About this Research Topic
The cores of local galaxy clusters, as the centers of giant dark matter halos, are unique locations in the universe. They are dense and biased regions, likely among the first to collapse after the Big Bang, that have undergone violent and multi-stage evolution throughout cosmic time to the present day. At the gravitational center of most cores today lies a single massive galaxy known as the Brightest Cluster Galaxy (BCG), which is a statistical outlier of the general massive galaxy population, exhibiting extreme luminosities and masses, as well as unusual properties such as enhanced dark matter content and excess rates of Active Galactic Nuclei. Surrounding each BCG is a low surface brightness feature called the Intracluster Light (ICL) - the combined light from intracluster stars unbound to any individual galaxy. The BCG and ICL are coupled—, indeed it is difficult to isolate where the BCG ends and the ICL begins—, and are likely linked through evolutionary history. This history is poorly understood, especially at early times, and is driven by the complicated physics of star formation, galaxy interactions, and mergers, Supermassive Black Hole feedback, and large-scale gas cooling. A simple picture has been proposed in which galaxy cluster cores collapse hierarchically, first forming galaxies that seed the BCG through cooling flows — then halted by AGN feedback — followed by a continual assembly of the stellar mass of both BCGs and the ICL through the dry shredding of captured galaxies. Still, this scenario is far from established, except perhaps at the lower (z < 1) redshifts, with new evidence pointing to complications, including recent and intense star formation, and the high-redshift (z > 1) regime remains poorly constrained.
The current level of our understanding of cluster core evolution suffers from the disparate studies of statistically small numbers of systems at high redshifts (z > 1). Compounding this is the complication that clusters themselves are selected using different, orthogonal techniques, and are a heterogeneous population. Add to this the difficulty of comparing model predictions to observational properties, and the fact that indeed even the definitions of a BCG and the ICL are often debated. This results in a field in which it can be difficult to place new evidence in the context of established results. By bringing together articles on selected topics from experts in the field this compendium will assess the current state of the field in a coherent and connected manner.
In this special issue, we aim to bring together state-of-the-art observations, theories, and models of galaxy cluster cores, to synthesize our current understanding of the growth of their components throughout cosmic history. We will include local fossil evidence, as well as direct observations and models of the involved evolutionary processes at earlier times. The broad goal is to highlight what is already known with strong certainty, what instead requires additional work to solidify, and the open questions that remain and that will be addressed by the next generation of instruments, surveys, and models. We envision a mixture of manuscript types, with reviews synthesizing our knowledge, original scholarship presenting new results, and data reports providing overviews of programs to the community which can further our understanding.
The current level of our understanding of cluster core evolution suffers from the disparate studies of statistically small numbers of systems at high redshifts (z > 1). Compounding this is the complication that clusters themselves are selected using different, orthogonal techniques, and are a heterogeneous population. Add to this the difficulty of comparing model predictions to observational properties, and the fact that indeed even the definitions of a BCG and the ICL are often debated. This results in a field in which it can be difficult to place new evidence in the context of established results. By bringing together articles on selected topics from experts in the field this compendium will assess the current state of the field in a coherent and connected manner.
In this special issue, we aim to bring together state-of-the-art observations, theories, and models of galaxy cluster cores, to synthesize our current understanding of the growth of their components throughout cosmic history. We will include local fossil evidence, as well as direct observations and models of the involved evolutionary processes at earlier times. The broad goal is to highlight what is already known with strong certainty, what instead requires additional work to solidify, and the open questions that remain and that will be addressed by the next generation of instruments, surveys, and models. We envision a mixture of manuscript types, with reviews synthesizing our knowledge, original scholarship presenting new results, and data reports providing overviews of programs to the community which can further our understanding.
Keywords: Brightest Cluster Galaxies, Intracluster light, Galaxy Clusters, Galaxy evolution, Galaxy Mergers, Star formation, Hierarchical structure formation, Stellar Mass, Active Galactic Nuclei, Feedback
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