8 mins read 03 Feb 2022

Astronomers Discover a Lonely, Wondering Black Hole

An international team of astronomers (including several New Zealand and Tasmanian researchers) have announced the discovery of an isolated black hole zipping through the Milky Way Galaxy.

Credit: Ohio State University.

Ever since black holes were considered theoretically (as a result of Einstein’s General Theory of Relativity in the early 20th Century) they’ve captivated our minds and imaginations. These mind-boggling objects, of which the laws of physics break down when being described, have spawned countless science fiction stories, and even more scientific papers.

But they’re rather tricky to spot. You see, the reason why they are called black holes (which is technically not an appropriate name, given that they are not actually holes) is that they are regions of space-time where the mass of an object is crushed down into incredible densities, producing fantastical gravitational fields around them. So strong is this field, that not even light – travelling at approximately 300,000 km/s can escape – so whatever goes into them, can never come out. And we don’t know what happens within their boundaries (that’s the part where the physics breaks down).

Now, a global team of astronomers – including scientists from New Zealand and Tasmania – have announced an incredible find. An isolated black hole (named MOQ-2011-BLG-191/OGLE-2011-BLG-0462), roaming through the Milky Way Galaxy all on its own. Historically, black holes have been discovered in binary systems, so this new black hole has the astronomy community abuzz - a random black hole just wondering around the Galaxy as it pleases - it’s a science fiction story waiting to be written. 

The discovery brings about a whole new range of questions, whilst answering others. We’ve always thought that there could be isolated black holes (those without a binary partner) roaming around the Milky Way, but how did they become isolated? And how did they get their kick to get going in the first place? Additionally, what methods could we use to detect them – especially given they don’t emit any light.

To detect MOQ-2011-BLG-191/OGLE-2011-BLG-0462 and confirm it is indeed a black hole, the team of astrophysicists used the Hubble Space Telescope (HST) to study this object for almost a year (roughly 270 days) as well as combining observations with a global network of ground-based telescopes, utilising a technique known as microlensing.

Ground-based photometric observations with a number of telescopes (listed, along with their aperture) showing the best-fited model light curve of MOQ-2011-BLG-191/OGLE-2011-BLG-0462. The top panel reflects the 300-day interval and the bottom panel showcases the seven days surrounding peak microlensing magnification. Credit: Sahu et al. 2022.

Now normally, black holes (which are small and invisible) are impossible to detect - though their surrounding regions like an accretion disc can give them away. To observe them, we need to observe their influence and impact on their surrounding regions, especially as they form part of a binary system.

For example, we could measure the doppler shift of a star orbiting a massive, invisible object and through an understanding of its orbital period, we could derive its mass and the mass of the invisible companion. Or we could measure the x-ray output of a powerful accretion disc, as it gets pulled in over the Roche lobe point from a companion star by the black hole (these are known as x-ray binaries and feature black holes or neutron stars as the compact remnant companion).

In the last few years, we’ve also been able to ‘feel’ stellar-mass black holes through the giant laser interferometers that measure gravitational waves – detecting the signal that represents the final few moments of each black hole’s life as it falls into a death spiral before merging to form an even larger black hole.

Using these methods, we are able to quantify the mass of a black hole by these impacts and influences on companion objects or the nearby region. But these methods don’t work for isolated black holes – because, without the binary companion, they’re just invisibly massive objects wandering through the cosmos.

Science Check – What is Microlensing?

When a background star’s light is bent around a foreground object, such as a Massive Compact Halo Object (MACHO), some of that additional light is directed towards our view, increasing the brightness of the background star. This is a microlensing event. Credit: Pietrzynski/Nature.

One way to find objects which emit or reflect very little light, let alone produce anything, is to watch for the effects these objects gravitational fields have on background stars. This is known as gravitational microlensing and usually involves telescopes on or near Earth keeping an eye on background stars in the hopes of catching one of them suddenly increasing then decreasing in brightness.

This occurs because the foreground object (which might be a planet, a star, or a compact remnant object like a neutron star or black hole) has sufficient gravity to bend the background distant star’s light, around the objects as it passes in front of it relative to our light of sight - lensing it, so to speak. The more massive the object, the more gravity it will have, and the more gravity it will have, the more dominant the microlensing effect will be.

This technique can be used to then determine the mass of an object by measuring how much the background starlight was lensed during the transit, thus, giving us a method to directly detect and measure the parameters of objects (even those which are invisible) without the need for a binary companion. In fact, there are many confirmed and candidate exoplanets that have been discovered using this method, as per the NASA Exoplanet Archive

A number of microlensing experiments have been established around the world to keep their eyes on the sky to determine if any distant star suddenly starts to flash brightly on and off again – as they could be microlensing events. This includes the MACHO project, an Australian and US collaboration that ended in 1999 and was searching for massive compact halo objects as part of the drive to find dark matter. Several other experiments have since succeeded MACHO, including the Polish collaboration called Optical Gravitational Lensing Experiment (OGLE) which uses a 1.3-metre telescope located in Chile, or the Microlensing Observations in Astrophysics project, a Japanese-New Zealand collaboration that utilises a 1.8-metre telescope in New Zealand. There’s also SuperMACHO which has been running since 2001. 

In 2021, Australian researchers - who were searching through decades-old data - were able to identify an intermediate-mass black hole (these are much more massive than the stellar-mass black holes, though less massive than the supermassive versions which reside in the heart of galaxies). The technique used in this discovery was once again microlensing. 

The Wondering Black Hole in the Milky Way

Hubble Space Telescope observations of MOQ-2011-BLG-191/OGLE-2011-BLG-0462 at different epochs during the study. The source star is indicated by the arrow in the first tile, when the magnification was a factor of about 12. Credit: Sahu et al. 2022.

In this latest pre-print paper (which means it is yet to undergo the rigorous peer-review process), astronomers have utilised several telescopes from around the world, and including New Zealand (Auckland, Farm Cove, Kumeu Observatory, and Vintage Lange) as well as Tasmania to combine data with measurements taken with the Hubble Space Telescope, capturing the light-curve of the background star over a roughly 300-day period. 

From this data, the science team are able to determine the parameters of the foreground object (the wondering black hole in this instance) and have found it to have a mass of about 7 times that of our Sun (for reference, our Sun’s mass is 1.98 x 1030 Kg) and determined its distance to be roughly 5,000 light-years from Earth (phew!) near the Scutum-Centaurus and Sagittarius-Carina spiral arms of the Milky Way. 

The study also outlines the ‘proper motion’ of the black hole - that is, the apparent motion of the object with reference to the centre of mass in the Solar system - and have determined that it’s moving at about 45 km/s across the Galaxy. 

This gives scientists an interesting scenario - how did this isolated object get this much velocity? It turns out that it likely came from the original supernova explosion that formed the black hole, in a process known as a ‘natal kick’. This occurs when asymmetrical forces are compressed during the core-collapse process, causing the remnant object to launch away from any binary companion it may have had, or the region of where it first existed. We see this occur with many pulsars, especially as they traverse through the supernova nebula. 

A pulsar (PSR J0002+6216) is here seen in this false-colour radio image being kicked out of the original supernova remnant from which it was birthed. This pulsar has received a natal kick and is now isolated from its original system. Credit: Schinzel et al. 2019.

And the reason why scientists think this might be a black hole is that it is too big to suit other models, such as that of a neutron star (which are usually capped at around 2.5 - 3 times the mass of our Sun), or a white dwarf (which has an upper limit of 1.44 times the mass of the Sun), as well as not emitting any light of its own. 

These latest findings are exciting as it gives us an example of detecting rogue, compact remnant massive objects as they traverse around the Galaxy, without the need to have to use a binary companion to determine their parameters. Additionally, whilst these are few and far between these days, a number of large new telescopes - such as the Nancy Grace Norman Space Telescope and the Rubin Observatory, will soon have the ability to make hundreds or thousands of these types of detections in the not too distant future. They’ll really start to showcase how many random black holes are zipping around out there.

Let the Sci-Fi story writers run wild.