My Master’s project was a 9-month investigation into the effects of magnetic fields on the properties of the circumgalactic medium (CGM). What is the CGM? This is the gas that envelopes galactic discs, it is thought to be a hot and diffuse medium which contains a high metal abundance (metals being any element heavier than hydrogen or helium). Observationally the CGM is difficult to study due to it being very diffuse additionally, it can sometimes be masked by outflows/inflows of gas in and out of the galaxy disc. Because of this difficulty cosmological simulations are a typical method for studying such a medium. Recent advancements in numerical methods and computing mean that simulating galaxy evolution is (to our understanding) very accurate, this has been shown many times. Cosmological simulations simulate the entire universe usually seen as the cosmic web (a universal web of gas and dust) and at the nodes of the web galaxies typically form and evolve. My project zoomed in on a selection of these nodes down to the halo scale of one or two galaxies. Halos are a virilised region which encapsulate galaxy discs and their CGM. These zoom-in regions were then restimulated at a higher resolution which increases the accuracy of the physics taking place in the halo.
Figure 1: 400 × 400 kpc Projections of Halo 6. Presenting gas Temperature (K) in the first column, hydrogen number density (cm-3) in the second column and metallicity, Z/Zsolar in the third column. The first row shows projections of Halo 6 in the absence of magnetic fields, B = 0. The second row show projections in the presence of magnetic fields, B>0. These projections highlight that magnetic fields have an effect on the properties of the halo gas.
My project had access to three different galaxy halos, halo 6, 12 and L8. Each was previously simulated once with magnetic fields and once without. In my investigation, I examined the effects of magnetic fields on the gas temperature, hydrogen number-density, metallicity, and the soft X-ray luminosity. These properties were selected as each are thought to directly affect the X-ray luminosity furthermore, the hot and dense regime of gas is thought to probe X-ray emission in the CGM. Projections of halo 6 can be seen in the image above (figure 1) for each gas property bar the X-ray emission since this was not examined until late in the project.
Magnetic fields are omnipresent in galaxies and although their effects are largely unknown observationally, they are thought to be dynamically important. Simulations allow us to see what galaxies look like with and without magnetic fields while in the real universe we don’t have this magic switch to turn them on and off. This investigation was building upon previous work done by Freeke van de Voort who wrote a paper on this topic, investigating the effects of magnetic fields on a range of simulation types and galaxies however, their investigation did not include the examination of the soft X-ray luminosity, this was new research.
To see my final written thesis on this project see the GitHub link at the top of the page. To get a taste for some of the plots that drove my investigation, see the figures below figures 2 & 3, which include figure captions to explain what they are and briefly what they show.
Figure 2: Radial profiles of each halo (distinguished by each column), for the median temperature (K), hydrogen number density, nH (cm-3 ), and metallicity, Z/Zsolar (distinguished by each row) out to the halo virial radius, R200c ~ 200 kpc. The median is calculated using 10 kpc bins out to 200 kpc, starting at 0-10 kpc. Halos with/without magnetic fields are presented by solid and dashed lines respectively. The radial profiles include the central disc and satellites usually seen by sharp rises or dips. The last column shows the mean of these properties for all three halos. In the presence of magnetic fields, the CGM is warmer, and the hydrogen-number density is more diffuse (however, the difference is small). Additionally, the abundance of metals is lower in the presence of magnetic fields.
Figure 3: Lx/SFR (ergs-1/Msolar yr-1) versus M*/SFR (Msolar/Msolaryr-1) for observational and simulated halos. The black circles with error bars are observational data points from the Chandra survey analysed in Wang et al. 2016. The red, green, and blue triangles are the simulated halos, 6, 12 and L8 respectively. The black dashed line is a loglog linear regression “best-fit” line. The left-hand panel contains the simulated halos in the presence of magnetic fields. The right-hand panel contains the simulated halos in the absence of magnetic fields.