A forward-modelling method to infer the dark matter particle mass from
  strong gravitational lenses
  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 $10^7$ 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, $m_\mathrm{DM}$. With 50 strong
lenses at Hubble Space Telescope resolution and a maximum pixel signal-to-noise
ratio of $\sim50$, the expected median 2$\sigma$ constraint for a CDM-like
model (with a halo mass cutoff at $10^{7}~{\rm M}_\odot$) is $m_\mathrm{DM} >
4.10 \, \mathrm{keV}$ (50% chance of constraining $m_{\rm DM}$ 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 $m_\mathrm{DM}$ 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).
