Gravitational Heating Helps Make Massive Galaxies Red and Dead

TL;DR

This study demonstrates that gravitational heating from infalling stellar clumps can effectively heat gas and quench star formation, leading to the formation of red, elliptical galaxies without requiring supernova or AGN feedback.

Contribution

It reveals gravitational heating as a key mechanism in galaxy evolution, showing its role in making massive galaxies red and dead independently of other feedback processes.

Findings

Gas temperature increases with decreasing redshift.

Gravitational heating can produce red elliptical galaxies by z~1.

Dark matter density decreases due to dynamical friction from infalling clumps.

Abstract

We study the thermal formation history of four simulated galaxies that were shown in Naab et al. (2007) to reproduce a number of observed properties of elliptical galaxies. The temperature of the gas in the galaxies is steadily increasing with decreasing redshift, although much of the gas has a cooling time shorter than the Hubble time. The gas is being heated and kept hot by gravitational heating processes through the release of potential energy from infalling stellar clumps. The energy is dissipated in supersonic collisions of infalling gas lumps with the ambient gas and through the dynamical capturing of satellite systems causing gravitational wakes that transfer energy to the surrounding gas. Furthermore dynamical friction from the infalling clumps pushes out dark matter, lowering the central dark matter density by up to a factor of two from z=3 to z=0. In galaxies in which the late formation history (z<2) is dominated by minor merging and accretion the energy released (E~5x10^{59} ergs) from gravitational feedback is sufficient to form red and dead elliptical galaxies by z~1 even in the absence of supernova and AGN feedback.