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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.   \\\\  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}. \\\\  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}.