Dark matter-rich early-type galaxies in the CASSOWARY 5 strong lensing system

TL;DR

This study analyzes a strong gravitational lensing system involving early-type galaxies, revealing a high dark matter fraction and suggesting the lens is part of a galaxy group, providing insights into dark matter and galaxy formation.

Contribution

It models the mass distribution of a lensing system using spectral and lensing data, highlighting the significant dark matter component and environmental effects.

Findings

Dark matter constitutes about 80% of total mass within 12.6 kpc.

The lens galaxies have nearly four times more dark matter than similar isolated galaxies.

The system is consistent with being a galaxy group influenced by its environment.

Abstract

We study the strong gravitational lensing system number 5 identified by the CASSOWARY survey. In this system, a source at redshift 1.069 is lensed into four detected images by two early-type galaxies at redshift 0.388. The observed positions of the multiple images are well reproduced by a model in which the total mass distribution of the deflector is described in terms of two singular isothermal sphere profiles. By modelling the lens galaxy spectral energy distributions, we measure the lens luminous masses and stellar mass-to-light ratios. These values are used to disentangle the luminous and dark matter components in the vicinity of the multiple images. We estimate that the dark over total mass ratio projected within a cylinder centred on the primary lens and with a radius of 12.6 kpc is 0.8 +/- 0.1. We contrast these measurements with the typical values found at similar distances (in units of the effective radius) in isolated lens galaxies and show that the amount of dark matter present in these lens galaxies is almost a factor four larger than in field lens galaxies with comparable luminous masses. Data and models are therefore consistent with interpreting the lens of this system as a galaxy group. We infer that the overdense environment and dark matter concentration in these galaxies must have affected the assembly of the lens luminous mass components. We conclude that further multi-diagnostics analyses on the internal properties of galaxy groups have the potential of providing us a unique insight into the complex baryonic and dark-matter physics interplay that rules the formation of cosmological structures.