I have been recounting the fantastic properties of black holes for a number of posts now, however I have not yet properly addressed one of the seemingly more mundane questions over their nature – what range of sizes can they come in? It is now definitively known thanks to the LIGO gravitational wave detector that black holes the weight of a few tens of solar mass exist. These are stellar mass black holes which form the death of a star that has grown so big it collapses inward under its own gravity. It is also known that supermassive black holes, with masses up to a billion times the mass of the sun, exist at the centre of galaxies, from observation of the motion of surrounding galactic matter. The astrophysical process that form such gargantuan beasts however is still unclear. Whether black holes of mass within this range, with mass hundreds to thousands of the sun, exist is a big open question. Such an extreme disparity in the size of black holes without a bridging population seems unlikely and the existence of such intermediate mass black holes (IMBHs) would help us better understand the astrophysical dynamics of black holes as well as gives us clues to crucial details over black hole formation. Determining the gravitational wave templates for intermediate black holes merges in order to spot their signals amongst the cosmic noise is the focus of my PhD, as such I have a strong vested interest in their existence.
Though there has not been any concrete proof of IMBHs so far, some evidence provides strong suggestion. Whilst gravitational waves are the medium with which we can detect black hole mergers, while they live as solitary beasts there are other ways in which we can detect their presence. As black holes consume matter many emit high-energy radiation in the form of X-rays. Observers have previously found strong X-ray emission from nearby galaxies, such as NGC 1313. From the analysis of the X-ray emission, its strength and periodicity, a constraint can be put on the mass of the source. In the case of NGC 1313, this came in at around 1,000 solar masses, firmly putting it in intermediate mass black hole territory. Further evidence that such strong X-ray emitting sources are indeed black holes comes when the radiation clearly does not have with a visible light counterpart, which would be the case should the source instead have been a star or galaxy. The radiation signals often also show periodicity, a phenomenon thought to be due to the consumption pattern of the black hole, whereby every time it rips out matter from encircling nearby stars it emits a strong burst of X-rays.
As detection of such signals is sporadic and not entirely conclusive, concrete proof of IMBHs will only come when we can detect a merger. This will either take the form of two intermediate black holes colliding, or intermediate mass-ratio inspirals (IMIRIs) when a stellar mass black holes falls into an IMBH or when an IMBH falls into a supermassive black hole. To detect gravitational waves signals from IMRIs requires innovative methods of modelling their gravitational waveform templates, an exciting new field which myself and my supervisors are currently working on. The next generation of gravitational wave detector, the space satellite LISA, will have a frequency detection range well suited to IMRI signals, so the race is on to provide accurate templates before its launch. It is also hoped that Advanced LIGO may be able to tune into the gravitational waves from IMRIs once we know what waveforms it is that we are looking for. The detection of IMBHs would be provide a vital bridge in our understanding of black holes, helping us piece together black hole formation, population and evolution over time. Stay tuned!