On the Morphology and Kinematics of the Ionised Gas in Brightest Cluster Galaxies

Abstract

Brightest Cluster Galaxies (BCGs) occupy a distinct position in the galaxy popula- tion, residing at the centres of galaxy clusters where they are embedded within dense intracluster media and subject to complex baryonic processes. This thesis presents a detailed investigation of the morphology, kinematics, and excitation mechanisms of the warm ionised gas in a representative sample of X-ray–selected BCGs from the Kaleidoscope survey, observed with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope. The high spatial and spectral resolution of MUSE enables spatially resolved mapping of the ionised gas, revealing a striking diver- sity of filamentary and clumpy structures, coherent velocity fields, and disturbed kinematics indicative of feedback-driven outflows and gas accretion. Emission-line diagnostics are employed to disentangle the relative contributions of active galactic nucleus (AGN) photoionisation, shocks, and star formation, thereby constraining the interplay between cooling flows and feedback in cluster cores.

Following the introduction to the field (Chapter 1), I present the properties of the Kaleidoscope survey—the largest census of X-ray–selected BCGs observed with MUSE—and describe the custom pipeline developed for data analysis (Chapter 2). A robust framework is then established for separating the sample according to the presence of optical line emission, yielding a line-emitting fraction of 35.6%, in good agreement with previous studies (Chapter 3). Building on the work of Hamer et al. [2016] based on VIMOS data, I apply morphological classifications to the Kaleido- scope sample (Chapter 4). The enhanced sensitivity and resolution of MUSE reveal a markedly different distribution of morphologies, motivating the development of a new algorithm designed to exploit the full capabilities of MUSE.

The most remarkable result is the identification of a systematic offset of ∼ 100km/s between the mean velocity of the ionised gas and the systemic stellar velocity (Chapter 5). I examine the finely tuned combinations of geometry, ob- scuration, and line-of-sight motions required to reproduce such an offset, and then probe the line-of-sight velocity distributions of the ionised gas in each system us- ing the velocity structure function (Chapter 6). This analysis enables a systematic classification of kinematic profiles across the sample.

I investigate the ionisation mechanisms powering the observed line emission using
standard diagnostic diagrams, including BPT line ratios (Chapter 7). The majority of systems lie along the LINER/Composite boundary, underscoring the complex interplay of star formation, AGN photoionisation, and shocks in BCGs. Finally, I examine the underlying stellar populations, placing constraints on star formation histories and evolutionary pathways of the BCGs in our sample (Chapter 8).