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It's 1902 and astronomer Max Wolf is confused. Scrutinizing photographic plates of the star- poor region near the north pole of the Milky Way, in the constellation Coma Berenices, he spots a dense patch of fuzzy nebulae. In his publication “The Fuzzy Objects at the Pole of the Milky Way” [1], Wolf provides careful descriptions of these objects, their sizes and morphologies — elliptical, spiral, core brightened He concludes with the understatement “It would be premature to speculate on this strange result. Nonetheless, I must not miss pointing it out for general attention.” (translation courtesy of Klaus Dolag, LMU and MPA).

Thirty years later, Wolf's fuzzy nebulae are shown to be “island universes,” first hypothesized by the German philosopher Immanuel Kant. They are each comparable in size to our own Milky Way galaxy, and at great distances from it. Wolf's cluster of galaxies is today known as the Coma cluster, and its central region of massive galaxies is shown in Figure 1.

Figure 1

Figure 1. Illustration of clusters of galaxies forming at the intersection of large scale filaments. Optical photograph courtesy of Bob Franke, Focal Pointe Observatory showing the inner 600 kpc of the Coma Cluster. Larger scale density structure courtesy of Richard Bower, from the EAGLE simulation [3].

It wasn't long before the next mystery appeared; using the Doppler shifts of galaxies in the clusters, astronomers calculated that the mass required to keep clusters like Coma gravitationally bound was ∼ 400 times that visible in stars. In 1933, the brilliant and eccentric Swiss astronomer Fritz Zwicky attributed the required extra gravitational force to some unknown “Dunkle Materie,” [2] whose nature remains a mystery to this day.

A brief glimmer of hope that the now so-called “dark matter” might have been found appeared in the late 1970s, with the discovery that clusters of galaxies were filled with an X-ray emitting plasma at a temperature of 107-8 degrees. But estimates of its mass quickly established that it, too, fell far short of being able to bind the cluster. Our current global mass budget for clusters is shown in Box 1. Dark matter, whatever its nature, dominates the overall cluster collapse and subsequent dynamical evolution. But a much richer story of clusters' continuing activity emerges when we examine the baryons — not only the star-filled galaxies, but the Intra-Cluster Medium (ICM), the dominant cluster-filling plasma.

MASS BUDGET. Total 1015 M “Virial” Radius ∼ 2 Mpc
Dark Matter:
Hot plasma (ICM)
Stars & cold gas
Cosmic Rays*
10-6 %
Cosmic Rays*
Magnetic Fields
Notes to Box 1: These values very roughly correspond to the volume within the “virial” radius, where the mass density is ≳ 200× the critical mass density of the universe (i.e. 200 × 10 H atoms/m3 at the current epoch). Cosmic rays (CR) have energies γmc2, where γ ≫ 1, and comprise the “relativistic plasma.”

Where do clusters fit in to the overall structure of the universe? Both computer simulations and observational studies of the three-dimensional distribution of matter show us that it is concentrated into filamentary structures that are evident approximately a billion years after the Big Bang. At the intersections of these filaments we find clusters of galaxies, with a continuing infall of dark and baryonic matter along the filaments fueling clusters' growth. Figure 1 provides a cartoon illustrating this picture.

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