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, $\alpha_\mathrm{L}$ ($S_\mathrm{L}\cdot
\Delta v \propto \nu^{\alpha_\mathrm{L}}$), between the H30alpha and the
H26alpha lines to study the structure of ultra-compact HII regions and to
identify maser emission ($\alpha_\mathrm{L}> 2.1$) from ionized winds. The
measured values of $\alpha_\mathrm{L}$ ranged from 1.0 to 2.9. The largest
$\alpha_\mathrm{L}$ 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.
