When we look into the night sky we are fooled by the bright and sparkling stars, and we think they make up most of the matter in the Universe. Among astronomers that’s commonly believed to not be true, since observations tell us that a significantly larger amount of mass fills the Universe in the form of dark matter. What dark matter is, and whether it exists at all, is difficult to answer and no one has found a definite answer yet. However, there are other forms of matter that hide from our naked eyes, or even from our large armada of telescopes. Planets, brown dwarfs, low-mass stars, neutron stars, white dwarfs and black holes are all emitting barely any electromagnetic radiation (a.k.a. light). Thus, only in very few, and nearby cases we can actually observe these objects. For most parts of the Universe, this hidden matter remains unseen and we have to add a certain amount of dark mass to the mass we see in stars based on the best of our knowledge.
It’s important to note at this point, though, that this hidden matter is by far not enough to explain the mass discrepancy in the Universe that led us to the hypothesis of dark matter. However, in certain parts of the Universe, hidden matter in the form of black holes, neutron stars and white dwarfs can make up a large fraction of the local matter density. One of these places is the Milky Way globular cluster NGC 6624. According to a paper I published recently with Miklos Peuten, Michael Brockamp and Pavel Kroupa, the center of this cluster should be filled to the brim with these dark remnants, as we call them. One third of all matter in this star cluster is hidden from our deepest observations, and dwells in the cluster’s center. There it causes such a strong gravitational acceleration on the neighboring stars, and especially on a low-mass X-ray binary called 4U 1820-30, that we can measure how these objects got accelerated over the past 20 years.
Space telescopes and radio antennae have monitored 4 special objects (the above mentioned X-ray binary and 3 pulsars) in the cluster over decades, and they tell us what the gravitational field in the cluster is like. By comparing this inferred gravitational field with the mass distribution in form of the stars, we can deduce that we’re seeing only 2/3 of the mass of the star cluster. Our combination of observations and modeling of this cluster allows us to exclude that this hidden matter is in the form of a huge black hole with several thousand times the mass of the Sun – although this was the kind of hidden object we were originally looking for. Our new interpretation of the observational material is no less spectacular, as dark remnants are usually not believed to be dynamically important in star clusters. We show that in fact they are, and that in some parts they can even entirely dominate the dynamics of the stars we see!