Do galaxies die? Different views from simulations and observations in the Local Universe

TL;DR

This paper compares simulation predictions and observational data on galaxy quenching, highlighting discrepancies in the existence of truly 'dead' galaxies and emphasizing the need for further research to resolve these differences.

Contribution

It critically examines the differences between simulated and observed galaxy populations regarding star formation activity, emphasizing the unresolved nature of truly quiescent galaxies.

Findings

Simulations predict many galaxies with no recent star formation.

Observations suggest even passive galaxies have residual star formation.

The true fraction of completely 'dead' galaxies remains uncertain.

Abstract

For years, the extragalactic community has divided galaxies in two distinct populations. One of them, featuring blue colours, is actively forming stars, while the other is made up of "red-and-dead" objects with negligible star formation. Yet, are these galaxies really dead? Here we would like to highlight that, as previously reported by several independent groups, state-of-the-art cosmological numerical simulations predict the existence of a large number of quenched galaxies that have not formed any star over the last few Gyr. In contrast, observational measurements of large galaxy samples in the nearby Universe suggest that even the most passive systems still form stars at some residual level close to $sSFR\sim10^{-12}~\text{yr}^{-1}$. Unfortunately, extremely low star formation poses a challenge for both approaches. We conclude that, at present, the fraction of truly dead galaxies is still an important open question that must be addressed in order to understand galaxy formation and evolution.