Spectral analysis of the nuclear stellar population and gas emission in six nearby galaxies

Abstract

Optical spectra were used to study the stellar population and gas emission, when present, in the central parts of the elliptical galaxies NGC 6958, NGC 6851 and ESO 185-G54, and the spiral galaxies NGC 5757, NGC 7412 and IC 4214. Stellar population properties were derived by comparing the equivalent widths of the absorption lines with those of galaxy templates built from star clusters, as well as by applying a method of stellar population synthesis based on the same star clusters. For the galaxies with emission lines, the underlying stellar population was subtracted to obtain the pure emission spectrum, and the resulting emission line intensities were measured to determine the mechanism responsible for the gas excitation. From this analysis we concluded that the galaxies have solar metallicity except the elliptical ESO 185-G54 that reached metallicity slightly above solar. The three ellipticals and the Sa galaxy IC 4214 have their nuclei dominated by old stellar populations, although IC 4214 appears to have some contribution from the young generation stars. Recent star formation ( t< 500 Myr) is prominent in the nucleus of the SBb galaxy NGC 5757, and is also present to a lesser degree in the Sbc NGC 7412. In these two galaxies, the recent star formation is responsible for the observed H ii type emission lines, while IC 4214 has LINER emission. Among the ellipticals, only in NGC 6958 did we detect ionized gas, whose spectra is typical of LINERs.