Sequential formation of supermassive stars and heavy seed BHs through the interplay of cosmological cold accretion and stellar radiative feedback

ABSTRACT Supermassive stars (SMSs) and heavy seed black holes, as their remnants, are promising candidates for supermassive black hole (SMBH) progenitors, especially for ones observed in the early universe $z\simeq 8.5-10$ by recent JWST observations. Expected cradles of SMSs are the atomic cooling haloes ($M_{\rm halo}\simeq 10^7\ \mathrm{M}_{\odot }$), where ‘cold accretion’ emerges and possibly forms SMSs. We perform a suit of cosmological radiation hydrodynamics simulations and investigate star formation after the emergence of cold accretion, solving radiative feedback from stars inside the halo. We follow the mass growth of the protostars for $\sim 3\ \mathrm{Myr}$, resolving the gas inflow down to $\sim 0.1\ \mathrm{pc}$ scales. We discover that, after cold accretion emerges, multiple SMSs of $m_{\star }\gtrsim 10^5\ \mathrm{M}_{\odot }$ form at the halo centre with the accretion rates maintained at $\dot{m}_{\star }\simeq 0.04\ \mathrm{M}_\odot \mathrm{yr}^{-1}$ for $\lesssim 3\ \mathrm{Myr}$. Cold accretion supplies gas at a rate of $\dot{M}_{\rm gas}\gtrsim 0.01-0.1\ \mathrm{M}_\odot \mathrm{yr}^{-1}$ from outside the halo virial radius to the central gas disc. Gravitational torques from spiral arms transport gas further inwards, which feeds the SMSs. Radiative feedback from stars suppresses $\mathrm{H}_2$ cooling and disc fragmentation, while photoevaporation is prevented by a dense envelope, which attenuates ionizing radiation. Our results suggest that cold accretion can bring efficient BH mass growth after seed formation in the later universe. Moreover, cold accretion and gas migration inside the central disc increase the mass concentration and provide a promising formation site for the extremely compact stellar clusters observed by JWST.