Possible detection of Baryon Acoustic Oscillations imprinted in the HI sources from the FASHI catalog

Abstract

Motivated by curiosity as to whether the observed Universe at low redshifts is well described by the concordance cosmological model, we investigate the large scale angular clustering of extragalactic neutral hydrogen HI sources in the local Universe. Using the second data release of the FAST All Sky HI Survey (FASHI), we analyze a sample of 142,204 HI sources up to $z<0.09$, with a median redshift of $\bar{z} = 0.034$, covering $\sim 40\%$ of the sky. We compute the 2-point angular correlation function, $w(\theta)$, and estimate its covariance using an analytical Gaussian model. We report the possible detection of a broad angular Baryon Acoustic Oscillation (BAO) signal, peaking near $50^\circ$. A comparison between a 5th-order polynomial fit and a quadratic baseline yields $\Delta \chi^2 = 9.06$ for $\Delta \mathrm{dof} = 3$ (difference of 3 degrees of freedom), corresponding to a nominal $1.9\,\sigma$ significance for a non-monotonic feature in $w(\theta)$ relative to the monotonic quadratic baseline, although this result alone does not establish a BAO detection. Our analyses show that, due to the projection effect, the expected transverse BAO scale shifts from $64^\circ$ in an infinitesimally thin redshift bin to $52^\circ$ in a bin of thickness $\Delta z = 0.09$, like the data we study, a value close to where our detection peaks, i.e., $\sim 50^\circ$. Furthermore, comparisons with an ensemble of lognormal mocks show that the observed broad bump is consistent with statistical fluctuations at the $1\sigma$ level, meaning that current uncertainties prevent us from unequivocally establishing it as a cosmological signal. Comprehensively, our study establishes a valuable observational baseline for wide-field HI surveys, showing the potential to trace primordial acoustic features even in the low redshift regime, where the transverse BAO signal is dominated by projection effects.