On the Sizes of Ionized Bubbles Around Galaxies During the Reionization Epoch. The Spectral Shapes of the Lyα Emission from Galaxies

• Published in: Astrophysical journal. Letters Vol. 954; no. 1; p. L14
• Main Authors: Hayes, Matthew J., Scarlata, Claudia
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-09-2023; IOP Publishing
• Subjects: Bubbles; Emission line galaxies; Galaxies; Galaxy evolution; High-redshift galaxies; Hydrogen atoms; Intergalactic media; Intergalactic medium; Ionization; Photons; Red shift; Reionization; Stars & galaxies
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/acee6a

Abstract
We develop a new method to determine the distance between a high-redshift galaxy and a foreground screen of atomic hydrogen. In a partially neutral universe, and assuming spherical symmetry, this equates to the radius of an ionized “bubble” (
R
B
) surrounding the galaxy. The method requires an observed Ly
α
equivalent width, its velocity offset from systemic, and an input Ly
α
profile for which we adopt scaled versions of the profiles observed in low-
z
galaxies. We demonstrate the technique in a sample of 23 galaxies at
z
> 6, including 8 at
z
= 7.2–10.6 recently observed with JWST. Our model estimates the emergent Ly
α
properties and the foreground distance to the absorbing intergalactic medium. We find that galaxies at
z
> 7.5 occupy smaller bubbles (∼0.5–1 pMpc) than those at lower
z
. With a relationship that is secure at 99% confidence, we empirically demonstrate the growth of ionized regions during the reionization epoch for the first time. We independently estimate the upper limit on the Strömgren radii (
R
S
), and derive the escape fraction of ionizing photons (
f
esc
LyC
) from the ratio of
R
B
/
R
S
, deriving a median value of 5%, which, on average, represents the lower end of the photon budget necessary for reionization.