A high quality Keck spectrum of the H \alpha line in NGC 4395 reveals symmetric exponential wings , f _ { v } \propto e ^ { - v / \sigma } , with \sigma \simeq 500 km s ^ { -1 } .
The wings extend out to \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 2500 km s ^ { -1 } from the line core , and down to a flux density of \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 10 ^ { -3 } of the peak flux density .
Numerical and analytic calculations indicate that exponential wings are expected for optically thin , isotropic , thermal electron scattering .
Such scattering produces exponential wings with \sigma \simeq 1.1 \sigma _ { e } ( \ln \tau _ { e } ^ { -1 } ) ^ { 0.45 } , where \sigma _ { e } is the electron velocity dispersion , and \tau _ { e } is the electron scattering optical depth .
The H \alpha wings in NGC 4395 are well fit by an electron scattering model with \tau _ { e } = 0.34 , and an electron temperature T _ { e } = 1.1 \times 10 ^ { 4 } K. Such conditions are produced in photoionized gas with an ionization parameter U \simeq 0.3 , as expected in the broad line region ( BLR ) .
Similar analysis of the [ O III ] \lambda 5007 line yields \tau _ { e } < 0.01 , consistent with the lower ionization in the narrow line region .
If the electron scattering interpretation is correct , there should be a tight correlation between \tau _ { e } and the ionizing flux on time scales shorter than the BLR dynamical time , or \sim 1 week for NGC 4395 .
In contrast , the value of \sigma should remain nearly constant on these time scales .
Such wings may be discernible in other objects with unusually narrow Balmer lines , and they can provide a useful direct probe of T _ { e } and \tau _ { e } in the BLR .