Cold-mode and hot-mode accretion in galaxy formation: an entropy approach

TL;DR

This study uses an entropy-based criterion to distinguish cold and hot accretion modes in galaxy formation, analyzing simulations with and without feedback, and compares results with a semi-analytic model.

Contribution

It introduces an entropy criterion for separating accretion modes and validates it against cosmological simulations and a semi-analytic model.

Findings

Entropy criterion effectively distinguishes accretion modes.

Feedback increases hot gas reaccretion and lowers cooling times.

GalICS 2.1 model reproduces the cold-to-hot accretion transition.

Abstract

We have analysed two cosmological zoom simulations with $M_{\rm vir}\sim 10^{12}{\rm\,M}_\odot$ from the NIHAO series, both with and without feedback. We show that an entropy criterion based on the equation of state of the intergalactic medium can successfully separate cold- and hot-mode accretion. The shock-heated gas has non-negligible turbulent support and cools inefficiently. In the simulations without feedback, only a small fraction ($\sim 20$ per cent) of the stellar mass comes from baryons that have been in the hot circumgalactic medium, although quantitative conclusions should be taken with caution due to our small-number statistics. With feedback, the fraction is larger because of the reaccretion of gas heated by supernovae, which has lower entropies and shorter cooling times than the gas heated by accretion shocks. We have compared the results of NIHAO to predictions of the GalICS 2.1 semianalytic model of galaxy formation. The shock-stability criterion implemented in GalICS 2.1 successfully reproduces the transition from cold- to hot-mode accretion.