Quenching vs. Quiescence: forming realistic massive ellipticals with a simple starvation model

TL;DR

This study shows that massive elliptical galaxies can form through a gradual starvation process without the need for sudden quenching, challenging traditional models of galaxy evolution.

Contribution

It introduces a simple starvation model demonstrating that gradual gas cooling suppression can produce realistic elliptical galaxies by z=0 without abrupt quenching events.

Findings

Galaxies can evolve into ellipticals through slow cooling suppression.

Color transition times are less than 1 Gyr, faster than previously thought.

Gradual starvation suffices to explain observed galaxy properties at z=0.

Abstract

The decrease in star formation (SF) and the morphological change necessary to produce the $z=0$ elliptical galaxy population are commonly ascribed to a sudden quenching event, which is able to rid the central galaxy of its cold gas reservoir in a short time. Following this event, the galaxy is able to prevent further SF and stay quiescent via a maintenance mode. We test whether such a quenching event is truly necessary using a simple model of quiescence. In this model, hot gas (all gas above a temperature threshold) in a $\sim10^{12} M_{\odot}$ halo mass galaxy at redshift $z\sim3$ is prevented from cooling. The cool gas continues to form stars at a decreasing rate and the galaxy stellar mass, morphology, velocity dispersion and position on the color magnitude diagram (CMD) proceed to evolve. By $z=0$, the halo mass has grown to $10^{13} M_{\odot}$ and the galaxy has attained characteristics typical of an observed $z=0$ elliptical galaxy. Our model is run in the framework of a cosmological, smooth particle hydrodynamic code which includes SF, early stellar feedback, supernova feedback, metal cooling and metal diffusion. Additionally, we post-process our simulations with a radiative transfer code to create a mock CMD. In contrast to previous assumptions that a pure "fade away" model evolves too slowly to account for the sparsity of galaxies in the "green valley", we demonstrate crossing times of $<$1 Gyr. We conclude that no sudden quenching event is necessary to produce such rapid colour transitions.