Research

List of Publications

A complete list of my refereed journal publications can be found on this ADS link.

Papers where I am either the lead author or have a significant contribution are listed below:

Ahad S. L., Reid R., Mpetha C. T., et al., “Cluster properties as a function of dynamical state in the DESI Legacy x UNIONS surveys.” 2025, submitted to ApJ. arXiv: 2512.14636.
Ahad S. L., Hoekstra H., Bahé Y.M., et al.,“The intragroup light in KiDS+GAMA groups: A stacking analysis.” 2025, A&A, 702, A271. arXiv: 2509.00233
Ellien, A., Montes, M., Ahad S. L., et al., “Euclid: Early Release Observations – The Intracluster Light of Abell 2390.”, 2025, A&A, 698, A134. arXiv: 2503.07484
Ahad S. L., Muzzin, A., Bahé Y.M., Hoekstra H.,“An environment-dependent halo mass function as a driver for the early quenching of z > 1 cluster galaxies.” 2023, MNRAS, 528, 6329. arXiv: 2307.01147
Ahad S. L., Bahé Y.M., Hoekstra H.,“How to interpret measurements of diffuse light in stacked observations of groups and clusters of galaxies.” 2023, MNRAS, 518, 3685. arXiv: 2210.14249
Ahad S. L., Bahé Y.M., Hoekstra H., et al.,“The stellar mass function and evolution of the density profile of galaxy clusters from the Hydrangea simulations at 0 < z < 1.5.” 2021, MNRAS, 504, 1999. arXiv:2010.16195

Image credit: ESA/Euclid/Euclid Consortium/NASA

Research Interests

Galaxy formation and evolution
Groups and clusters (and protoclusters) of galaxies
Intragroup and intracluster light
Star formation quenching at z>1
Detailed comparison of simulations and observations

Figure from Ahad et al. 2021 showing the evolution of concentration of galaxy cluster density profile (assuming an NFW profile) over cosmic time from simulations (blue shaded region) and observations (colored data points).

Galaxy groups and clusters

Groups and clusters sit at the nodes of the Cosmic Web and are prime sites to study structure formation and galaxy evolution in dense environments. My primary research interest is in understanding the formation and evolution of these systems and the galaxies within them. I have studied the redshift evolution of stellar mass functions and density profiles of galaxy clusters in simulations (Ahad et al. 2021). My current focus is to uncover the connection between cluster structural properties and their assembly history (Ahad et al. 2025, submitted to ApJ) in the UNIONS survey. I am also working on connecting high-z cluster progenitors, aka protoclusters to their descendant clusters from hydrodynamic simulations.

Intragroup and intracluster light (IGL/ICL)

IGL/ICL is the faint, diffuse stellar component bound to groups and clusters, formed through galaxy mergers and tidal stripping. The IGL/ICL assembly and stellar population inform us about the intracluster medium formation and the growth of the brightest cluster galaxy. Also, the shape of ICL is observed to be more aligned to the underlying cluster dark matter halo (measured by weak gravitational lensing) than the cluster galaxy distribution. My previous work has bridged the gap between theoretical prediction of ICL from hydrodynamic simulations and observations by using forward modelling (Ahad et al. 2023). I also developed a low-surface-brightness pipeline to enable ICL analysis in cosmology survey KiDS and performed a stacking analysis to measure the IGL in the KiDS+GAMA group sample (Ahad et al. 2025). I co-led Euclid Early Release ICL analysis of Abell 2390 and contributed to Euclid ICL forecasting and LSST readiness for ICL analysis papers, providing methods and benchmarks for forthcoming surveys.

Figure from Ahad et al. 2025. The shaded regions are showing our measurements of the IGL fraction in the KiDS+GAMA groups, compared to the total group light in the binned sample. The x-axis shows the average halo mass of the systems. The grey data points show measurements on individual systems from literature.

A typical starforming (left) and quiescent (right) galaxy. Both are from the local Universe. Image Credit: ESO/NASA.

Star-formation quenching in galaxies

Why massive cluster galaxies stop forming stars early is a key question for galaxy evolution. My research based on EAGLE and Hydrangea simulations shows that quenching at z>1 is closely tied to halo mass and central potential, not environment alone (Ahad et al. 2024). I am now investigating the physical processes that quench these massive galaxies.

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