A SEARCH FOR SUBMILLIMETER H₂O MASERS IN ACTIVE GALAXIES: THE DETECTION OF 321 GHz H₂O MASER EMISSION IN NGC 4945

Abstract

We present further results of a search for extragalactic submillimeter H 2 O masers using the Atacama Large Millimeter/submillimeter Array (ALMA). The detection of a 321 GHz H 2 O maser in the nearby type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H 2 O maser emission at 321 GHz toward the center of NGC 4945, a nearby type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H 2 O masers;however, the non-contemporaneous observations also show differences in velocity offsets. The subparsec-scale distribution of the 22 GHz H 2 O masers revealed by earlier very long baseline interferometry observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of ∼0 54 (∼30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3σ) high-velocity feature is redshifted by 579 km -s 1 with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of ∼0.020 pc (∼1.5×10 5 Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H 2 O masers. This detection confirms that submillimeter H 2 O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA.

Detection of 183 GHz H2O megamaser emission towards NGC 4945

“…The velocity scale is radio LSR. the 183 GHz transition found here is much higher than that found for 321 GHz emission towards NCC 4945 at ∼10 L (Pesce et al 2016;Hagiwara et al 2016). We speculate that this is due to the very different energy requirements of the two lines.…”

“…The velocities are radio heliocentric in Greenhill et al (1997) and so we use v LSR = v hel − 4.6 km s −1 to convert velocities. The black arrow denotes the velocity of a marginally significant feature in the 321 GHz spectrum reported by Hagiwara et al (2016) (the strongest features at 321 GHz occurred between about 650 to 750 km s −1 ). The velocity scale is radio LSR.…”

“…To date extragalactic mm/submm water maser emission has been detected for the 183 GHz transition towards NGC 3079 using the SMA (Humphreys et al 2005), Arp 220 using the IRAM 30 m and APEX (Cernicharo et al 2006;Galametz et al 2016), and for the 321 GHz transition towards Circinus and NGC 4945 using ALMA (Hagiwara et al 2013(Hagiwara et al , 2016Pesce et al 2016). There has also been a tentative, 5σ detection of 439 GHz H 2 O water maser emission towards NGC 3079 using the James Clark Maxwell Telescope (Humphreys et al 2005).…”

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).…”

SUBMILLIMETER H2O MEGAMASERS IN NGC 4945 AND THE CIRCINUS GALAXY

“…We note that Hagiwara et al (2016) offer a parallel analysis of the NGC 4945 data presented here. The observations and data reduction procedures are described in Section 2; in Section 3 we discuss the submillimeter emission and compare the 321 GHz masers to those at 22 GHz.…”