Tantalizing evidence of reionization relics in the eBOSS DR16 Ly α forest correlations: a preference for early reionization

ABSTRACT Cosmic reionization of H i leaves enduring relics in the post-reionization intergalactic medium, potentially influencing the Lyman $\alpha$ (Ly $\alpha$) forest down to redshifts as low as $z \approx 2$, which is the so-called ‘memory of reionization’ effect. Here, we re-analyse the baryonic acoustic oscillation (BAO) measurements from Ly $\alpha$ absorption and quasar correlations using data from the extended Baryonic Oscillation Spectroscopic Survey (eBOSS) Data Release 16 (DR16), incorporating for the first time the memory of reionization in the Ly $\alpha$ forest. Three distinct scenarios of reionization timeline are considered in our analyses. We find that the recovered BAO parameters ($\alpha _\parallel$, $\alpha _\perp$) remain consistent with the original eBOSS DR16 analysis. However, models incorporating reionization relics provide a better fit to the data, with a tantalizing preference for early reionization, consistent with recent findings from the James Webb Space Telescope. Furthermore, the inclusion of reionization relics significantly impacts the non-BAO parameters. For instance, we report deviations of up to $3\sigma$ in the Ly $\alpha$ redshift-space distortion parameter and $\sim 7\sigma$ in the linear Ly $\alpha$ bias for the late reionization scenario. Our findings suggest that the eBOSS Ly $\alpha$ data are more accurately described by models that incorporate a broad-band enhancement to the Ly $\alpha$ forest power spectrum, highlighting the importance of accounting for reionization relics in cosmological analyses.