Strong Gravitational Lensing and Multiwavelength Analysis of Groups of Galaxies: Characterising Intermediate Scale Dark Matter Structures

Abstract

Galaxy groups, with halo masses in the range $10^{13}$-$10^{14}\,\mathrm{M_\odot}$, occupy a critical intermediate regime between individual galaxies and massive clusters. Despite being the most common collapsed structures in the Universe, they remain among the least well characterised environments for dark matter studies. Strong gravitational lensing offers a uniquely direct, model-independent probe of their inner projected mass, yet confirmed group-scale strong lenses are rare and their analysis requires methodology that bridges the approaches used at galaxy and cluster scales.

This thesis presents a strong gravitational lensing analysis of the galaxy group ACO\,3827 ($z = 0.099$), a quadruply-imaged system in which four massive early-type galaxies jointly lens a background source at $z_\mathrm{s} = 1.24$. The analysis is implemented in \textsc{Lenstronomy} and follows a three-stage pipeline: parametric S\'ersic modeling of the group galaxy light and intra-group light, conjugate-point optimisation of the lens mass using identifiable arc features as astrometric constraints, and full surface brightness reconstruction fitting the lens mass and source light simultaneously.

The group galaxy light is well described by elliptical S\'ersic profiles with indices $n = 3.2$-$6.9$ and effective radii of $1.3''$-$3.2''$, consistent with massive early-type galaxies. A systematic comparison between single-galaxy and simultaneous four-galaxy fitting reveals a factor of $\sim\!1.8$ discrepancy in the inferred intra-group light background level, quantifying a fundamental degeneracy between the intra-group light and the extended S\'ersic wings of individual group members.

The conjugate-point mass model with four Singular Isothermal Ellipsoid profiles reproduces all conjugate groups with source-plane scatter well below $150$\,mas. Adding a group-scale halo as a fifth component reduces the mean scatter by $\approx\!35\%$, providing clear evidence that the individual galaxy halos alone do not account for the full projected mass within the lensing region. This improvement is confirmed by the full surface brightness reconstruction, in which the halo is required to correctly reproduce the caustic geometry and the positions of peripheral star-forming knots in the source plane. The source is reconstructed as a spatially extended star-forming galaxy spanning $\sim\!8.5$\,kpc, consistent with a massive disk galaxy at cosmic noon. These results establish that a smooth, extended group-scale dark matter component is robustly required by the data, and lay the groundwork for a full dark matter fraction measurement pending multi-band stellar mass estimates from Spectral Energy Distribution fitting.