Stacked residual spectra normalized by emission-line flux for Hβ (left) and Hα (right). The black solid curves are the normalized residual spectra, while the red dashed curves are the normalized gas-only (i.e., stellar-continuum subtracted) spectra for reference. The orange horizontal lines denote where the flux is zero. The F values (upper-left corners) indicate the fraction of integrated residual flux compared to the emission line flux, measured within the blue-shaded region of 20 Å width.