Past Events

The main aim of this thesis is to shed light on the effectiveness of star formation in the late evolution of BCGs, and investigate the intricate relationship between the star formation and the physical conditions of host clusters.

 

In the first study of this dissertation, we explored the nature of spatially extended blue diffuse light in NGC 1275. We employed a two-component spectroscopic decomposition technique that includes the inspection of stellar indices and full spectrum fitting to explore the physical properties of the stellar populations involved. We find that at least two stellar populations are required, one comprised of the underlying older stellar population and the other being a younger stellar population with age of only 160 Myr. An additional photometric investigation using three HST filters was performed to probe the morphology and color of the observed blue diffuse population. After a robust removal of the underlying old stellar population, a dynamically young stellar population of age no greater than 300 Myr is inferred extending up to 14 kpc from galaxy center. Considering the young age and dynamics of the stellar population, we argued that the most probable origin is associated with the gas deposited from the residual cooling flow.  

 

In the second part of the dissertation, we assessed the dependence of star formation, emission-line nebulae, and active galactic nuclei (AGNs) in CLASH BCGs on the ICM's thermodynamics and galaxy clusters' dynamical state. We constructed color images and employed photometric fitting techniques to determine the presence of star formation in BCGs. De-projected radial profiles of ICM temperature, density, and entropy were investigated in relation to the discovered star formation activity in BCGs. The overall results indicate that clusters having lower ICM core entropies and hence shorter cooling times at their cores preferentially host BCGs that display star formation, nebulae, and radio-luminous AGNs. To determine the dynamical state of galaxy clusters, relaxation parameters peakiness, alignment, symmetry from Mantz et al. 2015, and concentration, centroid shift, power ratio from Donahue et al. 2016 are utilized. We show that star-forming BCGs are clearly restricted to clusters judged by their ICM morphology as closest to being dynamically relaxed. These results clearly implicate that cluster mergers are the only mechanism able to impact the physical states of clusters on a global scale, altering the ICM thermodynamics of clusters to the condition causing temporary interruption of cooling and hence star formation in BCGs. The resumption of ICM cooling and star formation re-ignition only follows when the ICM returns to the state close to hydrostatic equilibrium. Such cyclic evolution of cluster states inevitably leads to episodes of persistent cooling-induced star formation in galaxy centers, potentially providing a significant pathway for BCG growth.