My research involves using cosmological simulations to probe the evolution of galaxies, groups and clusters. Some areas I am particularly focused on include:
- Infalling galaxies and galaxy groups
- Galaxy cluster outskirts ('infall regions')
- The effects of environment on galaxy properties
- Cluster dynamics and dynamical states
- Large-scale structure and cosmic filaments
- Connections between simulations and observations
I am involved in The Three Hundred project, an international collaboration using hydrodynamical zoom-in simulations of galaxy clusters.
Backsplash galaxies
'Backsplash galaxies' are a class of cluster galaxies that have previously passed through a cluster, but are now located beyond the radius of the cluster in its 'infall regions'.
Because these galaxies are found in the infall regions, they are observationally hard to distinguish from galaxies on their first infall. However, passing through a cluster environment strongly affects the properties of a galaxy, and so backsplash galaxies act as a contaminant when looking at samples of infalling galaxies around clusters.
In Haggar et al. (2020), we use simulations to show that the fraction of backsplash galaxies around a cluster varies between approximately 20% and 80%, and can be predicted using observable bulk properties of the cluster. For example, more dynamically relaxed clusters have a greater backsplash fraction.
Accounting for this backsplash contamination in large surveys will allow the effects of galaxy pre-processing to be disentangled from effects of the cluster, and help us further understand the complex relationship between the properties and environment of a galaxy.
Paths of an infalling galaxy, cluster member and backsplash galaxy, taken from our simulations. Black circle shows the radius of the cluster, and crosses show galaxy positions at z = 0.
Infalling groups
Cosmic structure grows heirarchically, with smaller objects merging together to form larger objects. One consequence of this is the accretion of galaxy groups (containing ~10 galaxies) by clusters (~1000 galaxies).
However, in Haggar et al. (2021) we find that the properties of simulated galaxy groups are dependent on the type of simulation, and specifically whether a dark matter-only or hydrodynamical simulation is being used. In particular, dark matter-only simulations do not model the central regions of galaxy groups well, leading to an apparent deficit in the number of galaxies there. We also show a similar effect in galaxy clusters.
In Haggar et al. (2023) we use hydrodynamical simulations to show that galaxy groups are highly disrupted when passing through a galaxy cluster. Groups that enter a cluster are quickly torn apart by tidal forces, so any groups observed nearby to a cluster are almost certainly on their first approach. The fate of individual galaxies in these groups -- whether they are tidally stripped, undergo a merger, etc. -- is strongly dependent on their location within their host group.
Fractional difference in the distribution of galaxies in the phase space of groups, between hydrodynamical and dark matter-only simulations. Dark red regions show areas with more galaxies in the hydrodynamical simulations, and dark blue for the dark matter-only simulations.