Evidence for line broadening by electron scattering in the broad-line region of NGC 4395

A high-quality Keck spectrum of the H alpha line in NGC 4395 reveals symmetric exponential wings, f(v) proportional to e(-v/sigma), with sigma similar or equal to 500 km s(-1). The wings extend out to greater than or similar to 2500 km s(-1) from the line core, and down to a flux density of less than or similar to 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 similar or equal to 1.1 sigma(e)(ln tau(-1)(e))(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 x 10(4) K. Such conditions are produced in photoionized gas with an ionization parameter U similar or equal to 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 timescales shorter than the BLR dynamical time, or similar to 1 week for NGC 4395. In contrast, the value of sigma should remain nearly constant on these timescales. 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.