The behemoth behind the brightness

Q. Your team expanded on research from nearly two years ago when you discovered the brightest quasar ever. What did you find this time?

Fuyan Bian (FB): What we found two years ago was the most luminous quasar astronomers have discovered so far at over 10¹⁴ (100 000 000 000 000!) times brighter than the Sun. For this follow-up research, we carried out extensive observations of the object using the VLT and Keck Observatory.

This quasar lies at the centre of a galaxy, and hot gas from its host galaxy is actively falling onto this central supermassive black hole, forming a brightly shining accretion disc around the black hole that astronomers call a quasar. The purpose of our follow-up observations was to view this hot gas in much higher detail to measure the mass of the supermassive black hole.

Christopher Onken (CO): We knew from our initial study that this quasar has a very high redshift, meaning it is very distant, and therefore the light left it long ago when the Universe was very young. In the latest research, we found that the quasar is powered by a black hole 34 billion times the mass of the Sun, making it the most massive black hole found in the early Universe and one of the biggest ever detected. This object has a mass of about half of all the stars in the Milky Way condensed into the space that doesn’t even stretch 1% of the distance between the Sun and the nearest star.

Q. So what exactly is a supermassive black hole, and how is this one different from others found in the past?

FB: There are many types of black holes with quite different evolutionary histories. Primordial black holes are very small and are still only predicted by theory; they’ve never been observed. Stellar black holes form from the last phase of a massive star exploding as a supernova.

CO: Higher in mass still we see supermassive black holes. With around 20 years of experience studying these objects we have found that every reasonably large galaxy has a black hole at its centre, ranging from a hundred thousand to a few billion times the mass of the Sun. What's interesting is that the black hole we found looks in all other aspects just like an ordinary black hole powering a quasar; it’s really only its extraordinary mass that sets it apart.

Q. Do you know how something this big formed?

CO: The basic answer is that we just don’t know definitively how these objects form.The growth of a black hole is a self-limiting process. They grow as surrounding matter in the accretion disc falls into them. However, the more they grow, the more light and radiation pressure the quasar produces, pushing away the feeding gas from around the black hole, slowing the progression of falling matter. So there is a limit to how fast black holes can grow.

In the case of particularly massive black holes, we are unsure if they started off being large when they initially formed or if they grew in some other way that the unusual conditions of the early Universe could have allowed for. There are various theories and ideas but this is something that we just don’t have a good enough understanding of yet.

FB: We tend to find these types of black holes at the early epoch of the Universe when the first galaxies were still forming. One theory for how they formed suggests that they started small, forming from black hole “seeds”’ during this early period, growing larger and larger to the point at which they are observable. The size of this black hole suggests the “seed” needed to be quite massive to let this black hole become so large in such a short period of time.

Q: How did you both feel when you realised the extraordinary mass of this black hole?

FB: I remember it really well. I remember putting the numbers from my measurements into my computer code and clicking the button and thinking, “oh wow this is actually very massive!” We had predicted that this black hole would be large, but we didn’t know exactly how large. We realised only later that it was one of the largest ever found when checking with one of our collaborators. It definitely felt as if all the hard work paid off.

SkyMapper Southern Sky Survey image of the brightest quasar ever detected (the faint red dot in the middle). This image is about five arcminutes on each side.

CO: Initially I was mostly worried about how reliable our estimate was. Once we had convinced ourselves that we did everything right and there weren’t any other strange explanations for our measurement it was quite amazing to think about.

Q. Why do you think it is important to study bright quasars and massive black holes?

FB: The largest black holes likely started to form around 100 million years after the Big Bang. The new data helps us understand the formation process for black holes as big as this one during this very early time period, and discover what the environment must have been like for something this big to have formed. It also tells us about how the first generation of stars formed in such an extreme environment, which is certainly very different from our Milky Way.

CO: Most supermassive black holes we find are in the nearby Universe. This is one of just a few we have identified that is located far away, at a time where the Universe is only around 1.25 billion years old. In the nearby Universe we find that the mass of a quasar is mainly proportionate to the mass of its host galaxy, but we don’t have nearly enough data to conclude this relation in the early Universe.

The hope is that by looking at these extreme early cases, even if our measurements have a bit of uncertainty in them, we can construct a clear enough picture to see if this relationship still holds true, that the galaxy is indeed also incredibly massive, growing at the same pace as the black hole.

In future research we plan to use ALMA to measure the cold gas within quasar host galaxies to find their mass as well. Hopefully we can uncover some clues about the relationship and physical processes that link quasars with host galaxies, revealing the properties of early galaxies themselves.

Q. You originally used data from survey telescopes to look at this object, then used spectroscopic instruments including X-shooter on the VLT. Why were further observations with spectroscopic instruments needed?

FB: The survey telescopes we used measure only the amount of total amount og light emitted by an object, so we used them to pick quasar candidates to observe in more detail, but their results are not actually very useful for detailed studies. For this, we have to use large, precise spectroscopic telescopes like the VLT.

The X-shooter instrument on the VLT.

Credit: ESO

CO: Indeed; the data from the Australian survey telescopes that we used only allow us to distinguish the quasars from stars in our own galaxy. In order to conduct “spectroscopic analysis” and really nail down the mass of these objects, we needed specialised equipment like the X-shooter spectrograph that splits light up into a spectrum of wavelengths.

Q. Can you explain in more detail how you used X-shooter to find the black hole’s mass?

CO: There are two key things you need to measure to estimate a black hole’s mass in this way. You need to figure out how fast the gas orbiting the black hole is moving and how far away from the black hole the gas is sitting. We measured the velocity of the gas by analysing a specific emission line on the spectrum of light coming from the quasar — the MgII doublet emission line. This singled out the light emitted by ionised magnesium near the black hole. The width of this line on a spectrum directly tells us how fast the gas is spinning around the central black hole.

FB: The X-shooter instrument has a wide enough coverage and resolution to give us an accurate measurement of both of these values — the velocity of the gas and its distance from the black hole — allowing us to weigh a black hole from billions of light years away.

Q. Why did you choose to use the VLT for your extra research? What benefit did this bring over other spectroscopic telescopes in Australia?

CO: Two main reasons. The first is that the VLT is much larger than any telescope in Australia. Secondly, the VLT’s X-shooter instrument is really unique in the world, and certainly unique in the facilities that Australia has access to. It is the best in the world for this particular kind of work, with very high wavelength coverage and high spectral resolution, making it perfect for our specific research where we wanted to clearly distinguish the magnesium lines we were after.

Light spectrum (energy at each wavelength) of the quasar SMSSJ2157 from the Keck/NIRES and VLT/X-shooter instruments. The MgII doublet emission line has restframe wavelengths of 2796 and 2803 Å. The grey line indicates the uncertainty at each wavelength.

Q. How was the strategic partnership between ESO and Australia beneficial to your research?

FB: I moved to ESO in 2018, the same year that Australia became a strategic partner with ESO. Teaming up with researchers from Australia was an opportunity for my research where I can contribute my expertise, observation experience and project design with the Australian team. A good combination and opportunity for us both.

CO: It’s great because it lets us do the science that we want to do. There’s really nothing local in Australia that can make the precise measurements that we wanted for this research. As a partner of ESO, we can use these world-leading instruments and facilities that also cover the southern sky. ESO instruments give us an opportunity to make fantastically detailed observations, including follow-ups to what we can observe using local telescopes. It’s more than what we could have asked for having access to these extra facilities.