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Do intermediate-mass black holes exist?

Periodic Reporting for period 4 - imbh (Do intermediate-mass black holes exist?)

Reporting period: 2020-03-01 to 2020-08-31

The consolidator grant focuses on how black holes form and in particular whether intermediate-mass black holes exist (masses intermediate between black holes 10 times more massive than the Sun as typically found in our galaxy, and the super-massive black holes found in the nuclei of galaxies of masses more than 1 million times the mass of the Sun). How black holes form is a general problem in astronomy that is not well understood. For instance, how supermassive black holes, black holes with masses higher than 1 million times the mass of the Sun are formed is unclear. Over the period of the grant, Earth-based gravitational wave detectors discovered black holes with masses up to 140-150 times the mass of the Sun, such black holes had not been found before. Our work contributed to the evidence that black holes in the mass range between 50-1 million times the mass of the Sun exist. Especially, our work on tidal disruption events where a star that comes too close to a black hole is ripped apart, has shown that often such black holes are less massive than 1 million times the mass of the Sun.

This is curiosity driven research. Black holes are objects that capture the mind of many in the public and researchers alike.
"The work has three aspects (three promising areas in astronomy where we think that we have a good chance of finding intermediate-mass black holes).

1) so called ultra-luminous X-ray sources emit more (X-ray) light than expected for a typical stellar mass black hole of 10 times the mass of the Sun. They do so because they accrete (=""eat"") gas from a neighbouring companion star. We are searching for systems where that companion star is a red-supergiant as those are so bright that we can determine their velocities using near-infrared spectroscopy using the largest telescopes on Earth. From those velocities we can deduce the mass of the black hole they circle around. We have finished the survey of nearby ultra-luminous X-ray sources to find those that have a red-supergiant companion star (published Lopez et al. 2017 and 2020). A second (new) thread of research has opened here after the gravitational wave detectors LIGO/Virgo showed that black holes with masses of 20-140 times the mass of the Sun exist. We have found several systems where there is a red-supergiant at the location of the ultra-luminous X-ray source. We currently are still in the process of determining the motion of those red-supergiants, as that will allow us to measure the mass of the black hole that accretes the gas. However, due to their large sizes, it takes a red-supergiant often more than a decade before it moves around the black hole, so we have to continue our measurements for a significant fraction of that period (hence, beyond the 5 year period that was covered by this grant).

2) in the formation of a galaxy like our milky way, many smaller galaxies have merged together. One of the theories on how supermassive black holes form state that those smaller galaxies should have hosted intermediate mass black holes. We are searching for the remnants of those smaller galaxies around our galaxy and around our neighbour galaxy the Andromeda galaxy as those remnants are expected to host the original black hole as well. These systems are called hyper-compact stellar clusters. We have used large ground based surveys for this and we will use Gaia's Data Release 2 (that became public in April 2018). We have also updated and expanded existing simulations on how we expect these hyper-compact stellar clusters to look like in order to have a database of potential images to compare our data with. Unfortunately, the data of Gaia's Data Release 2 was not sufficient for us to find a hyper-compact stellar cluster and Gaia's Data Release 3 has been delayed beyond the end of the grant. The good news is that our work shows that besides the Gaia satellite, the data from the Euclid satellite to be launched the end of 2022, should be ideal to find hyper-compact stellar clusters.

3) we use tidal disruption events to assess the mass of the black hole. Tidal disruption events are events where a star wanders too close to a black hole and gets shredded apart by the large difference in gravitational pull between the part close to the black hole and the part facing away from the black hole. Some types of stars namely white dwarfs can only be disrupted by intermediate mass black holes. So we are searching for such events. A fast flare is expected to occur up on disruption of such stars and we have provided the Gaia satellite that happens to gather data that is also suitable to search for such fast flares, with the tools needed to provide the scientific community with information of the occurrence of such flares. In addition, we investigate tidal disruption flares found by other means than by Gaia in order to learn about this new phenomena and to use it as a tool to investigate black holes. We have shown that indeed some of these are caused by intermediate mass black holes (see publication of Wevers et al. 2017; 2019). The phenomenology of these tidal disruption events is rich and as this is a relatively new kind of event that we can use to study black holes, a lot of additional information has been gained over the last 5 years. This has been summarized by a large group of researchers from all over the world in a series of publications that will together be made into a book to be published by Springer. The PI of the ERC grant has been the lead-Editor of this work and contributed to two of the chapters.

All our findings have been published in peer-reviewed scientific literature. The publications are all accessible through green open access (typically through arxiv.org)."
So far there is mostly suggestive evidence for the existence of intermediate-mass black holes and through our work we have i) added to the circumstantial evidence, and ii) solidified the evidence. In the grant rare and often difficult to find events have been used. We have developed methods to help find and identify these rare & hard to find events. This has been implemented in the ongoing Gaia Science Alerts project, as project using data from the Gaia satellite launched by the European Space Agency.