New Constraints on the free-streaming of warm dark matter from intermediate and small scale Lyman-$α$ forest data (1702.01764v4)

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.

• The paper improves hydrodynamical simulations of the Lyman-α flux power spectrum to derive stringent bounds, achieving a 5.3 keV lower limit at 2σ confidence with combined data.
• It analyzes two distinct quasar absorption samples from XQ-100 and HIRES/MIKE spanning redshifts 3–5.4 to effectively break parameter degeneracies.
• The study shows that unsmooth thermal histories can lower the mass limits, challenging models proposing lighter warm dark matter particles.

New Constraints on the Free-Streaming of Warm Dark Matter from Lyman-$$forest Data</h2> <p>The paper presents an in-depth analysis of the free-streaming properties of Warm Dark Matter (WDM) by examining data from the Lyman-$$\alpha$$forest, using two distinct samples of quasar absorption spectra observed through different spectrographs. The paper leverages the intermediate resolution XQ-100 sample with redshifts from$$z=3$to$4.2$$, and the high-resolution sample observed with HIRES/MIKE spectrographs from redshifts$$z=4.2$to$5.4$$.</p> <p>The researchers improved upon previous studies by refining the hydrodynamical simulations that model the dependence of the Lyman-$$\alpha$$flux power spectrum on WDM free-streaming, cosmological parameters, and the thermal history of the intergalactic medium (IGM). The outcome is a more stringent constraint on the equivalent mass of thermal relic WDM particles. Specifically, the XQ-100 sample alone places a lower limit of 1.4 keV, while the re-analysis of the HIRES/MIKE sample yields 4.1 keV. A combined analysis of the two data sets provides a significantly tighter limit of 5.3 keV at the 2$$\sigma$$confidence level.</p> <p>An interesting aspect of this research is the consideration of astrophysical and cosmological parameter degeneracies. The combined analysis breaks degeneracies between parameter sets due to different non-overlapping redshift ranges and varying influences of thermal and free-streaming effects in different IGM temperature models. The paper also accounts for unforeseen changes in the IGM temperature, showing that an unsmooth thermal history can reduce the combined lower mass limit to 3.5 keV, though such scenarios are inconsistent with observational data and theoretical photo-heating expectations.</p> <p>The implications for the constraints on WDM suggest that thermal relics lighter than 5.3 keV (2$$\sigma$$C.L.) would result in inconsistencies with the observed Lyman-$$\alpha$$flux power spectrum. These constraints impose challenges for models proposing lighter WDM particles, such as those predicting sterile neutrinos or ultra-light bosons.</p> <p>There are several theoretical and practical implications of this research, underscoring the persistent tensions between current WDM models and observed small-scale structures in astrophysics, challenging the Cold Dark Matter (CDM) paradigm. The results also suggest further theoretical work to refine the models of IGM temperature evolution and spatial fluctuations of the UV background.</p> <p>Future research might exploit upcoming high-resolution Lyman-$$\alpha$ forest data to further constrain WDM models, particularly by refining thermal models of the IGM during the epoch of reionization. Combining observations with more advanced simulation techniques that account for the effects of radiation and hydrodynamic interactions in the IGM could shed additional light on the limitations of conventional DM models and explore alternative dark matter theories robustly.