Research
At the centres of galaxies is a supermassive black hole. In some of these galaxies, the black hole is deemed “active” as it emits large amounts of radiation beyond in thin streams called “jets”. These jets then blossom out into large mushroom clouds called lobes. The entire structure is called a radio galaxy and can be hundreds, thousands or even millions of lightyears across.
Image credit: Hercules A from NASA/ESA/NRAO
Some radio galaxies are order of magnitude smaller than typical radio galaxies. It is unclear whether these smaller radio galaxies are babies that haven’t had time to grow to the same size, or are frustrated, confined by a dense cloud of gas and dust. My PhD research aims to answer this question using a number of telescopes to observe these galaxies in several different “radio colours” and watch them varying.
Using the Murchison Widefield Array (MWA), I have conducted a survey of over 21,000 radio galaxies searching for variability across multiple radio colours. The MWA has a large fractional bandwidth allowing for measurements of the brightness of galaxies at multiple different radio colours.
My first paper used the fractional bandwidth of the MWA to identify over 300 radio galaxies that showed variability on a timescale of one year. In particular, it identified the “baby” radio galaxies as particularly variable compared to the rest of typical radio galaxies. The variability we detected, like GLEAM J223933-451414 pictured right, is unexpected on timescales of years. This suggests either our current models of radio galaxies is incomplete or we have observed a new behaviour for these galaxies.
In my first paper, we hypothesised several possibilities that could explain the variability we detected. The large number of variable galaxies that showed a simple increase or decrease in brightness across all radio colours could be explained due to a phenomena called “scintillation”. Scintillation is the process that causes stars to appear to twinkle in our night sky. The change in brightness of these galaxies was consistent with the light from the galaxy varying as it passes through our Milky Way galaxy before reaching our telescopes.
The galaxies that varied by different amount for different radio colours (or changed radio colours), cannot be explained by the effects of scintillation. For these galaxies, we concluded it was likely we are observing “down the barrel of the gun”, i.e. the streams of radiation coming from the black hole itself is directed right towards us on Earth. These galaxies are called “blazars” and can show rapid variability in various ways that could explain what we see. However, many of them were not known blazars.
In this paper, we concluded that the spectral information (or multiple radio colours) was a powerful tool for identifying variability and classifying how it is varying with colour. With more data, spectral variability could be used to infer detailed information of the galaxy itself and the intervening media that is otherwise unattainable.
The spectral energy distribution of radio galaxy GLEAM J223933-451414, identified in my first paper as a variable galaxy with changing colour.
After my first paper showed how valuable spectral variability, my second paper aimed to improve on the initial results. Combining simultaneous observations of the MWA and the Australia Telescope Compact Array (ATCA) over the course of 2020, I obtained broadband spectral information over several epochs.
These observations found it is uncommon for the variable galaxies identified in my first paper to show any variability at gigahertz frequencies on timescales of months. Such variability would be consistent with the core of the galaxy (or the accretion disc around the active black hole) itself varying. The lack of variability detected at these frequencies suggests it is unlikely the variability is intrinsic to the galaxy itself.
Conversely, the MWA observations identified variability in almost all galaxies monitored in this survey. Furthermore, each galaxy showed unique types of variability. Many showed gradual increases in brightness for all radio colours (as with the first paper), and we conclude this is consistent with scintillation again. However, some, like GLEAM J015445-232950 pictured left, showed changes in their spectral shape (or variations depending on radio colour). We calculate this is most likely due to changes in the optical depth of the galaxy. This is consistent with the black hole being surrounded by a dense inhomogeneous cloud of gas and dust and over time we are looking through different clumps of the cloud causing changes in the radio colours.
In this paper, we conclude that the spectral variability is able to determine the absorption mechanism of radio galaxies based on the different variability at different radio frequencies. We used the observed scintillation to calculate estimates on the source sizes and structures of the radio galaxies, providing high resolution details otherwise only attainable via long baseline interferometry, an expensive observing strategy. We also use the spectral variability to determine whether several radio galaxies are young or frustrated and estimate that spectral variability is the most reliable method for determining this for a large population so far.
The spectral energy distribution of radio galaxy GLEAM J015445-232950, identified in my second paper as variable due to a clumpy dense cloud surrounding it.