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\subsection{stellar feedback in galaxies and galaxy mergers to binary supermassive black holes}  Feedback encompasses a wide range of astrophysics, including radiation from stars heating and ionizing surrounding gas, thermal and kinetic energy injection from supernova explosions, heating from active galactic nuclei (AGN), and the impact of AGN jets. These processes can drive galactic winds, blowing gas out of galaxies, and slowing down star formation. Although SMBH have been suggested as a highly efficient mechanism for regulating star formation in galaxies, (Borys et al, 2005) suggest that observational evidence for feedback, in particular due to SMBH activity, is scarce.  Accretion onto SMBHs can release tremendous amounts of energy that can potentially drive large-scale outflows, and quench the star formation. Accretion is an important feedback source for galaxy formation as it releases vast amounts of energy – this is why AGN have high luminosities. (Karouzos et al, 2013) provides evidence for the existence of a positive correlation between the luminosity of the AGN component of a host galaxy and the host’s star-formation. This is said to be independent of the radio luminosity. It was also discovered that high radio-luminosity are associated with a decrease in star-formation rate. (Galaxy formation and evolution). Analytical models of galaxy formation that use cosmological N-body simulations have revealed that AGN feedback can be responsible for creating a population of red and dead massive galaxies (Croton et al. 2006; Bower et al. 2006; Cattaneo et al. 2006; Somerville et al. 2008).  Hot gas, cold gas, stars, and a supermassive black hole are the main components of a galaxy. Cooling, star formation, AGN accretion and feedback processes can transfer particles from one component to another, and in that way alter the efficiency of all the processes in the galaxy. For example, increased cooling of hot gas will lead to the production of more cold gas. This in turn increases the star formation rate, therefore increasing the rate of supernova. Energy released by from supernovae and ejections from AGN jets and reheats cold gas, leading to a suppression of star formation (negative feedback). In contrast, supernova blast-waves may also compress the surrounding cold gas, thus increasing star formation rate (positive feedback). The formation and evolution of galaxies depends on these feedback loops. (Mo et al, 2011).  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).