It turns out that the Milky Way’s satellite galaxies have fairly short lives. This contradicts the standard cosmological model, in which dark matter provides large galaxies with large numbers of moons. There are problems in modern cosmology that are quite difficult to solve. It’s the lack of dwarf galaxies. As a matter of fact, the stars at the edge of the galactic disk have a fairly high rotational speed around the center of the galaxy. This is a well-known effect of dark matter. Based on these velocities, we find that our Milky Way, whose visible disk is 100,000 light-years in diameter, must be immersed in a spherical halo of dark matter over 250,000 light-years in diameter.
But in this case, the dark matter in this halo should capture and hold onto many dwarf satellite galaxies (500 in the case of the Milky Way). In fact, astronomers have discovered only 12 of them. Similar phenomena can be observed in other galaxies. This clearly suggests a major problem in our understanding of dark matter, but what exactly the problem is is still debated. In some physics theories, the reason for this is that, in fact, dwarf satellite galaxies can only form if there is a large enough black hole at the center, and in such models our mass states that this is because the specific number of such objects per galaxy is limited. She doesn’t have 500 satellite galaxies that she can’t have. However, within the framework of today’s standard cosmological models there is no such restriction. Therefore, one of the most serious problems is the discrepancy between the theoretically predicted number of dwarf galaxies and the observed number.
A new paper published in the Monthly Notices of the Royal Astronomical Society offers a rather fresh perspective on the issue. The study authors used observations from the Gaia space telescope to compare the orbital energies of dwarf satellite galaxies. Such energy is directly influenced by the speed of satellite galaxies around the Milky Way. One of the lowest energies of its kind has been observed in the Sagittarius dwarf elliptical galaxy (closest to us of all moons). From its mechanics, it is clear that it became a satellite about 5 to 6 billion years ago. However, for satellites in orbits further away from our galaxy, the orbital energy situation is different. The energy is much higher than that of the aforementioned Sagittarius elliptical galaxy. In many of them, the stellar velocities have a very wide distribution, indicating severe instability of these galaxies. Based on this, researchers tried to estimate when it became a satellite of the Milky Way. Calculations showed a value well below 3 billion years. However, no truly old dwarf satellites dating back more than 6 billion years have been discovered.
Meanwhile, based on the standard cosmological model, a dark matter halo should ensure the accumulation of such satellites for at least the last 10 billion years. It turns out that the earlier satellites disappeared somewhere – the authors suggested that they collapsed due to dynamic instability caused by interaction with gas and gravity of the Milky Way and its environs.
The problem with this solution is that it virtually rules out the possibility of significant amounts of dark matter in dwarf galaxies. If it were there, it would stabilize the satellite galaxies and prevent them from collapsing over time. This is a rather unusual conclusion, since in the standard cosmological model it is completely unclear why many galaxies, at least in our neighborhood, can be free of dark matter. Moreover, such a model inevitably raises the question of where, in this case, many of the dwarf satellite galaxies it predicts are located—approximately 500 in the case of the Milky Way. If previously there seemed to be a dozen of them, now that number is approaching zero, exacerbating the problem of dwarf galaxies in cosmology.