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Paola Oliva-Altamirano added subsection_Lick_IDS_System_The__.tex
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\subsection{Lick/IDS System}
The stellar population parameters can also be estimated using the Lick/IDS system \citep[e.g.][]{WORTHEY97}. This method measures the equivalent width of the absorption features at a fixed IDS resolution ($>9$~\AA) to later compare them with stellar population models that provide the corresponding age and metallicity values \citep[e.g.][]{WORTHEY97,VAZDEKIS97, PROCTOR02, SCHIAVON07}.
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To probe the robustness of our results obtained using STECKMAP we test our VIMOS spectra with the Lick/IDS system (see also Appendix A). We use the same stellar models and absorption lines as the ATLAS$^{3D}$ team to measure the [$\alpha$/Fe] abundances, ages and metallicities i.e. \citet[][]{SCHIAVON07}, H$\beta$, Fe5010, and Mgb. In Fig \ref{fig:alpha} we show the central [$\alpha$/Fe] of our BCGs (green-filled and red-open squares) and the ATLAS$^{3D}$ sample (blue crosses) as a function of mass. We find the BCG [$\alpha$/Fe] values (median [$\alpha$/Fe]~$=0.17\pm0.04$) to be consistent with those of the ATLAS$^{3D}$ massive galaxies (median [$\alpha$/Fe]~$=0.24\pm0.03$).
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In Fig \ref{fig:met_age} we show the estimated metallicities and ages from both methods, STECKMAP and Lick indices. The Lick ages (median Age~$=10.0\pm1.1$~Gyr) and metallicities (median [Z/H]~$=0.18\pm0.02$) are consistent with those from STECKMAP (median Age~$=8.9\pm3.3$~Gyr, median $[$Fe/H$]_{[\alpha/Fe]=0}=0.13\pm0.07$) within 1$\sigma$ error. Our STECKMAP results are also consistent with the stellar populations analysed by \citet{GALLAZZI05} from the SDSS DR4 spectra (of the galaxies in our sample).
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We adopt as the final result the stellar population parameters measured with STECKMAP. We discuss these results in the following sections.
