My research projects Are angular momenta of disc galaxies oriented randomly? (PhD project)

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Background Nearly 40% (numbers) of the nearby galaxy popluation are disc galaxies. These disc galaxies are supported against gravity by ordered rotation of their stars, gas and dust. Understanding how these galaxies acquired their angular momenta is therefore an important piece in the puzzle of galaxy formation. Our currently best theoretical picture - the tidal-torque theory - explains this acquisition as an environmental effect. The forming proto-galaxy is not isolated in the universe, but embedded in the large-scale environment. Loosely speaking, this environment imprints the angular momentum onto the proto-galaxy via differential tidal foces. If disc galaxies obtained their angular momenta from their environment, then neighbouring disc galaxies should have resided in a very similar environments, thereby also feeling very similar tidal forces. Consequently, neighbouring disc galaxies should have similar angular momenta, that are not completely random! What are the problems? There are several problems involved: We need to be able to identify disc galaxies. (See my other projects about parametrisation and classification of galaxy morphologies.) Several important error sources need to be taken into account, e.g., classification uncertainties and redshift errors. What did I do? I estimated the autocorrelation function of angular-momenta of Scd galaxies (disc galaxies with no prominent bulge component). These galaxies were selected from the morphological classifications of Huertas-Company et al. (2011). I accounted for classification uncertainties analytically and numerically for other errors, e.g., in redshift estimates. This way, I obtained a marginal estimate of the autocorrelation function, which is statistically independent from the input data, i.e., it does not inherently assume that the observed input data were true values without errors. Given this marginal autocorrelation function, I tried to constrain free parameters in the prediction from the tidal-torque theory. Results I did not find statistically significant evidence that such autocorrelations do exist. Hence, I could not confirm the theoretical prediction. I showed that classification uncertainties and other errors, e.g., in redshift estimates, have to be taken into account. Otherwise, one obtains overly optimistic results which then pretend to confirm the theoretical prediction. Even though I did not find evidence for autocorrelations, I could nevertheless fit predicted autocorrelation functions to my data. The results are naturally far from decisive, but they loosely confirm the theoretically predicted autocorrelation length of 1Mpc. I concluded that more data is required in order to settle this question.