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

TL;DR

The paper tests whether warm dark matter can alleviate small-scale issues of ΛCDM by constraining the WDM free-streaming scale using high-redshift Lyman-α forest data from 25 QSOs observed with HIRES/MIKE, complemented by extensive hydrodynamical simulations that span multiple WDM masses and IGM thermal histories. Through a comprehensive likelihood analysis that marginalizes over IGM temperature, UV background fluctuations, metals, and cosmological parameters under Planck-inspired priors, they derive a robust 2σ lower bound of $m_{ m WDM} > 3.3$ keV for thermal relic WDM, with a best-fit near $33$ keV. The constraints imply a free-streaming mass of about $2\times10^8\,M_{}h^{-1}$ and leave little room for WDM to solve the small-scale crisis; adding SDSS Lya data does not improve the limit. Lighter WDM scenarios (0.5–2 keV) and typical sterile neutrino models are strongly disfavoured by the data, guiding future dark matter model-building and observational strategies.

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.

Figures (17)

• Figure 1: Ratio between the 3D non-linear matter power spectrum of 3 different WDM models (1, 2 and 4 keV, black, blue and orange curves) at 3 different redshifts ($z=3,\,4.2,\,5.4$, represented by the dot-dashed, dashed and continuous curves) and the corresponding $\Lambda$CDM model. The green curve represents the linear redshift independent suppression in terms of matter power for a $m_{\rm WDM}=2$ keV model obtained using Eq. 6 of Ref. Viel:2005qj. The arrows in the bottom part of the figure indicate the maximum value of the wavenumbers probed by the SDSS data and by the data set used in the present analysis. This figure refers to the reference (20,512) simulations.
• Figure 2: Transmitted flux along a set of random LOSs for the $\Lambda$CDM (green curve) and WDM 1 keV (black curve) and WDM 2 keV (blue curve) models at $z=4.6$. This figure refers to the reference (20,512) simulation without adding instrumental noise. The $\Lambda$CDM flux is clearly showing more substructure as compared to the WDM models.
• Figure 3: The ratio of the 1D flux power spectrum for 3 different WDM models (1, 2 and 4 keV represented in black, blue and orange) at 4 different redshifts ($z=4.2,4.6,5,5.4$ represented by the continuous, dotted, dashed and dot-dashed curves, respectively) to the corresponding $\Lambda$CDM flux power spectra. This figure displays the results from the (20,512) simulations. The mean flux is the same in all models and the shaded area shows the range of wavenumbers used in the present analysis.
• Figure 4: The ratio of the 1D flux power spectrum for 2 different WDM models (1 and 2 keV, represented in black and blue) at 2 different redshifts ($z=4.2,5.4$ represented by the continuous and dot-dashed curves, respectively) to the corresponding $\Lambda$CDM simulations. The thin curves refer to the (20,512) simulations, while the thick curves refer to the high resolution (20,768) models. The mean flux is the same for all models and the shaded area shows the range of wavenumbers used in the present analysis.
• Figure 5: The ratio of the 1D flux power spectrum for two $\Lambda$CDM models with different temperatures(HOT, roughly hotter by 3000 K with respect to the reference simulation, in orange and COLD, roughly colder by 3000 K with respect to the reference simulation, in black) and at four different redshifts ($z=4.2,4.6,5,5.4$ represented by the dot-dashed, dashed, dotted and continuous curves, respectively) to the corresponding $\Lambda$CDM simulations. The WDM 2 keV model is also shown in blue. The mean flux is the same for all models, and the shaded area shows the range of wavenumbers used in the present analysis.