Gas accretion at high redshift: cold flows all the way

ABSTRACT We study in detail how massive galaxies accrete gas through cosmic time using cosmological hydrodynamical simulations from the High-z Evolution of Large and Luminous Objects (HELLO) and the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) projects. We find that accretion through cold filaments at high z ($z\approx 2$–4) is a key factor in maintaining the high star-formation rates ($\gtrsim 100\, {\mathrm{M}}_{\odot }\, \mathrm{yr}^{-1}$) observed in these galaxies, and that more than 75 per cent of the total gas participating in the star formation process is accreted via this channel at high z even in haloes well above $10^{12}$ ${\mathrm{M}}_{\odot }$. The low volume occupancy of the filaments allows plenty of space for massive gas outflows generated by the vigorous star formation and AGN activity, with the cold incoming gas and the hot outflowing gas barely interacting. We present a model based on Bayesian hierarchical formalism able to accurately describe the evolution of the cold fraction accretion with redshift and halo mass. Our model predicts a relatively constant critical mass ($M_{\mathrm{c}}$) for cold-to-hot transition up to $z\sim 1.3$ and an evolving critical mass $\log (M_{\mathrm{c}}) \propto \log (1+z)^{1.7}$ at higher redshift. Overall, our findings provide deeper insight into the cosmic evolution of gas accretion modes and offer a robust framework for understanding how cold accretion contributes to galaxy growth across different epochs.