Nearby early-type galaxies with ionized gas - I. Line-strength indices of the underlying stellar population

With the aim of building a data-set of spectral properties of well studied early-type galaxies showing emission lines, we present intermediate resolution spectra of 50 galaxies in the nearby Universe. The sample, which covers several of the E and S0 morphological sub-classes, is biased toward objects that might be expected to have ongoing and recent star formation, at least in small amounts, because of the presence of the emission lines. The emission is expected to come from the combination of active galactic nuclei and star formation regions within the galaxies. Sample galaxies are located in environments corresponding to a broad range of local galaxy densities, although predominantly in low density environments. Our long-slit spectra cover the 3700 - 7250 angstrom wavelength range with a spectral resolution of approximate to 7.6 angstrom at 5550 angstrom. The specific aim of this paper, and our first step in the investigation, is to map the underlying galaxy stellar population by measuring, along the slit positioned along the galaxy major axis, line-strength indices at several, homogeneous galacto-centric distances. For each object we extracted 7 luminosity-weighted apertures (with radii 1.5 '', 2.5 '', 10 '', r(e)/10, r(e)/8, r(e)/4 and r(e)/2) corrected for the galaxy ellipticity and 4 gradients (0 <= r <= r(e)/16, r(e)/ 16 <= r <= r(e)/8, r(e)/8 <= r <= r(e)/4 and r(e)/4 <= r <= r(e)/2). For each aperture and gradient we measured 25 line-strength indices: 21 of the set defined by the Lick-IDS standard system ( Trager et al. 1998, ApJS, 116, 1) and 4 introduced by Worthey O Ottaviani ( 1997, ApJS, 111, 377). Line-strength indices have been transformed to the Lick-IDS system. Indices derived then include H beta, Mg-1, Mg-2, Mgb, MgFe, Fe5270, Fe5335 commonly used in classic index-index diagrams. The paper introduces the sample, presents the observations, describes the data reduction procedures, the extraction of apertures and gradients, the determination and correction of the line-strength indices, the procedure adopted to transform them into the Lick-IDS System and the procedures adopted for the emission correction. We finally discuss the comparisons between our dataset and line-strength indices available in the literature. A significant fraction, about 60%, of galaxies in the present sample has one previous measurement in the Lick-IDS system but basically restricted within the r(e)/8 region. Line-strength measures obtained both from apertures and gradients outside this area and within the r(e)/8 region, with the present radial mapping, are completely new.