Central dark matter trends in early-type galaxies from strong lensing, dynamics and stellar populations

TL;DR

This study investigates how the central dark matter content in early-type galaxies correlates with their size and age, using strong lensing, dynamics, and stellar population data at intermediate redshifts, revealing trends consistent with cosmological models.

Contribution

It combines multiple observational techniques to decompose galaxy masses and uncovers new correlations between dark matter fraction, size, and age in early-type galaxies.

Findings

Dark matter fraction increases with galaxy size.

Dark matter density decreases with galaxy size.

Dark matter fraction decreases with stellar age.

Abstract

We analyze the correlations between central dark matter (DM) content of early-type galaxies and their sizes and ages, using a sample of intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and by comparing them to a larger sample of z ~ 0 galaxies. We decompose the deprojected galaxy masses into DM and stellar components using combinations of strong lensing, stellar dynamics, and stellar populations modeling. For a given stellar mass, we find that for galaxies with larger sizes, the DM fraction increases and the mean DM density decreases, consistently with the cuspy halos expected in cosmological formation scenarios. The DM fraction also decreases with stellar age, which can be partially explained by the inverse correlation between size and age. The residual trend may point to systematic dependencies on formation epoch of halo contraction or stellar initial mass functions. These results are in agreement with recent findings based on local galaxies by Napolitano, Romanowsky & Tortora (2010) and suggest negligible evidence of galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.