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Credit: Tumlinson et al. 2017
Circumgalactic medium
Standard cosmology (Lambda-CDM) predicts that when a galaxy is formed, the baryons comprise approximately 16% of the total galactic mass. These baryons are shock heated, cooled, condensed to the central parts of the galaxy forming stars and the interstellar (ISM) medium. However, a large fraction of these baryons remain too hot to condense and form stars, staying in gaseous form within the gravitational bound of the galaxy. This gaseous medium is commonly referred to as the circumgalactic medium (CGM). It plays a pivotal role in the evolution of galaxies and is expected to contain a significant fraction of missing baryons. Therefore, constraining the amount, distribution and energetics of CGM has the potential to play an important role in the missing baryon problem as well as physical processes affecting galactic evolution.
Sunyaev-Zel'dovich (SZ) effect and X-ray emission
SZ effect is caused by the inverse Compton scattering of low energy cosmic microwave background (CMB) photons by the high energy intervening medium. The SZ effect is well studied and observed in the case of galaxy clusters, which are the largest and easiest to observe, followed by the studies of galaxy groups. The CGM in massive galaxies is shock heated to the temperatures (>106 K) where it could be observed through SZ effect and X-ray emission. The SZ effect and X-ray emission act as two complimentary tracers of the hot gas and they can help in breaking the density-temperature degeneracy.
Singh et al. 2018
Credit: Dolag et al. 2015, Teklu et al. 2015
Cosmological hydrodynamical simulations
Hydrodynamic simulations provide a unique opportunity to recreate the observed Universe, and therefore directly probe (a) the physical mechanisms in action in different density environments in a wide range of redshifts, (b) constrain the impact of projection effects and biases for a given observation, and (c) access a large variety of physical quantities playing a significant role in galaxy evolution which are otherwise difficult to observe or meaningfully constrain. These simulations are also essential to test the cosmology and baryon physics dependence of various observables which otherwise cannot be done with observations alone since we have access to only one observable Universe and simulations represent the only way to constrain them.
Galaxies in extreme environments
Passive (or quiescent) galaxies with old stellar populations are the dominant galaxy population in high density environments such as galaxy clusters. The transformation of a star-forming galaxy to a passive one, known as quenching is one of the major transformations that a galaxy undergoes during its life-cycle. Galaxy clusters represent the density peaks of cosmic web, and are therefore ideal locations for such transformations due to their extreme environment. Galaxy cluster samples selected through their intra-cluster medium (ICM) properties (tracing the mass of the cluster) such a SZ effect and X-ray emission provide ideal catalogues. In fact, thanks to their ICM selection, they represent almost unbiased laboratories where to study the properties of their galaxy content, whereas, optically selected cluster samples are much more prone to possible selection biases.
Credit: NASA-HST
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