OH Satellite-Line Masers in the Nucleus of NGC 253

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Abstract
We report the detection of 1720 and 1612 MHz OH ground-state satellite-line maser regions in the starburst galaxy NGC 253. We find 10 compact maser features, all of which are located along the narrow ridge of bright radio continuum emission within the central 10 arcsec2 of the nucleus. The compact OH features appear physically associated with regions of dense molecular gas, supernova remnants, and H II regions. The data for several of the features are consistent with collisional inversion from the interaction of supernova remnant shocks with molecular gas, similar to that found for Galactic supernova remnants by Frail et al. For many other features near the nucleus of NGC 253, we find conjugate behavior in the 1720 and 1612 MHz lines, with one line showing stimulated emission and the other showing stimulated absorption. These results suggest the importance of infrared pumping associated with the active star formation in NGC 253 and are consistent with large column densities of molecular gas [N(H2) 1022 cm-2] .
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