Probing Spatial Variations in IGM Transmission: Higher Ly$\alpha$ Visibility in an H$\alpha$-Selected Galaxy Overdensity at $z\simeq 6.2$

Abstract

The visibility of Ly$\alpha$ emission during the epoch of reionization depends on both galaxy properties and the transmission of Ly$\alpha$ photons through the intergalactic medium (IGM). Using JWST/NIRCam F470N imaging, we identify a prominent projected overdensity of H$\alpha$ emitters (HAEs) at $z\simeq 6.2$ in CEERS field. We use Subaru/HSC NB872 imaging to measure their Ly$\alpha$ fluxes and compare the Ly$\alpha$ visibility of galaxies within $2^{\prime }$ of the density center with that of galaxies outside this region. The Ly$\alpha$-emitter (LAE) fraction is $0.41_{-0.14}^{+0.15}$ in the inner region and $0.27_{-0.15}^{+0.19}$ in the outer region. Stacking the H$\alpha$ and Ly$\alpha$ images gives median Ly$\alpha$ escape fractions of $0.100_{-0.053}^{+0.091}$ and $0.063_{-0.063}^{+0.130}$ in the inner and outer regions, respectively. Both measures indicate higher Ly$\alpha$ visibility in the inner region, although only at modest significance. HAEs closer to the density center are systematically more massive and have redder UV slopes, which instead favor lower Ly$\alpha$ escape. The environmental difference in Ly$\alpha$ visibility is therefore difficult to explain solely by variations in underlying galaxy population. Our results are consistent with reduced Ly$\alpha$ attenuation through a more highly ionized IGM in the overdense region, as expected if large H\,{\sc ii} bubbles preferentially form around such galaxy overdensities. The wide-field LAE distribution provides complementary support for this scenario, as the HAE-defined density center coincides with the strongest LAE overdensity across the $1.4{\mathrm{deg}}^2$ HSC field. These findings highlight the importance of spatial variations in IGM attenuation in shaping Ly$\alpha$ visibility near the end of reionization.

12 pages, 4 figures, submitted to ApJ