Compact groups of galaxies
Sylvia Ploeckinger, 20 May 2020
Galaxy interaction critically depend on the presence (or absence) of dark matter. If each galaxy is embedded in a massive dark matter halo, the additional mass of the system leads to a faster merging process. In alternative theories - without dark matter - the individual galaxies can be in each others proximity for longer without merging into one massive galaxy. Comparing the abundance of compact groups of galaxies in a cosmological dark matter simulation with the observed abundance therefore serves as a test for the dark matter paradigm.
In Hartsuiker & Ploeckinger (2020) we extracted a sample of compact groups of galaxies from the EAGLE suite of cosmological hydrodynamic simulations and studied their abundance and dynamical evolution. The selection criteria closely follow those that are used in observations, so that the observed and simulated compact groups can be compared.
Sohn et al. (2015) have shown that the abundance of observed compact groups of galaxies is constant for z<0.2. If these galaxy groups indeed merge very fast, as expected in the cold dark matter scenario, there would be a need for a process that forms compact groups on a similar timescale to explain their constant abundance.
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| Figure 1. An example of a simulated compact group of galaxies extracted from the EAGLE simulation (left) and an observed compact group of galaxies (right). |
Results
We found that the EAGLE simulations that include cold dark matter can reproduce the observed constant abundance of compact groups. As this seems to contradict the traditional idea of a short timescale for the compact group members merge into a single galaxy, we explored the simulated groups in more detail.
With the current selection criteria, more than half of the compact groups only appear close on the sky, but are not as dense in their full 3D configuration, which is inaccessible in observations. Galaxy groups that are also compact in 3D typically coalesce on timescales of 2-3 Gyr.
Summarising, simulations of galaxies in a cold dark matter universe agree with the observed abundance of compact groups.