Dr Bron Reichardt Chu
Postdoctoral Research Associate
Centre for Extragalactic Astrophysics
Institute for Computational Cosmology
Department of Physics, Durham University
Research Interests
I use some of the largest optical telescopes on Earth to study the movement of gas through galaxies in order to understand how they have formed and evolved through cosmic time. The key question I am trying to answer is: how do stars affect the gas around them, and what does that mean for future generations of stars? To answer this question, I use three-dimensional data from Integral Field Spectrographs to trace outflowing gas from galaxies and relate it back to the underlying star formation processes. I also use high resolution data to pick out individual massive stars and star clusters, and investigate their impact on the surrounding gas. Although I am primarily an optical observer, I have an interest in combining multi-wavelength observations to get a clearer view of all phases of gas in galaxies. Recently, I have used my astronomical knowledge to build an image-processing pipeline that will be used as part of a project to locate and monitor satellites.
I did my PhD at the Centre for Astrophysics and Supercomputing (CAS) at Swinburne University of Technology working with Dr. Deanne Fisher. I did a Masters of Research in Astronomy at Macquarie University with Dr. Richard McDermid. I also worked on a couple of projects as an undergraduate with Dr. Lee Spitler.
My research focuses on star formation and its effect on the surrounding galaxy environment through feedback. Stellar feedback is the process where star formation self-regulates by introducing turbulence into the surrounding molecular cloud, slowing down any further star formation. Stellar feedback has been found to be necessary in simulations, otherwise they don't reproduce basic galaxy properties such as the galaxy mass function, galaxy sizes, and the Kennicutt-Schmidt Law. However, the physics behind stellar feedback is extremely difficult to constrain. One of the main challenges is that stellar feedback operates on a huge range of scales, from the immediate vicinity of individual stars all the way up to galaxy-wide winds.
I use star formation driven galaxy winds to constrain feedback models using data from the DUVET survey. DUVET is a survey of starbursting disk galaxies using observations taken with KCWI on the Keck Telescope in Hawaii. Star formation driven winds are an observational signature of the stellar feedback occurring within a galaxy. They are caused by the radiation from young massive stars and the explosions of supernovae in star-forming regions of galaxies. Gas and dust is pushed outwards, introducing turbulence and regulating the rate of star formation. If the outflow has a high enough velocity, it can escape the galaxy altogether and enrich the surrounding circumgalactic medium. It is especially important that we understand these processes in starbursting environments, since these are the type of environments where the majority of stars in the universe were created. However, the physical parameters regulating outflows are not yet fully understood.
Outflows cover the whole disk in a starbursting galaxy
I studied a face-on starbursting disk galaxy in our local Universe (IRAS 08339+6517) using observations taken with KCWI/Keck for the DUVET sample. Using KCWI means that we could get spatially resolved observations (about 500pc) and search for outflows across the entire face of the galaxy. To identify outflows I developed an automated fitting routine to systematically determine whether a double Gaussian fit was necessary using the Bayesian Information Criterion. Using this fitting routine (which I nicknamed Koffee), I found evidence for outflows in all spaxels within the half-light radius, and in 70% of spaxels within a radius containing 90% of the galaxy light.The relationship between the star formation rate surface density and the outflow velocity has historically been used to observationally discriminate between the subgrid models used to describe star formation driven outflows in simulations (e.g. Chen et al. 2010, Murray et al. 2011, Newman et al. 2012b). Two popular models in the literature are "energy-driven" and "momentum-driven" feedback models. Energy-driven models assume the outflowing gas and turbulence within the ISM are driven by mechanical energy from supernovae. The outflow velocity depends shallowly on the star formation activity within the galaxy (Chen et al. 2010, Kim et al. 2020). Momentum-driven feedback models drive outflows and turbulence through radiative pressure from young massive stars. These models have a steeper dependence of the outflow velocity on star formation activity (Murray et al. 2011). I found that for our galaxy (IRAS 08339+6517), the correlation between SFR surface density and outflow velocity had a shallow slope which matched the slopes we expect from energy-driven wind models. This is an important step towards fully characterising the dependence of star formation-driven feedback on the underlying stellar population.
For more information, check out my paper Reichardt Chu et al. 2022, MNRAS, 511, 5728.
MRes Thesis: Variations in the IMF
The stellar initial mass function (IMF) describes the original distribution of stellar masses in a stellar population. The IMF is critical in our understanding of galaxy evolution over cosmic time. My thesis presented preliminary results from the Fornax3D survey using the MUSE integral field spectrograph. Two techniques for finding the IMF were compared for the lenticular galaxy FCC167: full spectral fitting, and a more constrained approach focusing on a few key features. While the two methods agree reasonably well on radial variations for the age and abundance parameters in common, the derived IMF shape and its variation within the galaxy differ. You can find my Masters Thesis here.
Undergraduate Projects
As an undergraduate I worked on two separate projects with Dr. Lee Spitler. The first project involved cataloguing supermassive black holes at the centre of galaxies with a redshift greater than 3.0 using multi-wavelength catalogues. For the second project, I worked on the Huntsman Telephoto Eye (now the Huntsman Telescope) developing the initial pipeline to reduce data, combining exposures with different exposure times into a single image. During the project, I travelled to Siding Springs for an initial test observation run using the first telephoto lens acquired for the array.
Talks
Here are some videos of talks that I have given.
I won the Presentation Award for Best use of Slides for this talk.
About Me
I enjoy travelling and going on adventures with my husband. We've travelled together to New Zealand, Vietnam, Thailand and through parts of South America. And there's always more to see in my home country Australia! Some other things I enjoy doing are reading books, ice skating, singing, hiking, and doing sudokus.
Contact Me
Email me at: bronwyn.j.reichardtchu at durham.ac.uk