Journey to the Centre of a Galaxy: Active Galactic Nuclei

At the centre of all galaxies, there are giant black holes. These objects, known as active galactic nuclei, are some of the most powerful objects in the Universe. Sam Grafton-Waters, a third-year PhD student at MSSL, who studies these, tells us more.

Active galactic nuclei (AGN) are some of the most powerful extragalactic sources in the universe. They are supermassive black holes (SMBHs), with masses equal to 1 million to 1 billion times the mass of the sun, believed to be at the heart of every galaxy. AGN are `active’ because matter, in the form of a hot and energetic disk, known as an accretion disk, circles and falls onto the SMBH. Collisions between particles and fast moving material cause the disk to radiate UV and optical photons as it heats up. X-rays are believed to be produced in the hot electron corona situated in the vicinity of the black hole, most likely above it, possibly associated with the accretion disk. This is illustrated below. In addition, infrared radiation is emitted from the dusty torus that surrounds the central SMBH like a doughnut, making AGN extremely bright across the electromagnetic spectrum.

Artists impression of a supermassive black hole, circled by an accretion disk, with a bright electron corona above it, which is believed to be the origin of the X-rays. In some AGN, a powerful jet is present, as displayed by the blue twister. AGN are extremely complex and powerful objects that require multiple observations and telescopes to fully understand their underlying properties. Credit: MIT Kavli Institute for Astrophysics and Space Research.

Studying AGN and the host galaxies is fundamental in understanding the origins of SMBHs, the evolution of galaxy mergers and how black holes and galaxies co-evolve with each other. Studying AGN shows a relationship between the AGN’s black hole mass and the speed of the stars within the host galaxy, which is thought to be caused by feedback. Feedback is when matter from the host galaxy accretes onto the black hole, while the black hole also ejects matter, in the form of winds or jets, away into the inner galaxy. However, in this research field, there are many open questions. We do not know where the outflowing wind originates from and what drives it away from the central SMBH. We also do not know exaclty how these winds interact with the host galaxy in which the AGN is situated, in addition to how powerful or energetic the wind mechanism is to have an effect on the inner galaxy. Finally, the locations of the clouds within the wind and how far they are from the central SMBH are not fully understood. Investigating winds on a small scale allows us to infer information and understanding of the co-evolution on larger scales.

My name is Sam Grafton-Waters and I am a third year PhD student, working with my supervisors Graziella Branduardi-Raymont and Mat Page. Using high resolution X-ray spectra from observations taken by ESA’s XMM-Newton telescope (Which has just celebrated its 20th anniversary in December 2019), I investigate the outflowing winds that have been ejected away from the SMBH within individual AGN. I study these AGN with the reflection grating spectrometer (RGS) on board XMM-Newton (in addition to the EPIC-pn instrument), which shows absorption (troughs) or emission (peaks) features at particular energies or wavelengths that are signatures of a particular ion. An example RGS spectrum is shown below, displaying the emission and absorption features from a specific ion at a specific wavelength – as labelled. The height (or depth), broadness, and energy shift of these lines are extremely useful tools in characterising the plasma regions we are observing. The aim of my work is to determine the properties of ionised plasma that is outflowing away from the SMBH, in order to determine the number of clouds, how fast they are moving, how far they are from the SMBH, and how energetic or ionised the plasma is.

Although the Event Horizon Telescope was the first ever telescope to capture an image of a black hole, we are unable to simply take a picture of AGN and then understand all the processes going on. These objects are extremely far away, for a start, and we do not have the telescope power to decipher between each individual part in any great detail. This is why XMM-Newton and the RGS spectra are so vital in retrieving the information.

We can infer many properties of the plasma from fitting models to the high resolution spectra. When the model traces the shape of the features, we can determine the properties of the wind, by measuring certain parameters, such as how ionised the plasma is and how much material is in the wind. In most AGN, the outflowing wind travels at speeds of around 100 to a few 1000 kilometres per second. With these parameters, we can determine how many clouds there are within the wind and estimate the locations of the plasma and how much energy or mass is being carried by the wind. Working with a collaboration of astronomers, my first PhD project was to investigate the active galactic nucleus named NGC 7469, which we studied in optical, UV and X-ray photon energies to tie together an overall picture of this AGN. My particular work focused on analysing the RGS spectrum, which is shown below: the data is shown in black and the best fit model is displayed as the red line. Overall from this campaign, we identified between three and six absorbing clouds, known as the `warm absorber’, and two to three emitting plasma regions. The distances of the warm absorber regions are found between 2 and 80 pc (parsec: 1 pc = 30 million billion meters) and the emitting plasma regions are estimated to be less than 2.6 pc, from the supermassive black hole. Furthermore, we found that the X-ray warm absorber has out velocities consistent with the UV warm absorber of around a few hundred to a few thousand kilometres per second, and similar ionisations, suggesting that they are part of the same outflowing wind. For more details, see Papers 1, 2, 3, and 4.

Here is an example of an AGN RGS spectrum, in this case NGC 7469. Here you can clearly see peaks which are called emission lines, and troughs known as absorption lines. We then infer the properties of the plasma regions that produce these lines by fitting a model to the data (shown in red). This model contains certain parameters that we can use to infer the properties of what we are observing. The most important ion lines are labelled in blue. Credit: Grafton-Waters et al. 2019

Now I am looking at the AGN called NCG 1068. This time, I am comparing observations taken in 2000 and 2014 by XMM-Newton, analysing both RGS and EPIC-PN data. In the spectra of NGC 1068, we only see emission features because we are observing this AGN from the side, whereby the dusty torus surrounding the black hole blocks the X-ray source from our view. This is in contrast to the first AGN I studied, NGC 7469, where we observe this AGN from `face-on’, meaning we can see through the middle of the torus, essentially looking right at the black hole. Therefore, the material in the outowing wind blocks out some of the X-rays, which we detect as troughs – absorption lines. In NGC 1068, the X-rays that are absorbed by the plasma are re-emitted towards us, meaning we do not see any absorption because the material is not blocking the X-rays in the direction we are looking at – as shown below. After our analysis, we find no change in the plasma properties between 2000 and 2014, and we are currently comparing methods to estimate the distances of the plasmas from the SMBH.

Cartoon of a cut through of an AGN: central black hole surrounded by the accretion disk, an X-ray corona, the torus, and outflowing wind. For NGC 7469 (a type 1 AGN) we see absorption lines because the X-rays (red arrows) from the corona are blocked by the outflowing wind (green clouds). For NGC 1068 the torus completely blocks out the central black hole from view (type 2 AGN), and we do not see any absorption. So instead, these absorbed X-rays are re-emitted (purple arrows) in the direction 90 degrees to the direction of absorption, therefore we see only emission lines.

In conclusion, modelling the high resolution spectra of active galactic nuclei allow us to infer the properties of the outflowing wind. With this, we can use model parameters to estimate the distances of the clouds from the central supermassive black hole, and determine the power of these winds. This information can help us understand, on a larger scale, how these winds interact with the galaxy. However, given that the distances are so vast, we require more AGN to be investigated, so that we can build a picture and link all the results together to obtain better theory and knowledge of AGN.

Featured Image Credit: Caltech-Nustar (NASA)


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