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\section{Introduction}\label{sec:intro}
Brightest Cluster Galaxies (BCGs) are extremely luminous galaxies that are usually located in the centre of rich galaxy clusters. They have been shown to be distinct from other similarly luminous cluster galaxies \citep[e.g.][]{HAUSMAN78, POSTMAN95, LAZZATI98, LINDEN07}. In the hierarchical scenario of structure formation \citep{TOOMRE77,WHITE78} galaxies grow in mass and size by merging with their neighbours. BCGs are predicted to have a more active merger history than lower mass galaxies \citep [][]{WHITE78, KHOCHFAR03,DELUCIA07,OSER10,NAAB13}. These galaxies are often considered as the extreme end-point of massive galaxy evolution. However, despite being among the most luminous galaxies, and generally easy to detect, observations and theory have not reached a common point yet, and their evolution is still not fully understood.
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Observations suggest that the mass growth of BCGs evolves with time. BCGs accrete their mass at a fast rate until $z\sim0.5$, thereafter their mass growth slows down \citep{LIDMAN13, LIN13,OLIVA14,INAGAKI15}. Studies looking at BCG companions have concluded that their stellar mass grows through major mergers ($\ge1:3$ mass ratios) by a factor of $1.8\pm 0.43$ at $0.8 < z < 1.5$ \citep{BURKE13}, and mostly by minor mergers ($\leq1:4$ mass ratios) by a factor of $1.1$ at $z < 0.3$ \citep{EDWARDS12}. Major mergers are rare at low redshifts, yet still possible \citep[e.g.][]{BROUGH11,JIMMY13}.
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The recent accretion history of galaxies can be read through their stellar population gradients. In the canonical scenario, a galaxy's initial metallicity gradient is set by an initial starburst at $z\geq3$ and the metallicity decreases in the outskirts, as metallicity follows the changes in the gravitational potential \citep{SCOTT09, MCDERMID12}. This gradient can be disrupted by violent merging events (major mergers), or reinforced by minor mergers \citep{KOBAYASHI04, SPOLAOR09, COOPER10,PIPINO10}. \citet{HIRSCHMANN14} analysed the stellar populations of 10 massive halos ($10^{12}<$~M$_{\rm halo}<10^{13}$~M$_{\odot}$) from the high-resolution cosmological simulation of \citet{HIRSCHMANN13}. They found that major mergers do flatten the metallicity gradients. If, as predicted, BCGs have an active merger history, including several major mergers, they would be expected to have shallower metallicity gradients than lower mass galaxies. However, long-slit observations to date suggest that they have a wide range of gradients \citep{BROUGH07,LOUBSER12}.
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Integral Field Unit (IFU) spectroscopy is a valuable tool to explore the spatially-resolved kinematics and stellar populations of galaxies. The SAURON \citep{ZEEUW02} and ATLAS$^{3D}$ \citep{CAPPELLARI11a} surveys have used IFU spectroscopy to explore a significant sample of early-type galaxies in the local Universe. \citet{KUNTSCHNER10} and \citet{ATLAS3D} presented the stellar population analysis of the SAURON and ATLAS$^{3D}$ samples, respectively, finding that 40 per cent of the galaxies typically show contributions from young stellar populations connected to low mass, fast rotator systems. In contrast, they find that slow rotators are generally consistent with old ($\geq 10$ Gyr) stellar populations. The most massive systems (stellar mass $\geq 10^{10.5}$M$_{\odot}$) have the flattest metallicity gradients. However, the ATLAS$^{3D}$ sample contains 21 galaxies with dynamical masses greater than $10^{11.3}$M$_{\odot}$, and only one of those is a BCG (M87).
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\citet{JIMMY13} analysed the kinematics and photometry of a sample of 10 BCGs observed with the VIMOS IFU, of which 4 have close massive companions. If BCGs were the product of many minor mergers they would be expected to be slow-rotating galaxies. \citet{JIMMY13} found that 30 per cent of the BCGs in their sample are fast rotators. The simulations of \citet{NAAB13}, predict that angular momentum mostly depends on the gas content of the galaxies involved in the interaction, suggesting that the slow or fast rotation could be a temporary state. \citet{JIMMY13}, also find through photometric analysis \citep[$G-M_{20}$;][]{LOTZ08} that 40 per cent of the galaxies in the sample have undergone a minor merger within the last 0.2 Gyr.
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Our paper is the third in a series analysing the spatially-resolved spectra of BCGs \citep[after][]{BROUGH11,JIMMY13}. We present here the spatially$-$resolved stellar populations of the BCG sample presented in \citet{JIMMY13}. This is the first IFU analysis dedicated to the stellar populations of BCGs. We will investigate whether BCG accretion histories are different from those of the general massive early-type galaxy population by comparing our measurements to those from SAURON and ALTLAS$^{3D}$.
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We present the sample selection and observations in Section 2. Section 3 describes the stellar kinematic and photometric measurements from \citet{JIMMY13}. It also describes the early-type galaxy samples of SAURON \citep{KUNTSCHNER10} and ATLAS$^{3D}$ \citep{ATLAS3D} to which we will compare our sample BCGs throughout the paper. Section 4 presents the stellar population analysis. Section 5 summarises the main results. These are later discussed in Section 6. Our conclusions are presented in Section 7. The cosmology adopted throughout this paper is H$_0$=70 kms$^{-1}$ Mpc$^{-1}$, $\Omega_M$=0.3, $\Omega_{\Lambda}$=0.7.
