Detection of 183 GHz H₂O megamaser emission towards NGC 4945

Abstract

Aims. The aim of this work is to search Seyfert 2 galaxy NGC 4945, a well-known 22 GHz water megamaser galaxy, for H 2 O (mega)maser emission at 183 GHz. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to perform the observations. Results. We detected 183 GHz H 2 O 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 −1 . We estimate an isotropic luminosity of >1000 L , 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 −1 in the spectrum should arise from a distance of 0.022 pc from the supermassive black hole (1.6 × 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 H 2 O 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 H 2 O masers can indeed be useful 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.

Detection of 36 GHz Class I Methanol Maser Emission toward NGC 4945

“…Ellingsen et al (2014) observe offsets of 180 and 300 pc for the northeast and southwest components, respectively. Although the H 2 O maser emission observed in NGC 4945 is not physically associated with the 36.2 GHz maser emission, the redshifted components share an overlapping velocity range (Greenhill et al 1997;Hagiwara et al 2016;Humphreys et al 2016;Pesce et al 2016).…”

Subarcsecond imaging of the water emission in Arp 220

“…In extragalactic sources, the 22 GHz line is most commonly used to search for maser emission – more than 150 sources have been found to emit water maser emission, mostly in galaxies with active galactic nuclei (AGN, e.g., Lo 2005; Pesce et al 2016). The 183 GHz line has been detected towards NGC 3079 (Humphreys et al 2005), Arp 220 (Cernicharo et al 2006b; Galametz et al 2016), and most recently towards NGC 4945 (Humphreys et al 2016). In both NGC 3079 and NGC 4945, the 183 GHz maser emission is associated with the AGN.…”

Second Epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy

“…No maser emission is detected in the second epoch (2016 November) as the strength of the maser has decreased below the detection limit (3 σ) of 38 mJy. Also, a counterpart to the highly redshifted velocity feature at V LSR = 1138.6 km s −1 in the 183 GHz H 2 O maser spectrum (Humphreys et al 2016) was not found in the second epoch spectrum. In the Circinus galaxy, the most prominent H 2 O maser emission is detected with a peak flux density of 1.56 Jy at V LSR = 601.6 km s −1 with a signal-to-noise ratio (SNR) of >80 and a 1 σ rms level of ∼19 mJy per spectral channel (Figure 3).…”

“…H 2 O maser emission occurs at (sub-)millimeter wavelengths in external galaxies: the pioneering observational study of extragalactic (sub-)millimeter masers was the discovery of H 2 O maser in the 3 13 − 2 20 transition (ν rest = 183.31009 GHz) toward the LINER galaxy NGC 3079 (Humphreys et al 2005), which was later fol-lowed by the detection of a 183 GHz maser in the nearby AGN NGC 4945 (Humphreys et al 2016). Hagiwara et al (2013) conducted a search for H 2 O masers in the 10 29 −9 36 transition (ν rest = 321.226 GHz) toward H 2 O megamaser galaxies, which resulted in the first extragalactic detection of a 321 GHz H 2 O maser toward the center of the Circinus galaxy, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA).…”