This guest post was written by Elena Asencio, a first year PhD student at the University of Bonn. Elena works on testing and comparing different cosmological models at galaxy and galaxy cluster scales. She enjoys the overall experience of existing very much.
Authors: Mahmood Roshan, Neda Ghafourian, Tahere Kashfi, Indranil Banik, Moritz Haslbauer, Virginia Cuomo, Benoit Famaey, Pavel Kroupa
First Author’s Institution: Department of Physics, University of Mashhad
Paper Status: Accepted in MNRAS [open access on arXiv]
If you look at a face-on depiction of our beautiful Milky Way galaxy (see Fig.1), you might notice that this galaxy is not just a blob of dust, gas and stars, it has a certain structure. The bright central bulge and the spiral arms are perhaps their most striking features, but the Milky Way also presents another important substructure known as “bar”. Bars are an elongated, overdense aggregation of stars that go through the galactic centre and end near the edge of the central region of the galaxy, where the spiral arms begin to form. Those galaxies which include bars are known as barred spirals. Today’s paper makes us look at this type of galaxy with newfound appreciation, as it shows how galaxy bars can be used to test one of the most elusive components of the standard model of cosmology: dark matter.
The inclusion of dark matter into the standard cosmological model was originally motivated by a very curious discovery: objects in the outskirts of galaxies are observed to have a circular velocity that is way too high for them to stay within to the galaxy – given the gravitational attraction they experience from the observed mass of the galaxy. The solution that the scientific community came up with to explain this phenomenon is that there is more matter in galaxies than the observed one, so the gravitational attraction towards its substructures is stronger than expected. This is why these very fast objects are not escaping the galaxy. In other words, astronomers assumed the existence of a new type of matter that does not absorb, reflect or emit light and is, therefore “dark matter”. But then, how can we possibly detect matter that we can not see?
Dynamical friction in galactic bars: testing the presence of dark matter in galaxies
Dark matter may not interact with the electromagnetic force, however it does interact gravitationally with baryonic (observable) matter. In the case of galaxy bars, this implies that, as the bar moves in a dark matter dominated medium, the dark matter particles will slow down the bar by putting a drag to its movement. Figure 2 illustrates and further explains this effect, which is known as dynamical friction.
Because of dynamical friction, we expect that galactic bars will be moving slower if they are indeed immersed in a sea of dark matter particles, but slower…with respect to what? Given their higher mass, galaxy bars are more affected by dynamical friction than the stars moving individually around the galaxy in circular motion. The authors of the paper propose a method to determine the relative speed of the bars based on this fact. First, they measure the radius at which the bar is moving at the same circular velocity as the stars beyond the bar (the co-rotation radius). Then, they compare it with the radius of the bar itself. If the effect of dynamical friction is negligible on the bars, the co-rotation radius will be practically the same as the bar radius. However, if there is dynamical friction in the bar caused by the presence of dark matter, the pattern speed (circular velocity divided by the radius) of the bar will be slower and, therefore, the co-rotation radius will be larger than the bar radius in this case.
But, to which extent does the dark matter affect the location of the co-rotation radius? To answer that question, the authors looked at several cosmological simulations that follow the standard model and searched for barred spiral galaxies in them. By obtaining their properties of pattern speed and circular velocity in a very similar way in which these are inferred in real galaxies, they managed to make a clear comparison between the co-rotation and the bar radius ratio given by observations and that expected by the standard model of cosmology. Figure 3 shows the values that this ratio takes in observations and in several cosmological simulations.
From this, it is clear that the standard model of cosmology predicts a pattern speed for the galaxies that is way slower than what observations tell us. Not only that, but observations show that, in real galaxies, the ratio between the co-rotation and the bar radius is approximately one, just as expected from a system in which there is no significant dynamical friction from dark matter.
The predictions of the standard model of cosmology have failed to reproduce observations by a long shot. Now what?
The authors considered the possibility that observations could still be reconciled with the standard model if the dark matter possessed properties that suppressed dynamical friction on galaxy scales. But, up to today, all of the alternative models in which dark matter satisfies this condition seem to have additional problems in other areas.
Another option to consider is that galaxies do not have a dark matter halo. The most popular alternative to the dark matter hypothesis are the extended gravity theories. In these theories, gravity experiences a boost in the regime of low accelerations (e.g. far from the galaxy centre). Because of this, the gravity pull that the objects in the outskirts experience is stronger than in Newtonian gravity (the current gravity model). This allows them to orbit at high velocities without escaping the system. Even though in the extended gravity theories fast galaxy bars would arise naturally, it would still be necessary to have cosmological simulations of these models with a sufficiently high resolution to check whether they are also capable of reproducing other properties of the bars (e.g. strength, length, etc).
Such simulations do not yet exist, but it will be very interesting to find out whether the universe they describe resembles the observed one with a higher or with a lower accuracy than the standard cosmological model. After all, as the authors point out in their paper, the standard model has already shown to fail on several fronts: the planes of satellites, the Hubble tension, the observations of super voids and super clusters, and now, the galaxy bars too. Therefore, continuing to explore these alternative models in more detail seems to be right now our most sensible option.
Astrobite edited by Jenny Calahan
Featured image credit: NASA/JPL-Caltech/ESO/R. Hurt