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PhD student under the supervision of Barbara Catinella, Chris Power and Luca Cortese studying the neutral hydrogen (HI) gas in galaxies.


My research

The gas reservoirs of galaxies are their fundamental building blocks, so understanding them is essential to completing out picture of galaxy evolution. There are only ~500 observations of the HI in galaxies which resolve the distribution of the gas. The majority of data are spatially unresolved, in the form of the global HI emission line spectrum, which contain information about both the distribution and the motions of the gas.

The main goal of my PhD is to investigate the gas reservoirs of galaxies using these spectra by quantifying their smaller scale structure to trace disturbances, and relate this back to the properties of the galaxy.

Recently, I have been measuring the asymmetry of global HI spectra, as shown in Fig. 1, which quantifies the amount of excess emission on one side of the spectrum. The sample I have been using is the extended GALEX Arecibo SDSS Survey, or xGASS, a stellar mass-selected sample of galaxies in the local Universe with some of the deepest HI observations of a statistical sample of galaxies.

Figure 1. Visualisation of the asymmetry measurement which is calculated by taking the ratio of the green and orange shaded regions.


However, this measurement can be heavily affected by measurement noise to the extent that asymmetries can be created when they were not originally present. Fig. 2 shows the amount of scatter in a distribution of asymmetries due to noise for three different values of spectrum signal-to-noise (S/N). As the quality of the observation decreases, the uncertainty in an asymmetry measurement increases substantially. This means that if we try to compare the asymmetries in two samples we need to consider their S/N distributions: if one sample is very low S/N it will like be inferred to be more asymmetric than it actually is.


Figure 2. The asymmetry distribution of an initially symmetric spectrum with added measurement noise to reach three different S/N values.


With an understanding of how noise affects asymmetry we can compare how strong the asymmetries in xGASS are compared to what is expceted for noise. We find that xGASS shows higher asymmetry than what can be attributed to measurement noise. The gas reservoirs of galaxies are, on average, not regular or undisturbed systems.





Prior to my PhD, I completed both my Bachelor of Science degree with majors in Physics and Mathematics, and my Master of Physics in Astronomy and Astrophysics with Distinction, have been completed through The University of Western Australia and ICRAR. I have research experience in theoretical studies, between my Bachelors and Masters degrees I analysed simulations of dwarf galaxy mergers, however the majority of my experience is observational. My Masters thesis used Hubble Space Telescope images of resolved stellar populations to constrain the stellar initial mass function, to investigate whether all galaxies can form the biggest stars.

First author publications:

Watts, A.B., Power, C., Catinella, B., Cortese, L.,  Stevens, A.R.H., (2020), MNRAS, “Global H I asymmetries in IllustrisTNG: a diversity of physical processes disturb the cold gas in galaxies

Watts, A.B., Catinella, B., Cortese, L., Power, C., (2020), MNRAS, 492, p3672 – 3684 “xGASS: Robust quantification of asymmetries in global HI spectra and their relationship to environmental processes”

Watts, A.B., Meurer, G.R., Lagos, C.D.P., Bruzzese, S.M., Kroupa, P., & Jerabkova, T., (2018), MNRAS, 477, p5554-5567 “Star formation in the outskirts of DDO154: a top-light IMF in a nearly dormant disc”

Watts, A., & Bekki, K., (2016), MNRAS, 462, p3314-3324 “Formation and evolution of blue compact dwarfs: the origin of their steep rotation curves”

Other work:

Bruzzese, S. M.; Thilker, David A.; Meurer, G. R.; Bianchi, Luciana; Watts, A. B.; Ferguson, A. M. N.; Gil de Paz, A.; Madore, B.; Martin, D. Christopher; Rich, R. Michael, (2019), MNRAS, 491, p2366 – 2390  “The initial mass function in the extended ultraviolet disc of M83”



About Me outside the office

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