Strong Ly$\alpha$ Emission in an Overdense Region at $z=6.8$: A Very Large ($R\sim3$ physical Mpc) Ionized Bubble in COSMOS?

Abstract

Our understanding of reionization has advanced considerably over the past decade, with several results now demonstrating that the IGM transitioned from substantially neutral at $z=7$ to largely reionized at $z=6$. However, little remains known about the sizes of ionized bubbles at $z\gtrsim7$ as well as the galaxy overdensities which drive their growth. Fortunately, rest-UV spectroscopic observations offer a pathway towards characterizing these ionized bubbles thanks to the resonant nature of Lyman-alpha photons. In a previous work, we presented Ly$\alpha$ detections from three closely-separated Lyman-break galaxies at $z\simeq6.8$, suggesting the presence of a large ($R>1$ physical Mpc) ionized bubble in the 1.5 deg$^2$ COSMOS field. Here, we present new deep Ly$\alpha$ spectra of ten UV-bright ($\mathrm{M}_{\mathrm{UV}}^{} \leq -20.4$) $z\simeq6.6-6.9$ galaxies in the surrounding area, enabling us to better characterize this potential ionized bubble. We confidently detect (S/N$>$7) Ly$\alpha$ emission at $z=6.701-6.882$ in nine of ten observed galaxies, revealing that the large-scale volume spanned by these sources (characteristic radius $R = 3.2$ physical Mpc) traces a strong galaxy overdensity ($N/\langle N\rangle \gtrsim 3$). Our data additionally confirm that the Ly$\alpha$ emission of UV-bright galaxies in this volume is significantly enhanced, with 40% (4/10) showing strong Ly$\alpha$ emission (equivalent width$>$25 $\mathrm{\mathring{A}}$) compared to the 8$-$9% found on average at $z\sim7$. The median Ly$\alpha$ equivalent width of our observed galaxies is also $\approx$2$\times$ that typical at $z\sim7$, consistent with expectations if a very large ($R\sim3$ physical Mpc) ionized bubble is allowing the Ly$\alpha$ photons to cosmologically redshift far into the damping wing before encountering HI.