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\subsection{Galaxy Mergers} \subsection{Enhanced accretion and star formation in galaxy mergers}  If the predecessor dark matter halos contained central galaxies, the galaxies also merge, thus producing a new galaxy. Such a merger may be accompanied by strong star formation or AGN activity if the merging galaxies contained significant amounts of cold gas. (Mo et al, 2011). Black hole growth also depends on accretion and mergers. The merger of two galaxies increases the probability of the merger of the SMBHs at their centres, since black hole mergers are one of the ways through which black holes grow. Even so, the observation of systems with a combined SMBH are not common. (Dean et al, 2015)  These mergers instead produce a binary black hole which is a system consisting of two black holes in close orbit around each other. As the separation of the black holes decreases, they eject stars between them, creating a less dense core which is latter restored after the merger as large amounts of material are accreted towards the newly formed SMBH. (Volonteri et al, 2002). However, galaxy core-depletion can also be attributed to stars falling into to the SMBH(s) as a result of venturing too close to the SMBH(s) event horizon. (Graham 2005).  Theoretical expectations suggest that the closer AGN cores are the more AGN activity we observe because evidence in the X-ray electromagnetic range suggests that in dual AGN the luminosities of both AGN increase as the separation between the galaxies decrease. This suggests that AGN merging t is vital in powering the AGN. (Koss et al, 2011)  It has also been observed that the highest enhancements in AGN activity are seen for the equal-mass AGN pairs. The lower mass galaxies in unequal-mass pairs show relatively little enhancement in AGN activity. Reduced separation between AGN pairs has also been observed to lead to increased star formation (Ellison et al, 2011)