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

Abstract

We have searched for emission from the 36.2 GHz (  -4 3 E 1 0 ) methanol transition toward NGC 4945, using the Australia Telescope Compact Array. 36.2 GHz methanol emission was detected offset southeast from the Galactic nucleus. The methanol emission is narrow, with a line width <10 km s −1 and a luminosity five orders of magnitude higher than Galactic class I masers from the same transition. These characteristics combined the with physical separation from the strong central thermal emission suggests that the methanol emission is a maser. This emission is a factor of ∼90 more luminous than the widespread emission detected from the Milky Way central molecular zone. This is the fourth detection of extragalactic class I emission and the third detection of extragalactic 36.2 GHz maser emission. These extragalactic class I methanol masers do not appear to be simply highly luminous variants of Galactic class I emission and instead appear to trace large-scale regions of low-velocity shocks in molecular gas, which may precede, or be associated with, the early stages of large-scale star formation.

Investigations of the Class I methanol masers in NGC 4945

“…The 36.2-GHz (class I) methanol maser emission (see Table 3) is observed from the same position as reported in McCarthy et al (2017) in both follow up epochs, offset south-east from the galactic nucleus. The hybrid array configuration used for both 2017 observations allows for more accurate imaging of the methanol masing region, which wasn't possible with previous observations.…”

“…The following refers to the observing strategy for the 2017 June and October epochs. Details regarding the previously reported 2015 August observations (EW352 array configuration) can be found in McCarthy et al (2017). PKS B1934-648 was used for flux density calibration for both epochs and the bandpass was calibrated with respect to PKS B0537-441 and PKS B1921-293.…”

“…When the initial detection of 36.2-GHz maser emission in NGC 4945 was reported in McCarthy et al (2017), its galactic bar was thought to have a position angle of 33 o with an azimuth angle with respect to the plane of the galaxy of 40 o based on CO and HI observations by Ott et al (2001). Recent investigations by Henkel et al (2018) have concluded that the bar dynamics are drastically different than previously thought, with the bar elongated approximately eastwest in the plane of the sky.…”

Variability in extragalactic class I methanol masers: new maser components towards NGC 4945 and NGC 253

“…The two originally detected 36 GHz galaxies outside the Local Group, NGC 253 and NGC 4945 (Ellingsen et al 2014;McCarthy et al 2017) show overall properties that are quite different from the galaxies detected by us. Both NGC 253 and NGC 4945 host ongoing starbursts and are characterized by infrared luminosities a few times higher than those of the Milky Way, Maffei 2 or IC 342.…”

“…Nevertheless, the search for additional sources turned out to be difficult. For some time, NGC 4945 was the only additional galaxy detected in the 36 GHz CH 3 OH class I maser line beyond any reasonable doubt (at multiple epochs with more than one telescope; see McCarthy et al 2017McCarthy et al , 2018b. More recently, 36 GHz emission was also reported from IC 342 and NGC 6946 (Gorski et al 2018).…”