Evidence for the First Extragalactic Hydrogen Recombination Line Maser in NGC 253

We present the first evidence for extragalactic hydrogen recombination line maser emission in the H26 alpha transition toward the inner 13.5 pc nuclear region of the starburst galaxy NGC 253 using the Atacama Large Millimeter/submillimeter Array data. In regions with complex continuum emission (dust, free-free, and synchrotron) we propose to use the recombination line spectral index, alpha(L) (S-L . Delta v proportional to v(alpha L)), between the H30 alpha and the H26 alpha lines to study the structure of ultra-compact H II regions and to identify maser emission (alpha(L) > 2.1) from ionized winds. The measured values of alpha(L) ranged from 1.0 to 2.9. The largest alpha(L) can only be explained by maser emission. The measured flux density in the H26 alpha maser in NGC 253 suggests that we are observing hundreds of stars like MWC349A, a prototypical stellar wind where maser emission arises from its circumstellar disk. We briefly discuss the implication of the detection of maser emission in starburst galaxies like NGC 253.