Non-Halo Structures and their Effects on Gravitational Lensing
  Anomalies in the flux-ratios of the images of quadruply-lensed quasars have
been used to constrain the nature of dark matter. Assuming these lensing
perturbations are caused by dark matter haloes, it is currently possible to
constrain the mass of a hypothetical Warm Dark Matter (WDM) particle to be
$m_\chi > 5.2$ keV. However, the assumption that perturbations are only caused
by DM haloes might not be correct as other structures, such as filaments and
pancakes, exist and make up a significant fraction of the mass in the universe,
ranging between 5$\%$ -- 50$\%$ depending on the dark matter model. Using novel
fragmentation-free simulations of 1 and 3keV WDM cosmologies we study these
"non-halo" structures and estimate their impact on flux-ratio observations. We
find that these structures display sharp density gradients with short
correlation lengths, and can contribute more to the lensing signal than all
haloes up to the half-mode mass combined, thus reducing the differences
expected among WDM models. We estimate that non-halo structures can be the
dominant cause of line-of-sight flux-ratio anomalies in very warm, but already
excluded, $m_x \sim 1 \rm{keV}$ scenarios. For colder cases $m_x \gtrsim 3
\rm{keV}$, we estimate that non-haloes can contribute about $5 - 10\%$ of the
total flux-ratio signal.
