# The quantity of dark matter in early-type galaxies and its relation to the environment

## TL;DR

This study investigates how the dark matter content within early-type galaxies varies with environment, using SDSS data, and finds that denser environments tend to have galaxies with less dark matter relative to their dynamical mass.

## Contribution

It provides a comprehensive analysis of dark matter fractions in early-type galaxies across different environments and redshifts, considering various profiles and initial mass functions.

## Key findings

- Dark matter content depends on environment
- Galaxies in low-density environments have wider dark matter ranges
- Dark matter in dense environments is less than 55-75% of dynamical mass

## Abstract

We study the behavior of the dynamical and stellar mass inside the effective radius of early-type galaxies (ETGs) as a function of environment considering Newtonian dynamics, different surface--brightness profiles, different initial mass functions (IMF) and different redshift ranges. We use several samples of ETGs --ranging from 19,000 to 98,000 objects-- from the ninth data release of the Sloan Digital Sky Survey. We assume that any difference between the dynamical and stellar mass is due to dark matter and/or a non-universal IMF. The main results, considering samples in the redshift range 0.0024 $\leq\;z\;\leq$ 0.35 are: (i) the amount of dark matter inside ETGs depends on the environment; (ii) ETGs in low density environments span a wider dark matter range than ETGs in dense environments; (iii) the amount of dark matter inside ETGs in the most dense environments will be less than approximately 55\%--75\% of the dynamical mass; (iv) the accurate value of this upper limit depends on the impact of the IMF on the stellar mass estimation; (v) in the case of an ETGs sample which is approximately complete for log$({\bf M_{Virial}}/{\bf M_{Sun}}) > 10.5$ and in the redshift range 0.04 $\leq\;z\;\leq$ 0.08 we find that the amount of dark matter in the most dense environments will be less than approximately 60\%--65\% of the dynamical mass.