Dark winds on the horizon: Prospects for detecting neutrino and hot dark matter wakes in large-scale structure

Abstract

We explore the cosmological signatures of neutrino and Hot Dark Matter (HDM) wakes, which refers to the preferential accumulation of neutrinos (or, more broadly, HDM particles) downstream of moving cold dark matter structures. We improve on existing theoretical models, and provide forecasts for the detectability of the effect in future surveys under more realistic conditions than previously considered in the literature. We show that neutrino and HDM wakes are unlikely to be ever observed with the most natural tracer of a hot subcomponent of the total dark matter on cosmological scales, i.e. 2D weak lensing surveys. However, the effect can be detected at a high significance with idealistic 3D maps of a tracer of HDM, for sufficiently small values of the effective free-streaming length (e.g. present-day values of $k_{\textrm{fs},0} \gtrsim 0.1\textrm{Mpc}^{-1}$ to reach $\textrm{SNR} \gtrsim 1$, for a HDM species accounting for a percent of the total dark matter). HDM wakes are a smoking gun of the effects of free-streaming, which cannot be mimicked by changes to the background expansion history (such as allowing for the dark energy to be dynamical), and hence offer another avenue to search for massive neutrinos, and hot subcomponents of the total dark matter more broadly, in a way that complements traditional observables.

Figures (6)

• Figure 1: Cumulative signal-to-noise ratio of neutrino and HDM wakes, estimated by the reconstructed relative velocity field from measurements of galaxy-matter cross correlations, as a function of the present-day value of the effective free-streaming scale $k_{\textrm{fs},0}$, assuming a billion spectroscopic redshifts are collected out to $z_{\textrm{max}}=2$. The black curve follows from fixing the fractional contribution of hot to total matter density of $f_{\textrm{h}}= 0.01$, and the orange curve corresponds to the case of a single massive neutrino species. These results assume a 3D galaxy survey in combination with a noiseless 3D map of the total matter field (or any tracer of HDM, more broadly), at an effective redshift of $z_{\textrm{eff}}=1$.
• Figure 2: Cumulative signal-to-noise ratio of HDM wakes, estimated by the reconstructed relative velocity field from measurements of galaxy-matter cross correlations, as a function of the effective redshift under the same set-up as Fig. \ref{['fig:3d_scale']}, for an effective free-streaming scale $k_{\textrm{fs},0}=0.1\textrm{Mpc}^{-1}$ and galaxy bias $b(z_{\textrm{eff}})=1+z_{\textrm{eff}}$.
• Figure 3: Signal-to-noise ratio of HDM wakes as a function of the reconstructed long-wavelength relative velocity mode [as defined in Eq. (\ref{['eq:snrint_3d']})], and measured from galaxy-matter cross correlations, normalized to the cumulative signal-to-noise. The vertical dotted (orange) line shows the effective free-streaming scale at the effective redshift of the galaxy sample, and the set-up is the same as in Fig. \ref{['fig:3d_redshift']}.
• Figure 4: Correlation coefficient between galaxy and lensing converge fields. The lines on different shades of orange correspond to varying redshift bins (for the galaxy sample) in the 2D case with a total number of $N_{\textrm{z-bins}}=6$ equally spaced bins in the interval $0\leq z \leq 1.5$. The solid black line corresponds to the optimal combination of redshift bins. In all cases the lensing converge field is computed with the entire (LSST mock) source galaxy population out to $z=5$. We also include for comparison the three-dimensional galaxy-matter correlation coefficient, evaluated at $k=\ell/\chi(z_{\textrm{eff}}=1)$, as the dashed black line.
• Figure 5: Signal-to-noise ratios for neutrino and HDM wakes as a function of the number of tomographic redshift bins $N_{\textrm{z-bins}}$, as estimated by the reconstructed relative velocity field perpendicular to the light-of-sight, from measurements of 2D galaxy-matter cross correlations. We combine the different bins into single optimal two-dimensional fields [for both galaxy and relative velocity potential, see Eqs. (\ref{['eq:optimal_1']}) and (\ref{['eq:optimal_2']})]. The $N_{\textrm{z-bins}} \to \infty$ limit should reproduce the 3D case, up to some caveats discussed in the main text. We assume a fractional contribution of hot to total matter density of $f_{\textrm{h}}=0.01$, a present-day value for the effective free-streaming scale of $k_{\textrm{fs},0}=0.1\textrm{Mpc}^{-1}$, and an LSST-like galaxy redshift distribution with a total number density of $\bar{n} = 40 \ \textrm{arcmin}^{-2}$ [for the source galaxies used for lensing as well, see lines of text around Eqs. (\ref{['eq:lensing_kernel']}) and (\ref{['eq:total_galaxies_bin']})].