Investigations of the Class I methanol masers in NGC 4945

Abstract

We have used the Australia Telescope Compact Array (ATCA) to conduct further observations of the 36.2-GHz ($4_{-1}\rightarrow3_0$E) methanol transition towards the nearby active galaxy NGC 4945. These observations have led to a more accurate determination of the offset between the maser emission and the nucleus of NGC 4945 with a typical synthesised beam of $6^{\prime\prime} \times 4^{\prime\prime}$ ($108\times72$ pc). This corresponds to a factor of 4 improvement with respect to the major-axis of the beam. Other transitions of methanol and lines of other molecular species were obtained alongside the 36.2-GHz methanol emission, with strong detections of HC$_3$N (J = $4 \rightarrow 3$) and CS (J = $1 \rightarrow0$) presented here. We do not detect thermal methanol (5$\sigma$ upper limit of 5 mJy in a 6 km s$^{-1}$ channel) from the 48.4-GHz ($1_{0}\rightarrow0_0$A$^+$) ground-state transition, nor emission from the 44.1-GHz ($7_{0} \rightarrow 6_1 $A$^+$) class~I maser transition (5$\sigma$ upper limit of 6 mJy in a 3 km s$^{-1}$ channel). We also present a comparison of the class~I maser emission observed towards NGC 4945 with that from NGC 253 and towards the Galactic giant molecular cloud G 1.6-0.025.