Can Sterile Neutrinos Be the Dark Matter?

We use the Ly-$\alpha$ forest power spectrum measured by the Sloan Digital Sky Survey and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter scenario. Such a neutrino would be created in the early Universe through mixing with an active neutrino and would suppress structure on scales smaller than its free-streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino masses to describe the effect of a sterile neutrino on the Ly-$\alpha$ forest power spectrum. We find that the mass limit is $m_s>13\mathrm{keV}$ at 95% C.L. (9 keV at 99.9%), which is above the upper limit allowed by x-ray constraints, excluding this candidate from being all of the dark matter in this model.

Figure 1

Ratio of WDM power spectrum relative to CDM shown over the relevant observational range. From left to right, the sterile neutrino masses are 6.5, 10, 14, and 20 keV. The top left corner shows 1D (thick line) and 3D (thin line) linear power spectrum, while the other 3 panels show the ratios from hydrodynamic simulations at redshifts 2, 3, and 4. We used concordance cosmology with ${\Omega }_m=0.28$ and $H_0=71\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$.

Figure 2

To the left are the observed SDSS Ly-$\alpha$ forest flux power spectra ${\Delta }_F^2(k)=k{P}_F(k)/2\pi$ as a function of redshift from 2.2 (bottom) to 4.2 (top) in steps of 0.2. To the right are the power spectra from the high-resolution data compiled at redshifts 2.4, 3.0, and 3.9. For each redshift, the thick lines are from the best fitted CDM model, while the (generally lower at high $k$) thin lines are for the corresponding WDM model with the 6.5 keV sterile neutrino. The latter is discrepant with both SDSS and high-resolution data, with most of the distinguishing power coming from higher redshifts.