Warm Dark Matter as a solution to the small scale crisis: new constraints from high redshift Lyman-alpha forest data (1306.2314v2)

Abstract: 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.

• The paper presents robust constraints on warm dark matter, establishing a 3.3 keV lower limit at a 2σ confidence level while disfavoring 1, 2, and 2.5 keV candidates.
• It employs high-resolution hydrodynamical simulations of the Lyman-α forest from 25 quasars to analyze WDM free-streaming effects.
• The findings imply that modifying dark matter properties alone cannot resolve the small-scale structure issues in the ΛCDM model, inviting alternative approaches.

Warm Dark Matter as a Solution to the Small Scale Crisis: New Constraints from High Redshift Lyman-$\alpha$ Forest Data

The paper authored by Matteo Viel et al. focuses on addressing the small scale problems in $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cosmology by considering Warm Dark Matter (WDM) models. The analysis utilizes high-resolution Lyman-$\alpha$ (Ly$\alpha$) forest data from 25 quasars at redshifts $z > 4$, measured using Keck and Magellan telescopes. The critical objective of the research is to determine constraints on the free-streaming behavior of WDM by observing its impact on the Ly$\alpha$ forest flux power spectrum.

Methodological Approach

The authors employ a series of high-resolution hydrodynamical simulations under varying WDM mass scenarios, specifically testing WDM particle masses of 1, 2, and 4 keV if they are thermal relics. The simulated data is analyzed to probe different cosmological and astrophysical parameter spaces, leading to an estimation of the lower limits on the WDM particle mass. By addressing systematic errors and conducting an analysis with conservative error estimates, the credibility of the results is enhanced.

Key Findings

A significant finding of this paper is the lower limit of approximately 3.3 keV ($2\sigma$ confidence level) for WDM particles, based on the analysis of thermal relics. It also reveals that thermal relic WDM masses of 1 keV, 2 keV, and 2.5 keV are disfavored by the data with confidence levels of $9\sigma$, $4\sigma$, and $3\sigma$, respectively. The implications suggest a minimal role for WDM free-streaming in addressing challenges posited by the small scale structure crisis in $\Lambda$CDM frameworks.

Implications and Speculations

The research has noteworthy theoretical and practical implications. The marginalization of WDM suggests that simply altering dark matter properties is insufficient to resolve the small-scale issues within the $\Lambda$CDM paradigm. The finite suppression scale in the linear matter power spectrum indicates that mechanisms beyond WDM need to be explored to address the discrepancies observed in the galactic core density profiles, satellite galaxy counts, and other sub-Mpc scale structures.

Furthermore, the suppression of the mass function at scales below $\sim 2\times10^8\,h^{-1}$ M$_{\odot}$ is a robust marker that constrains the possible contributions of WDM to large-scale structure formation. The limitations identified would influence future studies in galaxy formation, emphasizing the need to consider baryonic physics in tandem with dark matter characteristics.

Future Outlook

The paper points towards several areas for future development. Improved high-redshift Ly$\alpha$ forest data with better resolution and coverage could refine the constraints on WDM masses. Simultaneously, constraints integrating next-generation cosmic microwave background measurements could provide complementary insights. Developments in numerical simulations that accurately capture non-linear baryonic effects and the consideration of alternative dark matter models such as mixed warm-cold dark matter or alternative thermal histories may offer avenues for addressing current cosmological puzzles.

In conclusion, this paper enriches the ongoing discourse regarding dark matter characteristics, emphasizing the limited scope for WDM as an isolated solution to the small-scale challenges faced by the $\Lambda$CDM model. It sets the stage for broader explorations into both theoretical models and observation-based validations in the field of cosmology.