Thesis Topic: Mapping the mass assembly of galaxies in groups and clusters

Thesis Supervisors: Magda Arnaboldi and Michael Hilker


In the framework of hierarchical structure formation, clusters of galaxies are expected to grow over time by accreting smaller groups. The fossil records of the mass assembly reside in the outer reaches of galaxies, where timescales are long. For example, in a typical brightest cluster galaxy, with velocity dispersion of ~ 200 km s-1 at a few tens of kpc, the orbital period of any closed orbits would be of the order of ~2 Gyr. Therefore, any accreted satellites at these radii in the last 4-7 Gyr would not have phase mixed yet and may be detectable from in deep photometry and kinematics measurements. The outer regions of galaxies are the realm of stellar halos and intracluster light (ICL, see Montes 2019 as a review). These components are extended, diffuse, and very faint (μg > 26–27 mag arcsec-2). Comparison with cosmological simulations inform us that the stars in the outer halos are accreted, either stripped from satellite galaxies, in the form of streams and tidal tails, or deposited at large radii because of violent relaxation during massive mergers (e.g., Murante et al. 2007). Because of the hierarchical nature of the accretion history, multiple stellar components and complex kinematics are expected. Because of the long relaxation time, the stellar halos and ICL still assembling at the present epoch.

From the observational side, the most recent deep imaging surveys of nearby clusters have enabled extensive analyses of the light and colour distribution of galaxies in dense environments, out to the regions of the stellar halos where the imprints of the mass assembly reside (see Ferrarese et al. 2012, Duc et al. 2015, Trujillo & Fliri 2016, Mihos et al. 2017, Iodice et al. 2021). Investigations of mass assembly in the outer regions of galaxies have been conducted using extended stellar kinematics (Barbosa et al. 2018, Veale et al. 2018, Greene et al. 2019), mapping of population properties (e.g. Coccato et al. 2010, 2011, Barbosa et al. 2021) and kinematics of discrete tracers like globular clusters (GCs; Coccato et al. 2013, Hilker et al. 2018) and planetary nebulae (PNe; Longobardi et al. 2018, Spiniello et al. 2018, Pulsoni et al. 2018, Hartke et al. 2018, 2020; Arnaboldi et al. 2020).

While cosmological simulations seem to provide a general framework which is in qualitative agreement with observations, there are still important tensions. For example, stellar haloes and intragroup/ intracluster light are significantly more metal poor than predicted by the aforementioned simulations (Hartke et al. 2018, 2020), hence pointing towards progenitors with significant lower mass than those predicted by simulations. Thus exploring the low surface brightness (LSB) universe in the cluster groups within 50 Mpc distance, where deep photometry and discrete tracers can be measured, is fundamental to constraint the mass assembly history of galaxies at all scales (from galaxies to clusters) and in all environments (in the low-density groups of galaxies and in rich clusters). Building on the latest results on stellar halo structures from the Illustris-TNG simulations (Pulsoni et al. 2021), we wish to map the extended stellar halos and ICL which trace the 3D space distribution of the dark matter at the cluster scale. From the baryon census of these bright cluster galaxies, including Xrays emitting gas, we will build the radial acceleration relation in order to constraint theories of modified gravity (see Brouwer et al. 2021).