The lens SW05 J143454.4+522850: a fossil group at redshift 0.6?

TL;DR

This study investigates the fossil group SW05 J143454.4+522850 at redshift 0.6, combining gravitational lensing and stellar population analysis to understand its mass composition and formation history.

Contribution

It provides the first detailed mass profile of this fossil group using lensing and stellar synthesis, revealing a higher stellar to dark matter ratio than typical galaxy groups.

Findings

SW05 is likely a fossil group with a high stellar to dark matter mass fraction.

The stellar mass fraction exceeds expectations from abundance matching.

Results suggest more efficient gas-to-star conversion in fossil groups.

Abstract

Fossil groups are considered the end product of natural galaxy group evolution in which group members sink towards the centre of the gravitational potential due to dynamical friction, merging into a single, massive, and X-ray bright elliptical. Since gravitational lensing depends on the mass of a foreground object, its mass concentration, and distance to the observer, we can expect lensing effects of such fossil groups to be particularly strong. This paper explores the exceptional system $\mathrm{J}143454.4+522850$. We combine gravitational lensing with stellar population-synthesis to separate the total mass of the lens into stars and dark matter. The enclosed mass profiles are contrasted with state-of-the-art galaxy formation simulations, to conclude that SW05 is likely a fossil group with a high stellar to dark matter mass fraction $0.027\pm0.003$ with respect to expectations from abundance matching $0.012\pm0.004$, indicative of a more efficient conversion of gas into stars in fossil groups.