Widespread methanol emission from the galactic center: The role of cosmic rays

Abstract

We report the discovery of a widespread population of collisionally excited methanol J = 4-1 to 30 E sources at 36.2 GHz from the inner 66′ × 18′ (160 × 43 pc) of the Galactic center. This spectral feature was imaged with a spectral resolution of 16.6 km s -1 taken from 41 channels of a Very Large Array continuum survey of the Galactic center region. The revelation of 356 methanol sources, most of which are maser candidates, suggests a large abundance of methanol in the gas phase in the Galactic center region. There is also spatial and kinematic correlation between SiO (2-1) and CH3OH emission from four Galactic center clouds: the +50 and +20 km s-1 clouds and G0.13-0.13 and G0.25 + 0.01. The enhanced abundance of methanol is accounted for in terms of induced photodesorption by cosmic rays as they travel through a molecular core, collide, dissociate, ionize, and excite Lyman Werner transitions of H 2. A time-dependent chemical model in which cosmic rays drive the chemistry of the gas predicts CH3OH abundance of 10-8 to 10-7 on a chemical timescale of 5 × 104 to 5 × 105 years. The average methanol abundance produced by the release of methanol from grain surfaces is consistent with the available data. © 2013. The American Astronomical Society. All rights reserved..