The density of dark matter haloes of early-type galaxies in low-density environments

TL;DR

This study investigates the dark matter content of two early-type galaxies in low-density environments using photometric and spectroscopic data, revealing lower dark matter fractions compared to similar galaxies in denser regions.

Contribution

It provides new detailed dynamical and stellar population analyses of early-type galaxies in low-density environments, highlighting differences in dark matter content.

Findings

Lower dark matter halo content compared to high-density environment galaxies

Dynamically derived mass is about twice the stellar mass from models

Galaxies' properties align with semi-analytical galaxy formation models

Abstract

New photometric and long-slit spectroscopic observations are presented for NGC 7113, PGC 1852, and PGC 67207 which are three bright galaxies residing in low-density environments. The surface-brightness distribution is analysed from the K_S-band images taken with adaptive optics at the Gemini North Telescope and the ugriz-band images from the Sloan Digital Sky Survey while the line-of-sight stellar velocity distribution and line-strength Lick indices inside the effective radius are measured along several position angles. The age, metallicity, and alpha-element abundance of the galaxies are estimated from single stellar-population models. In spite of the available morphological classification, images show that PGC 1852 is a barred spiral which we do not further consider for mass modelling. The structural parameters of the two early-type galaxies NGC 7113 and PGC 67207 are obtained from a two-dimensional photometric decomposition and the mass-to-light ratio of all the (luminous and dark) mass that follows the light is derived from orbit-based axisymmetric dynamical modelling together with the mass density of the dark matter halo. The dynamically derived mass that follows the light is about a factor of 2 larger than the stellar mass derived using stellar-population models with Kroupa initial mass function. Both galaxies have a lower content of halo dark matter with respect to early-type galaxies in high-density environments and in agreement with the predictions of semi-analytical models of galaxy formation.