Widespread methanol emission from the Galactic center: the role of cosmic rays

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.