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Paola Oliva-Altamirano added section_Stellar_Population_Analysis_label__.tex
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\section{Stellar Population Analysis}\label{sec:annuli}
The study of stellar populations requires high SNR spectra. In order to secure a high enough SNR we use our own python routine\footnote{http://astronomy.swin.edu.au/$\sim$poliva/codes/annuli\_stacking/spectra.py} to stack the spaxels within annuli for each galaxy. To identify the annuli we follow the total flux in the wavelength$-$integrated galaxy image. This indirectly maps the galaxy morphology, i.e. the shape of the annuli is determined by the galaxy's morphology. Each spectrum is shifted to rest frame wavelength before stacking using the velocity measurements obtained from pPXF. The spectra are then broadened to a reference velocity dispersion, $\sigma$ (the maximum velocity dispersion of the galaxy). This reduces the dilution of the spectral features due to rotational broadening and allows us to have a fixed and constant velocity dispersion when measuring the stellar population parameters. As a result, we have one spectrum per annulus per galaxy.
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From the 10 BCGs and 4 companion galaxies presented in \citet{JIMMY13}, the BCG 1153, and the companion of BCG 1066 have too low SNR for stellar population analysis. Our final sample thereby consists of 9 BCGs, 4 of them with close massive companions. For 2 of these (1027, 1048) it was possible to resolve the companions as well. In those cases we refer to the main galaxy as 1027A and 1048A, and the companions as 1027B, 1048B, and 1048C.
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In Fig \ref{fig:FOV} we show the annular distribution of 3 representative galaxies. The upper panels are the flux-collapsed VIMOS image. The lower panels are the annular distribution per galaxy. The annuli cover up to 1 R$_e$ in each galaxy. The central aperture has been defined as $0.2\pm0.03$~R$_e$, allowing the central annulus to contain two or more spaxels (for most of the galaxies). The median value of the SNR of the final sample stacked spectra is $\sim35$~\AA$^{-1}$ with the majority having SNR~$> 20$~\AA$^{-1}$. Only the outermost annuli in the companion galaxies 1048B and 1048C drop below this threshold (SNR~$= 15$~\AA$^{-1}$).
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The analysed rest wavelength range (3900-5600~\AA) covers the absorption line indices: Ca$_{\rm K}$ (Ca II 3933), Ca$_{\rm H}$ (Ca II 3968), H$\delta$, H$\gamma$, Ca$_{\rm G}$ (Ca I 4307), H$\beta$, Fe5015, Mgb5175, Fe5270. Our age and metallicity measurements are based on the full spectrum. However, we mask the weaker Balmer indices H$\delta$, and H$\gamma$ to ensure a clean comparison with the ATLAS$^{3D}$ data (which age measurements are based on H$\beta$). The enhanced lines Ca$_{\rm K}$, Ca$_{\rm H}$, and Ca$_{\rm G}$ provide a strong case for the existence of old stellar populations. Furthermore, where there are mixed stellar populations, the ratio between these lines indicates the relative importance of the young stellar populations \citep{SANCHEZ12}.
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In the following subsections we describe the method we implement to estimate the metallicities and ages.
