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\section{Discussion}\label{sec:disc}  BCGs represent the extremely massive end of the early-type galaxy population. These galaxies live in high-density environments commonly surrounded by many companions. We have presented here the first integral field analysis of the radial stellar populations of 9 BCGs up to 1~R$_e$.   %------------------------------------------------------------------------------------------------------------------------------------------------------------------  \subsection{Stellar Ages}  Hydrodynamical simulations of early-type galaxies (in less dense environments than BCGs) predict more massive galaxies to be older than less massive galaxies, such that at masses $>10^{10.5}$~M$_{\odot}$ the galaxies are older than $10$~Gyr \citep[e.g.][]{NAAB13,PEEPLES14,HIRSCHMANN13} and show passive evolution from $z=2$. We find that 3 out of 9 BCGs in our sample have old central ages ($> 12$ Gyr), in agreement with this prediction. These 3 galaxies are also consistent with the massive early-type galaxies from ATLAS$^{3D}$ (median Age~$=12.0\pm3.8$~Gyr).  \\\\  However, 6 out 9 BCGs in our sample have central intermediate ages (5 Gyr $\leq$ Age < 10 Gyr). Previous observations have shown that these intermediate ages in BCGs are not unusual. \citet{LOUBSER09} analysed a large sample of 49 BCGs, and found that 24 of them (49~per~cent of the sample) are younger than 9~Gyr old. \citet{FITZPATRICK14} found that, at fixed velocity dispersion and surface brightness, central galaxies in SDSS are younger than satellite galaxies. \citet{BARBERA14} also found that central galaxies have younger ages and higher metallicities than isolated early-type galaxies. Consistently, these 6 BCGs sit at the younger age limit of the massive early-type galaxies from ATLAS$^{3D}$ (Fig~\ref{fig:central}).   \\\\  Many hypotheses have tried to explain why such massive galaxies as BCGs have intermediate age stellar populations. A common prediction is that gas cooling from the intra-cluster medium may be forming stars which will result in young ages \citep[e.g.][]{EDWARDS07,ODEA08, BILDFELL08, LOUBSER14}. For most of the clusters considered here, X-ray imaging is not available, so that we cannot assess whether they host a cool core or not. Only two of our galaxies have been confirmed to be hosted by a non-cooling flow cluster \citep{WHITE97} and for those we find contradictory results: 2001 is one of the oldest galaxies in our sample, consistent with the cool core hypotheses, however, 1066 shows a young central region and a positive age gradient, suggesting that cool cores are not the explanation for the intermediate ages observed.   \\\\  The semi-analytical model of \citet{TONINI12} focusses on BCGs rather than on the general population of early-type galaxies, predicting that BCGs have more prolonged star formation as a result of their active merger history. The BCGs experience continuous bursts of star formation across cosmic time, generating many stellar populations super imposed on one another. Our age measurements are luminosity-weighted, which means they reflect the youngest stellar population of the galaxy. Therefore, the intermediate ages we find suggest that the last star formation event could have taken place at $z\sim1$ when galaxy mergers are more likely to be gas rich.   %------------------------------------------------------------------------------------------------------------------------------------------------------------------  \subsection{Metallicities}   The BCGs in our sample show very homogenous central metallicities (median $[$Fe/H$]_{[\alpha/Fe]=0}= 0.13\pm0.07$). These high metallicities are consistent with previous long-slit and fibre observations of BCGs \citep{BROUGH07,LINDEN07,LOUBSER09,EIGENTHALER13} and are in agreement with the hypothesis of continuous star formation events at high redshifts.   \\\\  We find that the BCGs we observe have a range of metallicity gradients, from flat to shallow (median $\Delta$[Fe/H]~$= -0.11\pm0.1$) similar to other massive early-type galaxies at similar mass ($\Delta$[Z/H]~$=-0.19\pm0.1$; Fig \ref{fig:grad_mass}).   \\\\  Hydrodynamical simulations predict that galaxies that form through dissipative core collapse have typical metallicity gradients $\Delta$[Fe/H]~$\sim-0.4$ \citep[e.g][]{KOBAYASHI04,HIRSCHMANN14}. This is significantly steeper than the gradients we observe. However, simulations also show that this initial gradient can later be affected by accretion of external stellar populations, i.e. mergers \citep{HIRSCHMANN13,MARTIZZI14}.  \\\\  Dissipationless major mergers make the stars lose their orbits and move randomly within the distribution of the galaxy, inducing stellar population mixing and flattening the gradients \citep{HOPKINS092}. This suggest that the BCGs as well as the massive SAURON galaxies have gone through at least one recent dissipationless major merger since $z<1$ \citep{KOBAYASHI04,HIRSCHMANN14}.   %------------------------------------------------------------------------------------------------------------------------------------------------------------------   \subsection{Merger Histories}  From our analysis we conclude that BCGs have diverse evolutionary paths. 3 out of 9 BCGs in our sample show old and metal-rich central stellar populations, and shallow metallicity gradients. This suggest that their stars were formed in-situ at $z>2$. Thereafter the galaxies grow in mass and size by at least one major merger and many minor mergers \citep[e.g.][]{KOBAYASHI04,HIRSCHMANN14}. These galaxies are similar to the massive early-type galaxies in the SAURON and ATLAS$^{3D}$ sample which also have old central stellar populations and shallow metallicity gradients.   \\\\  The rest of the sample (6 out of 9) BCGs have intermediate central ages, high central metallicities, and shallow metallicity gradients in BCGs. This implies that these galaxies have experienced active accretion histories throughout cosmic time, as predicted by semi-analytical and dark matter simulations \citep{DELUCIA07, TONINI12, LAPORTE13}. The dense environment where BCGs evolve allows them to experience many mergers. These mergers will trigger star formation at high redshifts, and will disrupt the metallicity gradients at $z<1$, given that the fraction of gas in the merging galaxies decreases with time.   \\\\  \citet{JIMMY13} found that 4 of the 9 BCGs studied here show photometric signatures of minor mergers. The effect of minor mergers are not apparent in the inner ($<1$~R$_e$) stellar population gradients studied here, as minor mergers only affect stellar population gradients at $>2$~R$_e$ \citep[][]{FOSTER09,BARBERA12,PASTORELLO14, HIRSCHMANN14}. However, the photometric results are evidence of the active merging activity of these galaxies. Furthermore, 4 of the galaxies in our sample have close massive companions, most of these companions are FRs and are gravitationally bound to their respective BCG. This suggests a potential future major merger \citep{JIMMY13}.   \\\\  Many studies have found similar results on the stellar populations of BCGs \citep[e.g.][]{WHILEY08, STOTT08}. \citet{WEN11} analysed the BCGs colours from different high-redshift data sets (CFHT, COSMOS, SWIRE) and found that BCGs are consistent with stellar population synthesis models in which the galaxy formed at $z>2$. However, a large fraction of the sample shows bluer colours on the g'~-~z' and B~-~m$_{\mu m}$ bands at $z\sim0.8$, indicating star formation at those epochs. Furthermore, some BCGs show low levels of star formation in the local Universe \citep[e.g.][]{LIU12,OLIVA14,FRASER14}. This is consistent with the hypothesis that BCGs have complex accretion scenarios.  \\\\  Thanks to the spatial extent of the IFU spectroscopy we were able to resolve 3 companion galaxies (1027B, 1048B, 1048C) from 2 of the BCGs (1027A, 1048A). We find that the companion galaxies similar stellar populations with their respective BCG. However, due to the fact that their effective radius are close to the seeing FWHM, their stellar population gradients are unreliable.  %------------------------------------------------------------------------------------------------------------------------------------------------------------------   \subsection{Connection Between Stellar Populations and Kinematics}  One of the advantages of using IFU spectroscopy for this analysis is that we can compare the stellar populations to the kinematics of the galaxies in our sample. We have analysed 7 slow and 2 fast rotating BCGs and their kinematics appear to be independent of their stellar populations. We do not find any correlation between the angular momentum of the galaxies and their stellar population gradients in this small sample. The SRs show a large scatter in their metallicity gradients. The 2 FRs have similar metallicity gradients, but within the range of the SRs.   \\\\  \citet{MOODY14} and \citet{NAAB13} showed that if the gas fraction in a major merger is less than $10$~per~cent (dissipationless), the galaxy tends to maintain the slow or fast rotation of their progenitors. The BCGs in our sample are likely to have preserved the slow or fast rotation of their progenitors, given the lack of gas observed at present epochs and the evidence of recent dissipationless major mergers.