Ionized gas in early-type galaxies: Its effect on Mgb and other stellar line-strength indices

In the light of the recent finding that nebular emission lines are commonly found in the inner regions of early-type galaxies, we evaluate their effect to stellar absorption-line indices. We derive analytical expressions for changes induced by the presence of nebular emission lines, both for atomic and molecular absorption-line indices. We find that the [NI] emission-line doublet at 5199 A can significantly affect Mgb line-strength measurements. For typical equivalent widths of the [NI] doublet in nuclei of early-type galaxies featuring nebular emission, the Mgb index is artificially enhanced by 0.4 - 2 A, which represents a significant fraction of the typical equivalent width of the Mgb line in early-type galaxies. We illustrate this effect in the case of NGC 2974.