2006ApJ...643..112L

A high-quality Keck spectrum of the Hα line in NGC 4395 reveals symmetric exponential wings, f_v∝e^–v/σ, 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^–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 σ≃1.1σ_e(lnτ^–1_e)^0.45, where σ_e is the electron velocity dispersion, and τ_eis the electron-scattering optical depth. The Hα wings in NGC 4395 are well fit by an electron-scattering model with τ_e=0.34 and an electron temperature T_e=1.1x10⁴ K. 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 τ_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 τ_eand the ionizing flux on timescales shorter than the BLR dynamical time, or ∼1 week for NGC 4395. In contrast, the value of σ 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_eand τ_ein the BLR.