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Accurately predicting the escape fraction of ionizing photons using rest-frame ultraviolet absorption lines

Chisholm, John; Gazagnes, S; Schaerer, Daniel; Verhamme, Anne; Rigby, Jane R.; Bayliss, Matthew B.; Sharon, Keren; Gladders, Michael D.; Dahle, Håkon
Journal article; PublishedVersion; Peer reviewed
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aa32758-18.pdf (1.551Mb)
Year
2018
Permanent link
http://urn.nb.no/URN:NBN:no-68617

CRIStin
1624965

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  • Institutt for teoretisk astrofysikk [724]
  • CRIStin høstingsarkiv [16854]
Original version
Astronomy and Astrophysics. 2018, 616:A30, DOI: http://dx.doi.org/10.1051/0004-6361/201832758
Abstract
The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early Universe. However, this escape fraction cannot be measured from high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to measure the escape fraction of high-redshift galaxies indirectly, it is unlikely that we will know what reionized the Universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within 1.4σ) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Lyα escape fraction and the optical [O III]/[O II] flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions (fesc ≤ 1%), but 11 have escape fractions >1%. Future studies will use these methods to measure the escape fractions of high-redshift galaxies, enabling upcoming telescopes to determine whether star-forming galaxies reionized the early Universe.

© 2018 ESO
 
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