The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories

TL;DR

This study investigates the relationship between dark matter content, galaxy size, and star formation history in early-type galaxies, revealing a connection between age and dark matter fraction that challenges simple models.

Contribution

It provides new evidence for cuspy dark matter haloes in early-type galaxies and introduces a novel constraint linking dark matter fraction to stellar age, suggesting complex galaxy formation processes.

Findings

Dark matter density scales with galaxy size as ~Reff^-2

Central dark matter fraction decreases with stellar age

Evidence supports cuspy dark matter haloes in early-type galaxies

Abstract

We examine correlations between the masses, sizes, and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar populations models. We confirm an anti-correlation between size and stellar age, and survey for trends with the central content of dark matter (DM). An average relation between central DM density and galaxy size of <rho_DM> ~ Reff^-2 provides the first clear indication of cuspy DM haloes in these galaxies -- akin to standard LCDM haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size-age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore we suggest that there may be a connection between age and halo contraction, and that galaxies forming earlier had stronger baryonic feedback which expanded their haloes, or else lumpier baryonic accretion that avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations.