Allow me to explain...

As I mentioned on my home page, my research is all about quasars, and how to work out how far away they are.

Quasars are galaxies that have in their centre an actively feeding black hole. Our own Milky Way Galaxy has a supermassive black hole (SMBH to its friends) at its centre, as do most other large galaxies, but while we can see stars orbiting the Milky Way's SMBH, they're not falling into it. A quasar's SMBH is actively feeding on the gas and dust around it, and as this matter falls in, it forms a structure called an accretion disk; roughly analogous to the vortex of water going down a plug hole.

Artist's concept of a black hole and accretion disk; https://www.nasa.gov/mission_pages/nustar/multimedia/pia16695.html

As the matter in the accretion disk swirls around, it heats up to immense temperatures, causing it to emit light, which is one of the things that hot things do. If you need convincing of this, turn a stove element on to high.

We see this light as radiation being emitted from the accretion disk (not from the black hole itself, which famously don't emit anything at all). By the time the light reaches us here on Earth, it's travelled huge distances, and passed through all manner of things in space, such as galaxy clusters, clouds of gas (mostly hydrogen), and dark matter. By examining the light, we can get lots of information about the things that it's passed through, a bit like being able to tell how much dust there is in the air by looking at mountains on the horizon. The first step to getting at this information is figuring out how far away the quasar is.

Since we can't directly measure the distance to things in space, we have to use a proxy for distance; this is called an object's redshift. We use one of two basic methods to measure redshift:

• spectroscopy, which is accurate and reliable, but is very slow and requires expensive, specialised equipment, and

• photometry, which is much faster and cheaper, but not as accurate.

In spectroscopy, we split an object's light up into a spectrum, the same way a prism projects a rainbow onto a wall. By looking at the shift in wavelength of each colour of light in the spectrum (i.e. its redshift), we can work out how far away the object was when it emitted the light:

This is the most reliable method we have of measuring the redshift, and therefore distance, of distant things in space. For 'close' objects, like planets and nearby stars, we use a different idea entirely for measuring distance. Unfortunately, measuring the spectrum for an object can be a slow process, taking hours or even days to collect enough light from dim, distant objects.

On the other hand, a photometric observation simply records the total brightness for each colour. This also means that we can collect data on every object in the image.

The thing is, our modern telescopes can take images of millions of galaxies at the same time. Gathering spectroscopic data on all of these objects will be far too time-consuming to be practicable, so photometry is a much more realistic method. Besides, not all telescopes have the equipment needed to measure a spectrum in the first place.

I've already figured out a more reliable way to use photometry to measure the redshift of a sample of galaxies (the University saw fit to give me an MSc for that work), and now my aim is to extend this into the radio part of the spectrum.