Elliptical galaxy masses out to five effective radii: the realm of dark matter

TL;DR

This study estimates elliptical galaxy masses out to five effective radii using tracers, revealing that dark matter fraction increases with galaxy mass and that potential profiles vary with mass, supporting models of galaxy formation.

Contribution

It provides the first comprehensive analysis of galaxy mass profiles out to five effective radii, highlighting the mass-dependent variation of dark matter content and potential profiles.

Findings

Dark matter fraction increases with galaxy mass.

Potential profiles are between isothermal and Keplerian regimes.

Models of star formation efficiency explain the mass trend.

Abstract

We estimate the masses of elliptical galaxies out to five effective radii using planetary nebulae and globular clusters as tracers. A sample of 15 elliptical galaxies with a broad variation in mass is compiled from the literature. A distribution function-maximum likelihood analysis is used to estimate the overall potential slope, normalisation and velocity anisotropy of the tracers. We assume power-law profiles for the potential and tracer density and a constant velocity anisotropy. The derived potential power-law indices lie in between the isothermal and Keplerian regime and vary with mass: there is tentative evidence that the less massive galaxies have steeper potential profiles than the more massive galaxies. We use stellar mass-to-light ratios appropriate for either a Chabrier/KTG (Kroupa, Tout & Gilmore) or Salpeter initial mass function to disentangle the stellar and dark matter components. The fraction of dark matter within five effective radii increases with mass, in agreement with several other studies. We employ simple models to show that a combination of star formation efficiency and baryon extent are able to account for this trend. These models are in good agreement with both our measurements out to five effective radii and recent SLACS measurements within one effective radii when a universal Chabrier/KTG initial mass function is adopted.