Constraints on the Correlation of IceCube Neutrinos with Tracers of Large-Scale Structure

TL;DR

This study tests whether the IceCube diffuse extragalactic neutrino flux correlates with the nearby large-scale structure by cross-correlating Northern-sky IceCube muon neutrinos with the unWISE-2MASS infrared galaxy catalog. Using a forward-modeling approach, it decomposes the cross-power spectrum into a correlated neutrino component and atmospheric/background terms, implemented across three energy bins with a full mask covariance and an ℓ_min of 10. The analysis finds no statistically significant cross-correlation (TS ≈ 2.31, p ≈ 0.23) and derives 90% upper limits on the correlated flux, implying that less than about 54% of the diffuse muon neutrino flux at 100 TeV can originate from sources tracing the unWISE-2MASS catalog. Interpreting the correlation strength in terms of the redshift evolution dN_s/dV ∝ (1+z)^k, the results disfavor strong negative evolution (k ≤ -1.75 at 90% CL) and favor models where neutrino sources have broader or different redshift distributions (e.g., star-formation-rate–like or constant comoving density) given plausible bias factors. The work highlights the potential of tomographic, redshift-resolved cross-correlations with richer tracers and larger data sets to reveal the evolution and identity of IceCube neutrino sources.

Abstract

The IceCube Neutrino Observatory has observed extragalactic astrophysical neutrinos with an apparently isotropic distribution. Only a small fraction of the observed astrophysical neutrinos can be explained by known sources. Neutrino production is thought to occur in energetic environments that are ultimately powered by the gravitational collapse of dense regions of the large-scale mass distribution in the universe. Whatever their identity, neutrino sources likely trace this large-scale mass distribution. The clustering of neutrinos with a tracer of the large-scale structure may provide insight into the distribution of neutrino sources with respect to redshift and the identity of neutrino sources. We implement a two-point angular cross-correlation of the Northern sky track events with an infrared galaxy catalog derived from WISE and 2MASS source catalogs that trace the nearby large-scale structure. No statistically significant correlation is found between the neutrinos and this infrared galaxy catalog. We find that < ~54% of the diffuse muon neutrino flux can be attributed to sources correlated with the galaxy catalog with 90% confidence. Additionally, when assuming that the neutrino source comoving density evolves following a power-law in redshift, $dN_s/dV \propto (1+z)^{k}$, we find that sources with negative evolution, in particular k < -1.75, are disfavored at the 90% confidence level

Figures (5)

• Figure 1: Left: The density of unWISE-2MASS galaxies on the sky. The gray region combines a $\pm 10\hbox{$^\circ$}$ region surrounding the galactic plane and a Planck dust map. The Large and Small Magellanic Clouds are masked as well. Right: The redshift distribution of the unWISE-2MASS catalog found by cross-matching sources with the GAMA redshift catalog. The GAMA redshifts are spectroscopically measured, so they are precise enough to neglect the statistical uncertainty. The catalog is magnitude-limited and has a tail at larger redshifts. The median redshift is 0.12, and the 10% and 90% percentiles are 0.05 and 0.22.
• Figure 2: If the neutrinos are sampled from the unWISE-2MASS catalog, then the neutrino flux can be estimated assuming the correlation bias parameters are equal, i.e., $b_\nu = b_g$. The derived fluxes for three different spectral indices are shown in the red dashed, black dot-dashed, and green dotted lines. The best fit diffuse flux of muon neutrinos is shown in the blue region. If the diffuse spectral index is assumed for the correlated neutrinos, the correlated flux is less than 54% of the total diffuse flux. The cross-correlation upper limits are plotted over the 90% sensitive energy range, which is defined as the upper and lower energy range that causes the analysis sensitivity to drop by 10%.
• Figure 2: The TS distribution for RA-scrambled synthetic data sets is shown above. The blue dotted line shows a $\chi^2$ distribution with two degrees of freedom. The unblinded TS is shown with a black star. The unblinded TS is 2.307, which is not statistically significant.
• Figure 3: Left: If the neutrino sources follow a distribution of the form $\frac{dN_s}{dV}\propto (1+z)^k$, the expected correlation strength, $f_\textrm{corr}$, can be calculated given the ratio of diffuse astrophysical neutrinos to total neutrinos, $f_\textrm{astro}$. The expected correlation strength for equal bias parameters (solid black) and $\frac{b_\nu}{b_g} = 2$ (dashed line) are shown. The latter case is a pessimistic model given that neutrino sources may form within the same dark matter halos as the unWISE-2MASS galaxies. The actual upper limit is shown by the gray dash-dot line. The area below the line is allowed by the upper limit. Right: If the neutrino sources follow a distribution of the form $\frac{dN_s}{dV}=(1+z)^k~e^{z/\xi}$ and astrophysical spectral index and sample purity are equal to the best fit diffuse muon neutrino flux ($\gamma=2.37$ and $f_\textrm{astro}=0.013$), the expected correlation strength can be calculated numerically using the Core Cosmology Library software. The expected correlation strength $f_\textrm{corr}$ for equal bias parameters is shown in solid black. The region in the lower right predicts correlations that are weaker than the observed upper limit. For $\xi \ll 1$, the limit on power law index approaches $k\ge -1.75$ which suggests a median observed neutrino redshift $z>0.2$.
• Figure 31: Left: The change in likelihood relative to the maximum likelihood value for the two astrophysical parameters is shown in color and with contours. The maximum likelihood value is on the boundary with $\gamma=4.0$. The test statistic is not statistically significant for the maximum likelihood parameter values. The expected value of $f_\textrm{corr}$ is given if all of the astrophysical neutrinos were correlated with the unWISE-2MASS catalog assuming the spectral index is the same as the diffuse measurement abbasiImprovedCharacterizationAstrophysical2022a. Right: The same likelihood is shown in terms of the differential flux at 100 TeV.