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

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 𝑠𝑆𝐹 𝑅 ∼ 10 −12 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.

The observed cosmic star formation rate density has an evolution which resembles a Γ(a, bt) distribution and can be described successfully by only 2 parameters

“…We note that there are great uncertainties for the simulated sSFRs (the SF R/M⋆ ratio) of passive galaxies (sSF R < 10 −11 M ⊙ /yr). The number density of the specific star formation rates, namely the specific star formation rate function predicted from the simulations is in severe tension with observations(Zhao et al 2020;Katsianis et al 2021;Corcho-Caballero et al 2021). This disagreement hints that galaxies are not quenched correctly in the simulations via Feedback and the results from the models should be treated with caution.…”