# What's so interesting about Coma-type galaxy clusters?

When the dynamical mass of a proto-cluster of galaxies is ~(1-2)x1014 Msolar, the proto-cluster is expected to evolve into a coma-type cluster.

In almost every paper I read about the confirmation of proto-clusters, the authors of the paper are (almost) always interested in proto-clusters (of galaxies) that will eventually evolve into a Coma-type cluster.

Why are astronomers and astrophysicists so interested in studying those type of structures (Coma-type galaxy clusters)? I mean is it interesting that a proto-cluster will eventually evolve into a Coma-type galaxy cluster? Are these structures rare in the universe?

• Out of curiosity, would you mind linking to one of these papers? Mar 30, 2020 at 22:59

I think the answer to this isn't that Coma-type galaxy clusters are interesting, it's that galaxy clusters are interesting. Chiang et al. (2014) talks about the importance of studying protoclusters as a means of understanding how galaxies evolve over time.

Galaxy clusters are extreme products of structure formation. They are ideal laboratories to study galaxy assembly, quenching, and sub/super-halo galaxy environments. It has become clear that to obtain a complete picture of cluster formation, we also need to find and study their progenitors at high redshifts that were still forming.

In their paper, they discuss the discovery of a large number of new protoclusters. They bin their protoclusters by mass and designate them as "Fornax-type" ($$M_{z=0} = 1-3\times 10^{14}\ M_{\odot}$$), "Virgo-type" ($$M_{z=0} = 3-10\times 10^{14}\ M_{\odot}$$), or "Coma-type" ($$M_{z=0} > 10^{15}\ M_{\odot}$$). Clearly, they're using present-day clusters as analogs to describe what these protoclusters may turn into as they continue to evolve. The fact that a protocluster is considered a Coma-type cluster, is only based on the mass. Astronomer's aren't so much interested in studying Coma-type protoclusters specifically, they're just studying protoclusters and the high mass protoclusters are considered Coma-type protoclusters.

It's very possible that you were preferentially seeing references to Coma-type clusters because Virgo- or Fornax-type clusters are harder to find (being of smaller mass) and so there is an observational bias that preferentially makes Coma-type protoclusters easier to find and subsequently publish about.