DETECTION OF 36 GHz CLASS I METHANOL MASER EMISSION TOWARD NGC 253

Abstract

We have used the Australia Telescope Compact Array (ATCA) to search for emission from the 4 −1 → 3 0 E transition of methanol (36.2 GHz) towards the center of the nearby starburst galaxy NGC 253. Two regions of emission were detected, offset from the nucleus along the same position angle as the inner spiral arms. The emission is largely unresolved on a scale of 5 ′′ , has a full-width half maximum (FWHM) line width of < 30 km s −1 , and an isotropic luminosity orders of magnitude larger than that observed in any Galactic star formation regions. These characteristics suggest that the 36.2 GHz methanol emission is most likely a maser, although observations with higher angular and spectral resolution are required to confirm this. If it is a maser this represents the first detection of a class I methanol maser outside the Milky Way. The 36.2 GHz methanol emission in NGC 253 has more than an order of magnitude higher isotropic luminosity than the widespread emission recently detected towards the center of the Milky Way. If emission from this transition scales with nuclear star formation rate then it may be detectable in the central regions of many starburst galaxies. Detection of methanol emission in ultra-luminous infra-red galaxies (ULIRGs) would open up a new tool for testing for variations in fundamental constants (in particular the proton-to-electron mass ratio) on cosmological scales.

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

“…The extragalactic 36.2 GHz emission observed from NGC 253 shares this property (∼10 km s −1 ; Ellingsen et al 2014); however, the megamaser emission toward Arp 220 is significantly more broad (Chen et al 2015). For NGC 4945 the 36.2 GHz methanol emission is spread over a velocity range of 50 km s −1 (from 660 to 710 km s −1 ), dominated by a narrow (FWHM line width of ∼8 km s −1 ) peak component at 674 km s −1 .…”

“…Similar to Ellingsen et al (2014), the 37.7 GHz transition was only included as it could be simultaneously observed with the 36.2 GHz emission and was not expected to be detected due to its rarity in Galactic sources. …”

“…Compared with the 9.0×10 7 and 9.5×10 11 Jy km s −1 kpc 2 observed toward NGC 253 and Arp 220, respectively (note that there is a typographical error in Table 2 of Chen et al 2015 and that the units for the integrated intensity are mJy km s −1 , not Jy km s −1 ; Ellingsen et al 2014;Chen et al 2015). It should be noted that all luminosity values reported here include a factor of p 4 , this was excluded from the value reported in Ellingsen et al (2014).…”

“…Photodesorption of methanol from cold dust by cosmic rays has been suggested as the mechanism producing the abundance of methanol required to produce these levels of observed methanol maser emission (Yusef-Zadeh et al 2013). It is reasonable to expect that a similar phenomenon may exist in other other galaxies, with Ellingsen et al (2014) suggesting that galaxies with enhanced star formation may display this emission over even larger volumes, making them more readily detectable.…”

Class I methanol megamasers: a potential probe of starburst activity and feedback in active galaxies

“…Miyoshi et al 1995). Compared to the rich and active methanol maser phenomena observed in our Galaxy, to date, extragalactic methanol maser emission from either class I (at 36.2 GHz) or class II (6.7 or 12.2 GHz) transitions has only been detected in a few galaxies, specifically the Large Magellanic Cloud, M31 and NGC253 (Green et al 2008;Ellingsen et al 2010Ellingsen et al , 2014Sjouwerman et al 2010). The isotropic luminosity of typical methanol masers observed in Galactic star formation regions is around 10 −5 L , and the methanol emission detected in nearby extragalactic sources is in all cases two or more orders of magnitude short of being a megamaser (i.e.…”