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Dark matter, black holes, and dwarf spheroidal galaxies

Our current understanding of the Universe suggests that it is composed of an invisible component called “dark matter“. This mysterious type of matter represents more than 25% of the entire matter and energy of which the Universe is made. The matter that we are used to “seeing” in our everyday life and that represents the building blocks for both our bodies and stars that shine in the sky, represents only 5% of the Universe. We call this “ordinary” or “baryonic” matter.

The fundamental law that regulates the interaction between bodies, composed of either dark matter or baryonic matter, is gravity. Roughly speaking, gravity keeps celestial bodies – such as the moon and the Earth – bounded together. Similarly, stellar systems can be composed of tens of stars, held together by gravitational pull. “Stellar clusters” are larger, with hundreds to a few millions of stars and “galaxies” are those having billions to thousands of billion stars. Our galaxy, the Milky Way, is a “spiral” galaxy weighing about 1012 times the mass of our Sun.

Although such a quantity seems to be definitively huge, observations made clear since the 1930s suggest that galaxies contain much more mass than is actually visible. Practically, two possible methods for evaluating the mass of a stellar system are either looking at the velocity of its stars, or looking at the total amount of light that it emits. Comparing these two quantities made clear in the majority of observations that the former method gives systematically higher mass values. This leads to the accepted picture that galaxies are embedded in large halos of dark matter. More importantly, dark matter distributes in a very characteristic way, having densities that steeply rises toward the centre of the halo. In particular, decreasing by a half the distance to the centre, the density increases twice.

Our own galaxy, the Milky Way, is likely to be contained within its dark matter halo. Interestingly, its neighbourhood is populated by a number of smaller galaxies, called “dwarf spheroidals“, which have masses of a few hundred million solar masses and orbits around it. These small galaxies are characterised by a very high fraction of dark matter, although they are much smaller than the Milky Way by a factor 10,000.

Even more interestingly, in these systems, the dark matter density seems to rise toward their centre following a more gentle trend. This represents a problem in cosmology called “the core/cusp problem”, as the standard theory predicts a general trend that is not observed in dwarf spheroidals.

Moreover, these galaxies do not host massive black holes at their centre, which occupy the majority of the nuclei of galaxies with masses above one million solar masses.

Our own galaxy, the Milky Way, is likely to be contained within its dark matter halo. Interestingly, its neighbourhood is populated by a number of smaller galaxies, called “dwarf spheroidals”, which have masses of a few hundred million solar masses and orbits around it.

In our work, we propose a mechanism that explains both the absence of massive black holes and the strange behaviour of dark matter distribution. Our theory relies upon the fact that nearly all the observed galaxies contain stellar systems, agglomerate of stars that may be composed of a few thousand up to a few million stars called star clusters. Star clusters move within the galaxy, interacting with the stars and the dark matter that compose the galaxy background. The sum of all the interactions causes a drift of their orbit, driving them toward the centre of the galaxy following a spiral pathway. At the same time, during the cluster orbital decay, its shape warps due to the same interactions.

The time over which orbital decay and the cluster warp takes place depends on the cluster mass, initial orbit, and initial velocity – but to give a general idea, we can state that, at fixed mass, the larger the initial orbit, the longer the decay time-scale.

It is a race against the clock, since the cluster can either be completely disrupted before accomplishing its orbital decay or it can reach the centre and settle there, leading to the formation of a very dense nucleus.

However, if they are sufficiently massive, these clusters influence the galactic nucleus, comprised of stars and dark matter, forcing it to re-arrange its configuration and leading to a much shallower density distribution.

Our results show that the star clusters undergo significant disruption due to the interaction with stars and dark matter that compose the galaxy. After a few Gyr, the star clusters are completely disrupted but, interestingly, they significantly changed the galaxy matter distribution. Indeed, during their disruption they cause the galaxy response that leads its total (stellar + dark matter) density to get shallower, leading also to a significant decrease in the galaxy central density.

NGC 147, a dwarf spheroidal galaxy of the Local Group by Ole Nielsen. CC BY-SA 2.5 via Wikimedia Commons.

This represents a complementary theory related to dwarf spheroidal galaxies, along with the possible contribution given by supernovae events to the nucleus structure or a possible modification of the classical theory of gravity.

How does this relate to massive black hole formation? It is widely thought that massive black holes form in the early life of a galaxy. One of the debated channels involve multiple collisions of stars, which drive the formation of a very massive star that possibly collapses to a black hole with masses of around 100-1000 solar masses and then rapidly grows by swallowing surrounding gas and stars, reaching the values currently observed (from few million to several billion solar masses).

What drives such a runaway process is the density of the nucleus, where stellar collisions start. The density decrease observed in our simulations make extremely unlikely the possible starting of the massive black hole seed formation phase, giving a quite general explanation of the causes of the lack of massive black holes in dwarf spheroidals.

Hence, our work proposes a four-step mechanism that occurs in the very early life of the dwarf, which can be summarised as follows:

  1. The dwarf spheroidal forms with a steep density profile and some star clusters form following the overall galactic distribution;
  2. Due to gravitational interactions between stars and the clusters, the latter moving on inner orbits are efficiently disrupted, while those moving in the galactic outskirt are almost untouched;
  3. During their infall, clusters exert on the galactic nucleus a force, which causes its readjustment. In consequence of this, the resulting nucleus is much less dense than its initial configuration, possibly obstaculating the formation of a massive black hole seed;
  4. Clusters moving on the outer orbits undergo orbital decay and eventually reach the galactic centre, driving the formation of a bright nucleus.

Featured image credit: Galaxy. CCO Public Domain via Pixabay.

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