Simulated galaxies without dark matter

The image shows the gas distribution of a small region (1.31 Million lightyears x 0.56 Million lightyears) in the Illustris-1 simulation at the present time. Tidal dwarf galaxies are dark matter-free (red circles) while primordial dwarf galaxies are dark matter-dominated (blue circles). For clarity, we have not highlighted all dark matter-dominated dwarf galaxies in the image. Source: Illustris-1 Simulation, RG Kroupa/University of Bonn

According to the standard model of cosmology (also called LCDM), dark matter is the most prominent part of the Universe and makes up 80 percent of its mass. Dark matter acts like a glue that hold galaxies together: Its strong gravitational forces prevent galaxies from being destroyed by their own centrifugal forces. This however is not the case for all galaxies in the Universe. "In 2012 we postulated that some types of dwarf galaxies have to be dark matter-free or at least dark matter-deficient," explains Prof. Dr. Pavel Kroupa from Charles University in Prague and the University of Bonn. "We are now able to identify simulated dwarf galaxies without dark matter and thus verify the dual dwarf galaxy theorem using the Illustris simulation. This is currently one of the largest and most modern galaxy evolution simulations."

Compared to "normal" galaxies like our Milky Way or the Andromeda, dwarf galaxies are much smaller and contain only around 100 million stars. In contrast, our Milky Way has around 100 billion stars. In the standard model of cosmology, most of the dwarf galaxies are primordial and form shortly after the Big Bang via gas condensation. These dwarf galaxies are in heavy dark matter halos that formed before the first stars were born.

Dwarf galaxies without dark matter are more compact

Dwarf galaxies without dark matter are formed during the interaction of two normal massive galaxies. Such an event causes huge tidal forces, which eject gas from the outer regions of the interacting galaxies. This ejected gas accumulates and forms clumps, which grow continually in mass and produce stars. "If these objects are massive enough, we call them tidal dwarf galaxies," explains Moritz Haslbauer. "Due to their special formation scenario, these types of dwarf galaxies have no or at least very little dark matter."

The astrophysicists have now verified the dual dwarf galaxy theorem in the Illustris simulation, in which they found both dark matter-dominated and dark matter-free galaxies.

"This study shows that, as expected, tidal dwarf galaxies with very little dark matter do form in the standard model of cosmology. This calls for systematic observational campaigns to find more dwarf galaxies in different environments and to thoroughly investigate their properties to test the predictions of the standard model," says Dr. Behnam Javanmardi from the Institute for Research in Fundamental Sciences in Tehran and the Paris Observatory. However, the results of the Illustris simulation are in tension with current observational data. Simulated primordial dwarf galaxies of a given mass have larger radii than simulated tidal dwarf galaxies. The reason is that the latter form naked without the help of dark matter and thus have to be much more compact than dwarf galaxies (that have a huge amount of dark matter) in order to remain gravitationally bound. This behavior is in conflict with observed dwarf galaxies. "All known dwarf galaxies have a very similar mass-to-radius ratio – we do not observe such significant size differences in the real Universe," explains Dr. Jörg Dabringhausen from Charles University in Prague.

Prof. Kroupa concludes that this is in conflict with the standard model of cosmology, indicating that dark matter probably does not exist. In fact, so far dark matter is only a hypothetical concept in order to explain certain observations. It has never been directly detected. "We expect that Newtonian gravity is insufficient to explain galaxies correctly. Instead of adding dark matter, we suppose that Newton’s law of gravity is not valid on the scales of galaxies and has to be modified," explains Indranil Banik from the University of Bonn. "Moreover, modified Newtonian dynamics would also explain why all observed dwarf galaxies have a similar mass-to-radius ratio since none of them would have any dark matter."

Publication:
M. Haslbauer, J. Dabringhausen, P. Kroupa, B. Javanmardi and I. Banik: Galaxies lacking dark matter in the Illustris simulation; Astronomy & Astrophysics; https://arxiv.org/abs/1905.03258