2024MNRAS.527.7400F

This is a model-independent analysis that investigates the statistical isotropy in the Local Universe using the Arecibo Legacy Fast ALFA (ALFALFA) survey data (0 < z < 0.06). We investigate the angular distribution of H I extragalactic sources from the ALFALFA catalogue and study whether they are compatible with the statistical isotropy hypothesis using the two-point angular correlation function (2PACF). Aware that the Local Universe is plenty of clustered structures and large voids, we compute the 2PACF with the Landy-Szalay estimator performing directional analyses to inspect 10 sky regions. We investigate these 2PACF using power-law best-fitting analyses, and determine the statistical significance of the best-fitting parameters for the 10 ALFALFA regions by comparison with the ones obtained through the same procedure applied to a set of mock catalogues produced under the homogeneity and isotropy hypotheses. Our conclusion is that the Local Universe, as mapped by the H I sources of the ALFALFA survey, is in agreement with the hypothesis of statistical isotropy within $2\sigma$ confidence level, for small- and large-angle analyses, with the only exception of one region - located near the Dipole Repeller - that appears slightly outlier ($2.4\sigma$). Interestingly, regarding the large angular distribution of the H I sources, we found three regions where the presence of cosmic voids reported in the literature left their signature in our 2PACF, suggesting projected large underdensities there, with number-density contrast δ ≃ -0.7. According to the current literature, these regions correspond, partially, to the sky position of the void structures known as Local Cosmic Void and Dipole Repeller.