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The most frequently proposed model for the origin of quasars holds that thehigh accretion rates seen in luminous active galactic nuclei are primarilytriggered during major mergers between gas-rich galaxies. While plausible fordecades, this model has only begun to be tested with statistical rigor in thepast few years. Here we report on a Hubble Space Telescope study to test thishypothesis for $z=2$ quasars with high super-massive black hole masses($M_\mathrm{BH}=10^9-10^{10}~M_\odot{}$), which dominate cosmic black holegrowth at this redshift. We compare Wide Field Camera 3 $F160W$ (rest-frame$V$-band) imaging of 19 point source-subtracted quasar hosts to a matchedsample of 84 inactive galaxies, testing whether the quasar hosts have greaterevidence for strong gravitational interactions. Using an expert rankingprocedure, we find that the quasar hosts are uniformly distributed within themerger sequence of inactive galaxies, with no preference for quasars inhigh-distortion hosts. Using a merger/non-merger cutoff approach, we recoverdistortion fractions of $f_\mathrm{m,qso}=0.39\pm{}0.11$ for quasar hosts and$f_\mathrm{m,gal}=0.30\pm{}0.05$ for inactive galaxies (distribution modes, 68%confidence intervals), with both measurements subjected to the sameobservational conditions and limitations. The slight enhancement in distortedfraction for quasar hosts over inactive galaxies is not significant, with aprobability that the quasar fraction is higher of$P(f_\mathrm{m,qso}>f_\mathrm{m,gal}) = 0.78$ ($0.78\,\sigma{}$), in line withresults for lower mass and lower $z$ AGN. We find no evidence that majormergers are the primary triggering mechanism for the massive quasars thatdominate accretion at the peak of cosmic quasar activity.

Do the Most Massive Black Holes at $z=2$ Grow via Major Mergers?