHz suggests that 183 GHz emission also arises from the active galactic nucleus (AGN) central engine. If the 183 GHz emission originates from the circumnuclear disk, then we estimate that a redshifted feature at 1084 km/s in the spectrum should arise from a distance of 0.022 pc from the supermassive black hole (1.6 x 10(5) Schwarzschild radii), i.e. closer than the water maser emission previously detected at 22 GHz. This is only the second time 183 GHz maser emission has been detected towards an AGN central engine (the other galaxy being NGC 3079). It is also the strongest extragalactic millimetre/submillimetre water maser detected to date. Conclusions: Strong millimetre 183 GHz water maser emission has now been shown to occur in an external galaxy. For NGC 4945, we believe that the maser emission arises, or is dominated by, emission from the AGN central engine. Emission at higher velocity, i.e. for a Keplerian disk closer to the black hole, has been detected at 183 GHz compared with that for the 22 GHz megamaser. This indicates that millimetre/submillimetre water masers can indeed be useful probes for tracing out more of AGN central engine structures and dynamics than previously probed. Future observations using ALMA Band 5 should unequivocally determine the origin of the emission in this and other galaxies.

arXiv comment4 pages, accepted by A&A Letters

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Cited by in corpus (15)

• Key Science Goals for the Next-Generation Event Horizon Telescope
• SEPIA - a new single pixel receiver at the APEX Telescope
• The ALMA Phasing System: A Beamforming Capability for Ultra-High-Resolution Science at (Sub)Millimeter Wavelengths
• Molecular line emission in NGC 4945, imaged with ALMA
• Sub-arcsecond imaging of the water emission in Arp 220
• Detection of 36 GHz Class I Methanol Maser Emission Towards NGC 4945
• Water megamaser emission in hard X-ray selected AGN
• 183 GHz water megamasers in active galactic nuclei: a new accretion disk tracer
• Second Epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy
• Central X-ray point-sources found to be abundant in low-mass, late-type galaxies predicted to contain an intermediate-mass black hole
• Investigations of the Class I methanol masers in NGC 4945
• The extended HI halo of NGC 4945 as seen by MeerKAT
• ALMA detection of millimetre 183 GHz H2O maser emission in the Superantennae galaxy at z ~ 0.06
• ALMA Sub-arcsec-resolution 183 GHz H2O and Dense Molecular Line Observations of Nearby Ultraluminous Infrared Galaxies
• ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945