Measurement of the Lick indices in early-type galaxies: Line-of-sight velocity distribution corrections for IC 1459

Abstract

In this paper we analyze the measurements of the absorption line-strength Lick indices in the early-type galaxy IC 1459. We use the long-slit spectra of the elliptical galaxy IC 1459 from which its kinematics had previously been extracted to calculate the Lick indices for the observed spectral region (Mg2, Fe5270, Fe5335 and H?). We apply the usual procedure and correct the indices to the Lick spectral resolution and for the zero velocity dispersion. The procedure applied in this paper also corrects to non-Gaussian line-of-sight velocity distribution (LOSVD) observed in this galaxy, especially in its outer parts. The findings of Kuntschner (2004) were tested and it is shown that the departures from the Gaussian LOSVD may indeed cause erroneous determinations of the Lick indices. The impact of the introduction of non-Gaussian LOSVD differs for different indices. For the galaxy IC 1459 it is shown that the iron indices are especially sensitive when the correction due to anistropies is introduced: the corrections for Fe5270 and Fe5335 are ~10 and ~19 percent larger, respectively, than the corrections obtained in case of a pure Gaussian. The corrections for Mg2 index are shown to be negligible and the corrections of the H? index due to anisotropies are also small (below ~ 4 per cent at most).