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We propose the formation of massive pristine dark-matter (DM) halos withmasses of $\sim 10^8~M_\odot$, due to the dynamical effects of frequent mergersin rare regions of the Universe with high baryonic streaming velocity relativeto DM. Since the streaming motion prevents gas collapse into DM halos anddelays prior star formation episodes, the gas remains metal-free until the halovirial temperatures $\gtrsim 2\times 10^4~{\rm K}$. The minimum cooling mass ofDM halos is boosted by a factor of $\sim 10-30$ because frequent major mergersof halos further inhibit gas collapse. We use Monte Carlo merger trees tosimulate the DM assembly history under a streaming velocity of twice theroot-mean-square value, and estimate the number density of massive DM haloscontaining pristine gas as $\simeq 10^{-4}~{\rm cMpc}^{-3}$. When the gasinfall begins, efficient Ly$\alpha$ cooling drives cold streams penetratinginside the halo and feeding a central galactic disk. When one stream collideswith the disk, strong shock forms a dense and hot gas cloud, where the gasnever forms H$_2$ molecules due to effective collisional dissociation. As aresult, a massive gas cloud forms by gravitational instability and collapsesdirectly into a massive black hole (BH) with $M_\bullet \sim 10^5~M_\odot$.Almost simultaneously, a galaxy with $M_{\star, \rm tot}\sim 10^6~M_\odot$composed of Population III stars forms in the nuclear region. If the typicalstellar mass is as high as $\sim 100~M_\odot$, the galaxy could be detectedwith the James Webb Space Telescope even at $z\gtrsim 15$. These massive seedBHs would be fed by continuous gas accretion from the host galaxy, and grow tobe bright quasars observed at $z\gtrsim 6$.

Massive black hole and Population III galaxy formation in over-massive dark matter halos with violent merger histories