IONIZED-GAS IN EARLY-TYPE GALAXIES - ITS EFFECT ON MG-B AND OTHER STELLAR LINE-STRENGTH INDEXES

In the light of the recent finding that nebular emission lines are com monly found in the inner regions of early-type galaxies, we evaluate t heir effect to stellar absorption-line indices. We derive analytical e xpressions for changes induced by the presence of nebular emission lin es, both for atomic and molecular absorption-line indices. We find tha t the [NI] emission-line doublet at 5199 Angstrom can significantly af fect Mg b line-strength measurements. For typical equivalent widths of the [N I] doublet in nuclei of early-type galaxies featuring nebular emission, the Mg b index is artificially enhanced by 0.4-2 Angstrom, w hich represents a significant fraction of the typical equivalent width of the Mg b line in early-type galaxies. We illustrate this effect in the case of NGC 2974.