Yay!
After many hours of pouring over, revising, reviewing and rerevising, I have finally submitted my masters thesis. Nevertheless, I have no doubt it will take some time to get to the stage where the final viva examination can be completed, as I believe there is an approval process to go through.
Still, that’s a milestone after a couple of years of trawling through Zwicky Transient Facility (ZTF) archives for evidence of Tidal Disruption Events (TDEs) in Compact Stellar Systems (CSS).
Hey! More acronyms 😉
Okay, so I anticipate I’ll be doing quite a few science pieces over the next few years to explain what I’ve been up to with my research and on my way to my PhD which has been a lifetime dream, but here’s a taster of what I’ve been working on over the last few months.
Black Holes
Most people have heard of black holes, and many people are even aware there are different varieties. With masses from 3 to 100 times that of our Sun, Stellar Mass Black Holes form from the collapsed remnants of giant stars which have run out of fuel (hydrogen) in their interiors. The lack of fusion energy to resist the gravitational pull of the matter that makes up the star causes it to collapse in on itself, and no known force can stop that collapse. The density of the matter gravitational pull of these objects is so great, that the speed which must be attained to escape from its ‘surface’ exceeds that of light, hence the term black hole.
Super-massive Black Holes, are found in the centres of most galaxies and have masses upwards of 1 million times that of our Sun. How these behemoths form is still one of the unanswered questions in astronomy.
The characteristics of black holes at these extremes of mass are similar, albeit related to their mass and therefore, it is possible that black holes between these mass extremes also exist, i.e. from 100 to 1 million times the mass of our Sun.
These intermediate mass black holes are what I have been searching for in my research.
However, all black holes are intrinsically invisible objects, making them rather difficult to observe, so how do we approach the task of unmasking their elusivity.
Tidal Disruption Events (TDEs)
Black holes create the most extreme gravitational fields in the cosmos, the strength of which is related to their mass. This mass distorts the fabric of space-time, and like water falling into a reservoir glory hole, the matter speeds up as it falls into the gravitational well. As it speeds up, a disc will tend to form and friction will heat the matter as it spirals inwards, causing it to glow hotter towards its eventual demise across the one-way threshold, known as the event horizon. Thus black holes can be detected from the ‘glow’ of this accreting matter.
Other means of detecting the presence of these compact objects involve measuring the gravitational influences of the extreme environment surrounding the object where, for example, stars and gas would move faster than if the black hole was not present.
Black holes are elusive though. Detecting the emission glow from accreting matter rely upon there being gas or dust in the vicinity of the compact object without which the object will be quiescent and truly invisible. Detection by gravitational influence, relies upon stars being within the ‘sphere of influence’ of the black hole, and even then this method is only possible at the relatively short distances of the Local Group of galaxies. Increased resolution telescopes may increased the probability of detection through this method.
A quiescent black hole may suddenly start emitting if a happless star strays too close to it. Such an occurence is known as a tidal disruption event, or TDE, and is where the extreme gravity strips the gases from a star and forms a temporary accretion disc, emitting a high intensity flare of radiation over some weeks to months. These are the emissions I have been searching for in observatory archive data.
Zwicky Transient Facility (ZTF)
When we look up to the heavens every night, we could be forgiven for thinking that the sky is static. Indeed, ancient astronomers knew only of the motion of the planets which moved predictably through the night sky – the very word planet comes from the Greek meaning ‘wanderer’. Until the early 17th century only the occasional comet spoiled that view, at which point Brahe, Kepler and Galileo were amongst the first to methodically document variations on the night sky from nova, comets, sunspots and moons around Jupiter. Even so, repetitive surveying of large swathes of sky with sufficient cadence to capture the dynamic and variable nature of the sky had to wait until the last couple of decades.
The Zwicky Transient Facility or ZTF, is based around the 48″ Samuel Oschin telescope on Mt. Palomar, California, the telescope of which was used for some of the earliest photographic surveys of the northern hemisphere in the 1950’s. Things have come a long way since then and the ZTF now trains its 48° FOV on the northern sky repeatedly every 3 days.
The variability which can now be analysed in what might otherwise appear to be constant targets has added an extra dimension to astronomical observations, and more importantly has also afforded the opportunity to identify targets of a transient nature, such as supernova, cataclysmic variables, kilonova, the optical counterparts of fast-radio bursts and gravitational wave events, and of course TDEs.
Now, TDEs are rarer than unicorn droppings – the current theoretical rates are predicted to be 1 per galaxy per 10,000 years, yet despite this, over 98 TDEs have been detected (to March 2022) – of these 35 have had their discovery credited to the ZTF.
So now we can find them, the problem becomes, where do we look?
Compact Stellar Systems
If galaxies are the cities of the universe, and stars are the houses, then compact stellar systems are the outlying towns and villages surrounding the metropolis. These compact stellar systems or CSS, consist mainly of what are known as globular clusters, dense collections varying from 10,000 to 1,000,000 stars which are tightly bound together, making them very stable and long lived. More massive cluster structures also exist know as ultra-compact dwarfs (UCDs) and Compact Ellipticals (CEs). These environments are so densely populated with stars, it has long been thought that black holes may form in them and then ‘sink’ to the core of the cluster where mergers may cause the black holes to grow.
Some of these larger systems may also be dwarf galaxies which have had their outer layers stripped off after tidal interaction with more massive galaxies, like our own Milky Way galaxy. Having originally formed following a galaxy evolution process, these larger systems are also likely to contain black holes in their cores, following the logic that most larger galaxies contain supermassive black holes.
As these stellar systems have a huge number of stars in a comparatively small volume, i.e. they are densely populated, the probability of interaction between stars, or even mergers, is increased. If they do contain black holes in their cores, then the probability of tidal disruption events, between the black hole and stars, is also more probable. The mass range spanned by globular clusters, and compact stellar systems in general, in comparison to larger galaxies, is strongly suggestive that any putative, nuclear black holes would be of intermediate-mass, and this is the key reason why I have been concentrating my search on these type of environments.
So, finally, we know why, where and how we would look for these tidal disruption events, in a quest to provide evidence for the existence of intermediate-mass black holes. So, what did I find?
Nothing like a cliff-hanger to whet the appetite for the next episode… 🙂
