Abstract
We present new measurements of the free-streaming of warm dark matter (WDM) from Lyman-$\alpha$ flux-power spectra. We use data from the medium resolution, intermediate redshift XQ-100 sample observed with the X-shooter spectrograph ($z=3 - 4.2$) and the high-resolution, high-redshift sample used in Viel et al. (2013) obtained with the HIRES/MIKE spectrographs ($z=4.2 - 5.4$). Based on further improved modelling of the dependence of the Lyman-$\alpha$ flux-power spectrum on the free-streaming of dark matter, cosmological parameters, as well as the thermal history of the intergalactic medium (IGM) with hydrodynamical simulations, we obtain the following limits, expressed as the equivalent mass of thermal relic WDM particles. The XQ-100 flux power spectrum alone gives a lower limit of 1.4 keV, the re-analysis of the HIRES/MIKE sample gives 4.1 keV while the combined analysis gives our best and significantly strengthened lower limit of 5.3 keV (all 2$\sigma$ C.L.). The further improvement in the joint analysis is partly due to the fact that the two data sets have different degeneracies between astrophysical and cosmological parameters that are broken when the data sets are combined, and more importantly on chosen priors on the thermal evolution. These results all assume that the temperature evolution of the IGM can be modelled as a power law in redshift. Allowing for a non-smooth evolution of the temperature of the IGM with sudden temperature changes of up to 5000K reduces the lower limit for the combined analysis to 3.5 keV. A WDM with smaller thermal relic masses would require, however, a sudden temperature jump of $5000\,K$ or more in the narrow redshift interval $z=4.6-4.8$, in disagreement with observations of the thermal history based on high-resolution resolution Lyman-$\alpha$ forest data and expectations for photo-heating and cooling in the low density IGM at these redshifts.
Highlights
New constraints on the free-streaming of warm dark matter from intermediate and small scale Lyman-α forest data
For the reference analysis case we model the mean flux parameters independently for each redshift bin, the number of which varies for each data set (XQ-100 has 7, MIKE/HIRES has 4 and the combined analysis has 10 redshift bins)
We complement these parameters with an additional 9 parameters: 5 parameters describing either cosmology or astrophysics (σ8, neff, zrei, mass of the WDM (mWDM), fUV) and 4 parameters describing the thermal state of the intergalactic medium (IGM), using a power-law T − ρ relation, T = T0∆γ−1
Summary
New constraints on the free-streaming of warm dark matter from intermediate and small scale Lyman-α forest data. The further improvement in the joint analysis is partly due to the fact that the two data sets have different degeneracies between astrophysical and cosmological parameters that are broken when the data sets are combined, and more importantly on chosen priors on the thermal evolution These results all assume that the temperature evolution of the IGM can be modelled as a power law in redshift. We will provide constraints on the free-streaming length of dark matter from modelling the XQ-100 flux-power spectrum as well as from a combined analysis with new modelling of the HIRES/MIKE flux power spectrum presented in [7] These two data sets have a small redshift overlap and it can be expected that a combined analysis will further break degeneracies with remaining uncertainties in the parameters describing the thermal evolution of the IGM, the evolution of the mean flux and cosmological parameters. Apart from changing the nature of the DM several studies have shown that baryonic physics could help in alleviating or even solving the small scale tensions [27, 28]
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