Halo shapes constrained from a pure sample of central galaxies in KiDS-1000

We present measurements of f_h, the ratio of the aligned components of the projected halo and galaxy ellipticities, for a sample of central galaxies using weak gravitational lensing data from the Kilo-Degree Survey (KiDS). Using a lens galaxy shape estimation that is more sensitive to outer galaxy regions, we find f_h = 0.50 ± 0.20 for our full sample and f_h = 0.55 ± 0.19 for an intrinsically red sub-sample (that therefore has a higher stellar mass), rejecting the hypothesis that round halos and/or galaxies are not aligned with their parent halo at 2.5σ and 2.9σ, respectively. We quantify the 93.4% purity of our central galaxy sample using numerical simulations and overlapping spectroscopy from the Galaxy and Mass Assembly survey. This purity ensures that the interpretation of our measurements is not complicated by the presence of a significant fraction of satellite galaxies. Restricting our central galaxy ellipticity measurement to the inner isophotes, we find f_h = 0.34 ± 0.17 for our red sub-sample, suggesting that the outer galaxy regions are more aligned with their dark matter halos than the inner regions. Our results are in agreement with previous studies and suggest that lower mass halos are rounder and/or less aligned with their host galaxy than samples of more massive galaxies, studied in galaxy groups and clusters.