Evidence for Line Broadening by Electron Scattering in the Broad Line Region of Ngc 4395

A high quality Keck spectrum of the Hα line in NGC 4395 reveals symmetric exponential wings, fv ∝ e, with σ ≃ 500 km s. The wings extend out to > ∼ 2500 km s from the line core, and down to a flux density of < ∼ 10 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 σ ≃ 1.1σe(ln τ e ), where σe is the electron velocity dispersion, and τe is the electron scattering optical depth. The Hα wings in NGC 4395 are well fit by an electron scattering model with τe = 0.34, and an electron temperature Te = 1.1 × 10K. Such conditions are produced in photoionized gas with an ionization parameter U ≃ 0.3, as expected in the broad line region (BLR). Similar analysis of the [O III] λ5007 line yields τe < 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 τe and the ionizing flux on time scales shorter than the BLR dynamical time, or ∼ 1 week for NGC 4395. In contrast, the value of σ 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 Te and τe in the BLR. Subject headings: galaxies: active — galaxies: Seyfert — quasars: emission lines — galaxies: individual NGC 4395