# Galaxy structure with strong gravitational lensing: decomposing the internal mass distribution of massive elliptical galaxies

## TL;DR

This paper demonstrates how strong gravitational lensing can decompose the internal mass structure of massive elliptical galaxies, revealing stellar and dark matter components to test galaxy formation models.

## Contribution

It introduces a method combining photometric decomposition with lensing analysis to separately measure stellar and dark matter distributions in elliptical galaxies.

## Key findings

- Galaxies consist of a central bulge and an extended stellar envelope.
- Lensing effects enable a clean separation of baryonic and dark matter components.
- Detected rotational offsets and lopsidedness provide insights into galaxy evolution.

## Abstract

We investigate how strong gravitational lensing can test contemporary models of massive elliptical (ME) galaxy formation, by combining a traditional decomposition of their visible stellar distribution with a lensing analysis of their mass distribution. As a proof of concept, we study a sample of three ME lenses, observing that all are composed of two distinct baryonic structures, a `red' central bulge surrounded by an extended envelope of stellar material. Whilst these two components look photometrically similar, their distinct lensing effects permit a clean decomposition of their mass structure. This allows us to infer two key pieces of information about each lens galaxy: (i) the stellar mass distribution (without invoking stellar populations models) and (ii) the inner dark matter halo mass. We argue that these two measurements are crucial to testing models of ME formation, as the stellar mass profile provides a diagnostic of baryonic accretion and feedback whilst the dark matter mass places each galaxy in the context of LCDM large scale structure formation. We also detect large rotational offsets between the two stellar components and a lopsidedness in their outer mass distributions, which hold further information on the evolution of each ME. Finally, we discuss how this approach can be extended to galaxies of all Hubble types and what implication our results have for studies of strong gravitational lensing.