The Role of Cold Flows and Reservoirs in Galaxy Formation With Strong Feedback

TL;DR

This study investigates how cold and hot gas flows contribute to galaxy formation, highlighting the importance of hot gas reservoirs and feedback processes in regulating star formation over cosmic time.

Contribution

It provides a detailed analysis of bimodal gas accretion modes and their impact on star formation, emphasizing the role of hot reservoirs and feedback in galaxy evolution.

Findings

Cold-mode dominates early gas inflows and star formation.

Hot-mode accretion builds a hot gas reservoir in halos.

Feedback cycles regulate star formation and gas cycling.

Abstract

We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations (Stinson et al. 2010). Accreted gas is bimodal with a natural temperature division at $10^5$ K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z=2-4 and declining exponentially below z$\sim$2. Hot-mode accretion peaks near z=1-2 and declines gradually. Hot-mode exceeds cold-mode accretion at z$\sim$1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2-8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z=1-2 from the accretion peak at z=2-4.