The VIRUS-dE Survey. II. Cuspy and Round Halos in Dwarf Ellipticals—A Result of Early Assembly?

Abstract

We analyze the dark matter (DM) halos of a sample of dwarf ellipticals (dEs) and discuss cosmological and evolutionary implications. Using orbit modeling, we recover their density slopes and, for the first time, halo flattening. We find that the “cusp-core” tension is mild; on average, dEs have central slopes slightly below the Navarro–Frenk–White predictions. However, the measured flattenings are still more spherical than cosmological simulations predict. Unlike brighter early-type galaxies, the total density slopes of dEs are shallower, and their average DM density does not follow their scaling relation with luminosity. Conversely, dE halos are denser and the densities steeper than in late-type galaxies. We find average DM density and slope are strongly correlated with the environment and moderately with the angular momentum. Central, nonrotating dEs have dense and cuspy halos, whereas rotating dEs in Virgo’s outskirts are more cored and less dense. This can be explained by a delayed formation of the dEs in the cluster outskirts, or alternatively by the accumulated baryonic feedback that the dEs in the outskirts have experienced during their very different star formation history. Our results suggest halo profiles are not universal (they depend on assembly conditions) and they evolve only mildly due to internal feedback. We conclude dEs in the local Universe have assembled at a higher redshift than local spirals. In these extreme conditions, star formation and halo assembly, for example, were very different, suggesting no new dEs are formed at present.

Keywords

• Galaxy structure
• Galaxy dark matter halos
• Galaxy formation
• Dwarf elliptical galaxies
• Dark matter distribution
• Virgo Cluster