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

We present the first detection of extragalactic hydrogen recombination line maser emission in the H26alpha transition toward the inner 13.5 pc nuclear region of the starburst galaxy NGC 253 using ALMA data. In regions with complex continuum emission (dust, free-free and synchrotron) we propose to use the recombination line spectral index, $α_\mathrm{L}$ ($S_\mathrm{L}\cdot Δv \propto ν^{α_\mathrm{L}}$), between the H30alpha and the H26alpha lines to study the structure of ultra-compact HII regions and to identify maser emission ($α_\mathrm{L}> 2.1$) from ionized winds. The measured values of $α_\mathrm{L}$ ranged from 1.0 to 2.9. The largest $α_\mathrm{L}$ can only be explained by maser emission. The measured flux density in the H26$α$ 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.