Tightening weak lensing constraints on the ellipticity of galaxy-scale
  dark matter haloes
  Cosmological simulations predict that galaxies are embedded into triaxial
dark matter haloes, which appear approximately elliptical in projection. Weak
gravitational lensing allows us to constrain these halo shapes and thereby test
the nature of dark matter. Weak lensing has already provided robust detections
of the signature of halo flattening at the mass scales of groups and clusters,
whereas results for galaxies have been somewhat inconclusive. Here we combine
data from five surveys (NGVSLenS, KiDS/KV450, CFHTLenS, CS82, and RCSLenS) in
order to tighten observational constraints on galaxy-scale halo ellipticity for
photometrically selected lens samples. We constrain $f_\rm{h}$, the average
ratio between the aligned component of the halo ellipticity and the ellipticity
of the light distribution, finding $f_\rm{h}=0.303^{+0.080}_{-0.079}$ for red
lenses and $f_\rm{h}=0.217^{+0.160}_{-0.159}$ for blue lenses when assuming
elliptical NFW density profiles and a linear scaling between halo ellipticity
and galaxy ellipticity. Our constraints for red galaxies constitute the
currently most significant ($3.8\sigma$) systematics-corrected detection of the
signature of halo flattening at the mass scale of galaxies. Our results are in
good agreement with expectations from the Millennium Simulation that apply the
same analysis scheme and incorporate models for galaxy-halo misalignment.
Assuming these misalignment models and the analysis assumptions stated above
are correct, our measurements imply an average dark matter halo ellipticity for
the studied red galaxy samples of $\langle|\epsilon_\rm{h}|\rangle=0.174\pm
0.046$, where $|\epsilon_{h}|=(1-q)/(1+q)$ relates to the ratio $q=b/a$ of the
minor and major axes of the projected mass distribution. Similar measurements
based on larger upcoming weak lensing data sets can help to calibrate models
for intrinsic galaxy alignments. [abridged]
