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We present updated constraints on the free-streaming of warm dark matter (WDM) particles derived from an analysis of the Lya flux power spectrum measured from high-resolution spectra of 25 z > 4 quasars obtained with the Keck High Resolution Echelle Spectrometer (HIRES) and the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We utilize a new suite of high-resolution hydrodynamical simulations that explore WDM masses of 1, 2 and 4 keV (assuming the WDM consists of thermal relics), along with different physically motivated thermal histories. We carefully address different sources of systematic error that may affect our final results and perform an analysis of the Lya flux power with conservative error estimates. By using a method that samples the multi-dimensional astrophysical and cosmological parameter space, we obtain a lower limit mwdm > 3.3 keV (2sigma) for warm dark matter particles in the form of early decoupled thermal relics. Adding the Sloan Digital Sky Survey (SDSS) Lya flux power spectrum does not improve this limit. Thermal relics of masses 1 keV, 2 keV and 2.5 keV are disfavoured by the data at about the 9sigma, 4sigma and 3sigma C.L., respectively. Our analysis disfavours WDM models where there is a suppression in the linear matter power spectrum at (non-linear) scales corresponding to k=10h/Mpc which deviates more than 10% from a LCDM model. Given this limit, the corresponding \"free-streaming mass\" below which the mass function may be suppressed is 2x10^8 Msun/h. There is thus very little room for a contribution of the free-streaming of WDM to the solution of what has been termed the small scale crisis of cold dark matter.

In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at \\(\\langle z \\rangle \\simeq 2.8\\) through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the CIV forest, our deep spectrum is sensitive at \\(3\\,\\sigma\\) to lines with column density down to \\(\\log N_{\\rm CIV} \\simeq 11.4\\) and in 60 per cent of the considered redshift range down to \\(\\simeq11.1\\). In our sample, all HI lines with \\(\\log N_{\\rm HI} \\ge 14.8\\) show an associated CIV absorption. In the range \\(14.0 \\le \\log N_{\\rm HI} <14.8\\), 43 per cent of HI lines has an associated CIV absorption. At \\(\\log N_{\\rm HI} < 14.0\\), the detection rates drop to \\(<10\\) per cent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range \\(\\log N_{\\rm HI} \\ge 14\\), we observe a fraction of HI lines with detected CIV a factor of 2 larger than the fraction of HI lines lying in the circum-galactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter haloes with \\(\\langle M\\rangle \\sim10^{12}\\) M\\(_{\\odot}\\). The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity \\(\\log Z/Z_{\\odot} \\ge -3\\) should be of the order of \\(\\sim 10-13\\) percent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources.