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We simulate an equal-mass merger of two Milky Way-size galaxy discs withmoderate gas fractions at parsec-scale resolution including a new model forradiative cooling and heating in a multi-phase medium, as well as starformation and feedback from supernovae. The two discs initially have a$2.6\times10^6\mathrm{~M_{\odot}}$ supermassive black hole (SMBH) embedded intheir centers. As the merger completes and the two galactic cores merge, theSMBHs form a a pair with a separation of a few hundred pc that graduallydecays. Due to the stochastic nature of the system immediately following themerger, the orbital plane of the binary is significantly perturbed.Furthermore, owing to the strong starburst the gas from the central region iscompletely evacuated, requiring $\sim10$~Myr for a nuclear disc to rebuild.Most importantly, the clumpy nature of the interstellar medium has a majorimpact on the the dynamical evolution of the SMBH pair, which undergogravitational encounters with massive gas clouds and stochastic torquing byboth clouds and spiral modes in the disk. These effects combine to greatlydelay the decay of the two SMBHs to separations of a few parsecs by nearly twoorders of magnitude, $\sim 10^8$ yr, compared to previous work. In mergers ofmore gas-rich, clumpier galaxies at high redshift stochastic torques will beeven more pronounced and potentially lead to stronger modulation of the orbitaldecay. This suggests that SMBH pairs at separations of several tens of parsecsshould be relatively common at any redshift.

Orbital Decay of Supermassive Black Hole Binaries in Clumpy Multiphase Merger Remnants