We propose the formation of massive pristine dark-matter (DM) halos with masses of $sim 10^8~M_odot$, due to the dynamical effects of frequent mergers in rare regions of the Universe with high baryonic streaming velocity relative to DM. Since the streaming motion prevents gas collapse into DM halos and delays prior star formation episodes, the gas remains metal-free until the halo virial temperatures $gtrsim 2times 10^4~{rm K}$. The minimum cooling mass of DM halos is boosted by a factor of $sim 10-30$ because frequent major mergers of halos further inhibit gas collapse. We use Monte Carlo merger trees to simulate the DM assembly history under a streaming velocity of twice the root-mean-square value, and estimate the number density of massive DM halos containing pristine gas as $simeq 10^{-4}~{rm cMpc}^{-3}$. When the gas infall begins, efficient Ly$alpha$ cooling drives cold streams penetrating inside the halo and feeding a central galactic disk. When one stream collides with the disk, strong shock forms a dense and hot gas cloud, where the gas never forms H$_2$ molecules due to effective collisional dissociation. As a result, a massive gas cloud forms by gravitational instability and collapses directly 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 typical stellar mass is as high as $sim 100~M_odot$, the galaxy could be detected with the James Webb Space Telescope even at $zgtrsim 15$. These massive seed BHs would be fed by continuous gas accretion from the host galaxy, and grow to be bright quasars observed at $zgtrsim 6$.
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