Strong Lyman-α emission in an overdense region at z = 6.8: A very large (R ∼3 physical Mpc) ionized bubble in COSMOS?

Ryan Endsley, Daniel P. Stark

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


Our understanding of reionization has advanced considerably over the past decade, with several results now demonstrating that the intergalactic medium 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 ≥ 7 as well as the galaxy overdensities which drive their growth. Fortunately, rest-ultraviolet (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 α detections from three closely separated Lyman-break galaxies at z ∼ 6.8, suggesting the presence of a large (R > 1 physical Mpc) ionized bubble in the 1.5 deg2 COSMOS field. Here, we present new deep Ly α spectra of 10 UV-bright (M UV ≤-20.4) z ∼ 6.6-6.9 galaxies in the surrounding area, enabling us to better characterize this potential ionized bubble. We confidently detect (S/N > 7) Ly α 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/N 3). Our data additionally confirm that the Ly α emission of UV-bright galaxies in this volume is significantly enhanced, with 40 per cent (4/10) showing strong Ly α emission (equivalent width >25 Å) compared to the 8-9 per cent found on average at z ∼7. The median Ly α equivalent width of our observed galaxies is also ≈2 times that typical at z ∼7, consistent with expectations if a very large (R ∼3 physical Mpc) ionized bubble is allowing the Ly α photons to cosmologically redshift far into the damping wing before encountering H i.

Original languageEnglish (US)
Pages (from-to)6042-6054
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Apr 1 2022


  • Dark ages
  • First stars
  • Galaxies: high-redshift
  • Reionization

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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