Search

The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power-law plus 'external shear', which notionally accounts for neighbouring galaxies and cosmic shear along our line of sight. A small amount of external shear could come from these sources, but we show that the vast majority does not. Except in a handful of rare systems, the best-fitting values do not correlate with independent measurements of line-of-sight shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit external shear is aligned with the major or minor axis of 88 per cent of lens galaxies; and the amplitude of the external shear increases if that galaxy is discy. We conclude that 'external shear' attached to a power-law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models. [ABSTRACT FROM AUTHOR]

The cold dark matter (DM) model predicts that every galaxy contains thousands of DM subhaloes; almost all other DM models include a physical process that smooths away the subhaloes. The subhaloes are invisible, but could be detected via strong gravitational lensing, if they lie on the line of sight to a multiply imaged background source, and perturb its apparent shape. We present a predominantly automated strong lens analysis framework, and scan for DM subhaloes in Hubble Space Telescope imaging of 54 strong lenses. We identify five DM subhalo candidates, including two especially compelling candidates (one previously known in SLACS0946 + 1006) where a subhalo is favoured after all of our tests for systematics. We find that the detectability of subhaloes depends upon the assumed parametric form for the lens galaxy's mass distribution, especially its degree of azimuthal freedom. Using separate components for DM and stellar mass reveals two DM subhalo candidates and removes four false positives compared to the single power-law mass model that is common in the literature. We identify 45 lenses without substructures, the number of which is key to statistical tests able to rule out models of, for example, warm or self-interacting DM. Our full analysis results are available at https://github.com/Jammy2211/autolens_subhalo. [ABSTRACT FROM AUTHOR]

Observations suggest that the dark matter and stars in early-type galaxies 'conspire' to produce a surprisingly simple distribution of total mass, ρ(r) ∝ ρ−γ, with γ ≈ 2. We measure the distribution of mass in 48 early-type galaxies that gravitationally lens a resolved background source. By fitting the source light in every pixel of images from the Hubble Space Telescope , we find a mean |$\langle \gamma \rangle =2.075_{-0.024}^{+0.023}$| with an intrinsic scatter between galaxies of |$\sigma _\gamma =0.172^{+0.022}_{-0.032}$| for the overall sample. This is consistent with and has similar precision to traditional techniques that employ spectroscopic observations to supplement lensing with mass estimates from stellar dynamics. Comparing measurements of γ for individual lenses using both techniques, we find a statistically insignificant correlation of |$-0.150^{+0.223}_{-0.217}$| between the two, indicating a lack of statistical power or deviations from a power-law density in certain lenses. At fixed surface mass density, we measure a redshift dependence, |$\partial \langle \gamma \rangle /\partial z=0.345^{+0.322}_{-0.296}$|⁠ , that is consistent with traditional techniques for the same sample of Sloan Lens ACS and GALaxy-Lyα EmitteR sYstems (GALLERY) lenses. Interestingly, the consistency breaks down when we measure the dependence of γ on the surface mass density of a lens galaxy. We argue that this is tentative evidence for an inflection point in the total mass-density profile at a few times the galaxy effective radius – breaking the conspiracy. [ABSTRACT FROM AUTHOR]

A fundamental prediction of the cold dark matter (CDM) model of structure formation is the existence of a vast population of dark matter haloes extending to subsolar masses. By contrast, other dark matter models, such as a warm thermal relic (WDM), predict a cutoff in the mass function at a mass which, for popular models, lies approximately between 107 and |$10^{10}\, {\rm M}_\odot$|⁠. We use mock observations to demonstrate the viability of a forward modelling approach to extract information about low-mass dark haloes lying along the line of sight to galaxy–galaxy strong lenses. This can be used to constrain the mass of a thermal relic dark matter particle, m DM. With 50 strong lenses at Hubble Space Telescope resolution and a maximum pixel signal-to-noise ratio of ∼50, the expected median 2σ constraint for a CDM-like model (with a halo mass cutoff at |$10^{7}\, {\rm M}_\odot$|⁠) is |$m_\mathrm{DM} \gt 4.10 \, \mathrm{keV}$| (50 per cent chance of constraining m DM to be better than 4.10 keV). If, however, the dark matter is a warm particle of |$m_\mathrm{DM}=2.2 \, \mathrm{keV}$|⁠ , our 'approximate Bayesian computation' method would result in a median estimate of m DM between 1.43 and 3.21 keV. Our method can be extended to the large samples of strong lenses that will be observed by future telescopes and could potentially rule out the standard CDM model of cosmogony. To aid future survey design, we quantify how these constraints will depend on data quality (spatial resolution and integration time) as well as on the lensing geometry (source and lens redshifts). [ABSTRACT FROM AUTHOR]