A Search for Great Walls of Cosmic Baryons in the Northern Sky

Abstract

The dispersion measures (DMs) of fast radio bursts (FRBs) encode the total ionized-gas column densities along their sightlines. Most observed FRBs originate at distances where the cosmological principle applies. Thus, variations in the DM distribution of FRBs observed in different regions on the sky trace local sources of anisotropy, such as the warm ionized medium and circumgalactic medium of the Milky Way, and local large-scale structure. We present a map of extragalactic DM variations across the Northern Sky using a few thousand FRBs from the second CHIME/FRB catalog. We detect an excess of ∼150 pc cm−3 above the global mean, extended over ∼30° scales and centered near α ≈ 12h, δ ≈ 55°. This excess, termed Wall 1, is robust to variations in sample selection and jackknife resampling, and cannot be explained by Galactic-disk DM-model uncertainties. The excess is likely too large to correspond to anisotropy in the Milky Way halo. The signal spatially coincides with the Ursa Major supercluster and associated large-scale structures. A secondary, more tentative Wall 2 near α ≈ 2h, δ ≈ 45° is spatially coincident with the Perseus–Pisces supercluster. Given the current data quality, however, the probability of chance coincidence is likely too high (∼10%−20%) to claim confident associations. Although previously unknown forms of sky-dependent instrumental variations cannot be excluded, these results highlight the potential of using FRB DMs to detect baryon overdensities associated with local large-scale structure. The results have important implications for near-field baryon mapping and FRB cosmology.