Constraints on warm dark matter from weak lensing in anomalous quadruple lenses

We investigate the weak lensing effect by line-of-sight structures with a surface mass density of less than or similar to 10(8) M-circle dot arcsec(-2) in QSO-galaxy quadruple lens systems. Using high-resolution N-body simulations in warm dark matter (WDM) models and observed four quadruple lenses that show anomalies in the flux ratios, we obtain constraints on the mass of thermal WDM, m(WDM) >= 1.3 keV (95 per cent CL) assuming that the density of the primary lens is described by a singular isothermal ellipsoid (SIE). The obtained constraint is consistent with those from Lyman alpha forests and the number counts of high-redshift galaxies at z > 4. Our results show that WDM with a free-streaming comoving wavenumber k(fs) <= 27 h Mpc(-1) is disfavoured as the major component of cosmological density at redshifts 0.5 less than or similar to z less than or similar to 4 provided that the SIE models describe the gravitational potentials of the primary lenses correctly.