Double-peaked Lyman-$\hat{I}\pm$ emission at z=6.803: a reionisation-era galaxy self-ionising its local HII bubble

Abstract

We report the discovery of a double-peaked Lyman-alpha profile in a galaxy at $z=6.803$, A370p\_z1, in the parallel Frontier Field of Abell 370. The velocity separation between the blue and red peaks of the Lyman-$\hat{I}\pm$ profile ($\hat{I}”v=101_{-19}^{+38}(\pm 48){\text{km s}}^{-1}$) suggests an extremely high escape fraction of ionising photons $>59(51)\%(2\ddot{I}ƒ)$. The spectral energy distribution indicates a young ($50$ Myr), star-forming ($12\pm 6{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}$) galaxy with an IRAC excess implying strong [OIII]+H${\hat{I}}^2$ emission. On the basis of the high escape fraction measured, we demonstrate that A370p\_z1 was solely capable of creating an ionised bubble sufficiently large to account for the blue component of its Lyman-alpha profile. We discuss whether A370p\_z1 may be representative of a larger population of luminous $z\simeq 7$ double-peaked Lyman-alpha emitting sources with high escape fractions that self-ionised their surroundings without contributions from associated UV-fainter sources.

Accepted for publication in MNRAS. 7 pages, 4 figures, 3 tables