The impact of Warm Dark Matter of the ages of galaxies

According to the current concordance cosmological paradigm, the Universe is dominated by a so-called Cold Dark Matter and a Dark Energy (Λ-CDM scenario) and the smallest structures are predicted to be the first to collapse and then later interact and merge to form larger structures. While this paradigm is able to explain several observational evidences, especially on the large-scale properties of the Universe, it still shows shortcomings on galactic and sub-galactic scales. Some of these problems are related to an excess of small-scale structures in Λ-CDM models compared to observations. In particular, Λ-CDM-based simulations of structure formation overpredict by a factor of 5-10 the number of satellite galaxies orbiting larger galaxies such as the Milky Way. Also, several physical properties of galaxies are not easily reproduced in Λ-CDM-based simulations, such as the fact that smaller galaxies are dominated by younger stellar populations with respect to larger galaxies.

While these problems can be ascribed to an incomplete understanding of the detailed physical processes regulating star formation in galaxies, they can also be related to the history of structure collapse and growth which is governed by the nature of Dark Matter. In particular, in a Warm Dark Matter scenario, the constituents of Dark Matter are lighter particles, with larger velocities, than in the standard Cold Dark Matter model, resulting in a suppression of the growth of structures on small scales.

Nicola Menci (INAF-Rome) and collaborators have delevoped a model (the Rome semi-analytic model) that connects the physics of gas and star formation in galaxies with the merging history of dark matter halo in a Warm Dark Matter scenario. With such a model, and assuming a mass of the dark matter particle of 0.75 keV, Menci et al (2012) correctly reproduced the abundance of small, faint galaxies observed today.
 
Can a Λ-WDM cosmology also reproduce the observed relation between the mean stellar age of galaxies and their mass, without modifying the star formation recipes? This question has been addressed by Francesco Calura (INAF-Bologna), Nicola Menci (INAF-Rome) and Anna Gallazzi (AstroFIt fellow at INAF-Arcetri) in a paper recently appeared on Monthly Notices of the Royal Astronomical Society, "The ages of stellar populations in a warm dark matter universe". 

The authors have run two sets of simulations, which adopt the same recipes for star-formation and gas recycling, one based on a Λ-WDM cosmology and one based on a Λ-CDM cosmology. The predicted relations between the mean stellar age and galaxy mass are compared to that observed in local galaxies using the largest spectroscopic survey to date, SDSS, by Gallazzi et al (2005). While in the Λ-CDM cosmology the relation is very flat, due to a too early formation of small galaxies, in the Λ-WDM cosmology the ages of stellar population in galaxies decrease with decreasing galaxy mass, as observed, and the overall frequency of young galaxies is also reproduced. While improvements in the modeling of the physics regulating gas cooling and recycling and star formation are still needed to accurately reproduce the observations, this work highlights the impact of dark matter structure formation on the star formation histories of galaxies.

Edited by Anna Gallazzi