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

TL;DR

This study measures the alignment between galaxy shapes and their dark matter halos using weak lensing data from KiDS, finding significant alignment especially in outer galaxy regions and for red, higher-mass galaxies.

Contribution

It provides new measurements of halo-galaxy alignment ratios for a pure central galaxy sample, emphasizing the importance of outer galaxy regions in alignment studies.

Findings

Approximately 50% halo-galaxy alignment for the full sample.

Red, higher-mass galaxies show stronger alignment with halos.

Outer galaxy regions are more aligned with dark matter halos than inner regions.

Abstract

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 $f_{\rm h}=0.50\pm0.20$ for our full sample and $f_{\rm h}=0.55\pm0.19$ for an intrinsically red (and therefore higher stellar-mass) sub-sample, rejecting the hypothesis of round halos and/or galaxies being un-aligned with their parent halo at $2.5\sigma$ and $2.9\sigma$, 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 $f_{\rm h}=0.34\pm0.17$ for our red sub-sample, suggesting that the outer galaxy regions are more aligned with their dark matter halos compared to 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.