These pictures show formation of an object similar to our Milky Way (MW) galaxy from redshift (z) of 9.0, when the Universe was only 540 million years old, to the present (z0.0), when the universe is 13.47 billion years old. Other epochs are z=5.0 (universe is 1.16 billion years old), and z=1.0 (universe is 5.75 billion years old).
At the present day epoch the MW-like object is near another object of comparable mass. This second object is a counterpart of the Andromeda galaxy and the configuration (distance, masses) is similar to the Milky Way - Andromeda pair that astronomers call "The Local Group of Galaxies". The region shown is centered on the most massive progenitor of the Milky Way for epochs z>0, and is centered on the center of mass of the galaxy pair at z=0. The size of the region is about 1 million parsecs of more than 3 million light years. The pictures show the distribution of dark matter with brighter regions corresponding to regions of higher densities.
The combined mass of the two systems at the present time epoch is equal to 5 times 10^12 masses of the sun.
At the present-day epoch, two modes of dark matter distribution can be seen:
- diffuse "halos" around the two massive systems and
- numerous small dense clumps
The masses of the smallest clumps seen here are comparable to the masses of the smallest galaxies observed in the Local Group. However, the number of clumps seen in simulations exceeds the observed number of galaxies by a factor of more than ten (only two dozen are observed in such a volume). Andrey Kravtsov's paper explains this large discrepancy in the numbers by showing that only about 10% of the seen dark matter clumps have had conditions during evolution conducive to forming a luminous galaxy.
Simulations were performed at the National Center for Supercomputer Applications (NCSA) at Urbana-Champaign by Andrey Kravtsov (The University of Chicago) and Anatoly Klypin (New Mexico State University). Visualizations by Andrey Kravtsov.