Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars

Abstract

We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile -under the assumption of a Chabrier stellar initial mass function -shows significant variation from galaxy to galaxy. Those with an outermost image beyond ∼ 10 kpc are very well fit by a projected NFW profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than e dm = 0.2, their stars can extend to e * > 0.2. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear (γ 0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.

Dark matter haloes of massive elliptical galaxies at z ∼ 0.2 are well described by the Navarro–Frenk–White profile

“…The lenses with large misalignment (∆PA> 12 deg) also have relatively larger (γ ext 0.05) external shear within the sample. This result is consistent with previous studies (Kochanek 2002;Ferreras et al 2008;Treu et al 2009;Gavazzi et al 2012;Sluse et al 2012;Bruderer et al 2016;Shajib et al 2018). The absence of systems with large misalignment angle and low external shear is consistent with the prediction of galaxy formation models that highly misaligned orbits in isolated galaxies are unstable and thus rare (e.g., Adams et al 2007;Debattista et al 2015).…”

The inner structure of early-type galaxies in the Illustris simulation

“…The inner dark matter slope is notably steeper than the predicted logarithmic slope of unity for an NFW profile at small radii. A mean slope of γ PL dm (0.5 R eff , R eff ) ∼ 1.49 is well consistent with both observations (e.g., Sonnenfeld et al 2012;Cappellari et al 2013;Oguri et al 2014;Bruderer et al 2016) and published simulations (e.g., Johansson et al 2012;Remus et al 2013). Fig.…”

“…This may also explain the systematically lower ellipticity ratios between the luminous and the total matter distributions of the simulated galaxies. However, it is also noteworthy to point out that recent lensing studies that have either applied non-parametric lens modelling (e.g., Bruderer et al 2016) or adopted novel techniques to extract galaxy morphology data from adaptive optics observations (e.g., Rusu et al 2016) found that the total matter distributions are systematically rounder than the stellar distributions in central regions of early-type galaxies, in agreement with our simulation results.…”

Quantifying Environmental and Line-of-sight Effects in Models of Strong Gravitational Lens Systems

“…In some cases it may be reasonable to put priors on the lens galaxy ellipticity and orientation based on the light profile (e.g., Bolton et al 2008), but in lensing there is not always a strong relation between the light and mass (Bruderer et al 2015, and references therein). Therefore we do not place any priors on the lens galaxy parameters.…”