Abstract
Abstract Observations made with the Jansky Very large Array (JVLA) at an angular resolution of ∼0.″1 have detected class I methanol maser emission from the 36.2 GHz transition toward the starburst galaxy NGC 253. The methanol emission is detected toward four sites which lie within the regions of extended methanol emission detected in previous lower angular resolution (a few arcseconds) observations. The peak flux densities of the detected compact components are in the range 3–9 mJy beam−1. Combining the JVLA data with single-dish observations from the Shanghai Tianma Radio Telescope (TMRT) and previous interferometric observations with the Australia Telescope Compact Array (ATCA), we show that the 36.2 GHz class I methanol emission consists of both extended and compact structures, with typical scales of ∼6″ (0.1 kpc) and ∼0.″05 (1 pc), respectively. The strongest components have a brightness temperature of >103 K, much higher than the maximum kinetic temperature (∼100 K) of the thermal methanol emission from NGC 253. Therefore, these observations conclusively demonstrate for the first time the presence of maser emission from a class I methanol transition in an external galaxy.
Highlights
Methanol masers are very common in star-forming regions within our Galaxy
If we assume that all of the detected methanol emission originates from a region with an angular size less than 0 1, we can put a lower limit on the brightness temperature of the four components in the range of 440–1200 K considering only their peak flux density, and 1.2 × 104–2.8 × 104 K km s−1 when integrated over the velocity range
The spatial separations of the components detected by the Jansky Very large Array (JVLA) and Australia Telescope Compact Array (ATCA) are reasonable, because the ATCA observations have a lower angular resolution of 0 9 × 3 4 and contain emission from both compact and more extended methanol emission, which may result in differences in the measured locations on scales comparable to the synthesized beam
Summary
Methanol masers are very common in star-forming regions within our Galaxy. More than 1000 sources have been found to show methanol maser emission from either class I or class II transitions, or both (e.g., Green et al 2009; Chen et al 2014; Yang et al 2017). At the angular resolution of the combined ATCA array data, the lower limit on the brightness temperature of the 36.2 GHz methanol emission of the brightest components is only a few K (or 220 K km s−1) from the peak flux density (or the integrated intensity; Ellingsen et al 2017a) These values are in the range of the rotational and kinetic temperatures observed for molecular emission under thermal conditions in the central region of NGC253 (e.g., Martín et al 2006; Meier et al 2015). Existence of the class I methanol masers in NGC253 using the JVLA A-array observations at an angular resolution of ∼0 1
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