The VIRUS-dE Survey II: Cuspy and round halos in dwarf ellipticals -- A result of early assembly?

TL;DR

This study investigates the dark matter halos of dwarf ellipticals, revealing their density slopes and shapes, and how these properties relate to environment and formation history, challenging the universality of halo profiles.

Contribution

First measurement of halo flattening in dwarf ellipticals, showing their density profiles depend on assembly conditions and environment, with implications for galaxy formation models.

Findings

dEs have slightly shallower density slopes than NFW predictions

Halo flattenings are more spherical than cosmological simulations suggest

dEs in outskirts are more cored and less dense than central dEs

Abstract

We analyze the dark matter (DM) halos of a sample of dwarf Ellitpicals (dE) and discuss cosmological and evolutionary implications. Using orbit modeling we recover their density slopes and, for the first time, the halo flattening. We find the `cusp-core' tension is mild, on average dEs have central slopes slightly below the Navarro Frenk White (NFW) predictions. However, the measured flattenings are still more spherical than cosmological simulations predict. Unlike brighter ETGs 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 LTGs. We find average DM density and slope are strongly correlated with the environment and moderately with the angular momentum. Central, non-rotating 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 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 (e.g. star-formation, halo assembly) were very different, suggesting no new dEs are formed at present.