---
_id: '11507'
abstract:
- lang: eng
  text: 'Lyman-α (Lyα) is intrinsically the brightest line emitted from active galaxies.
    While it originates from many physical processes, for star-forming galaxies the
    intrinsic Lyα luminosity is a direct tracer of the Lyman-continuum (LyC) radiation
    produced by the most massive O- and early-type B-stars (M⋆ ≳ 10 M⊙) with lifetimes
    of a few Myrs. As such, Lyα luminosity should be an excellent instantaneous star
    formation rate (SFR) indicator. However, its resonant nature and susceptibility
    to dust as a rest-frame UV photon makes Lyα very hard to interpret due to the
    uncertain Lyα escape fraction, fesc, Lyα. Here we explore results from the CAlibrating
    LYMan-α with Hα (CALYMHA) survey at z = 2.2, follow-up of Lyα emitters (LAEs)
    at z = 2.2 − 2.6 and a z ∼ 0−0.3 compilation of LAEs to directly measure fesc, Lyα
    with Hα. We derive a simple empirical relation that robustly retrieves fesc, Lyα
    as a function of Lyα rest-frame EW (EW0): fesc,Lyα = 0.0048 EW0[Å] ± 0.05 and
    we show that it constrains a well-defined anti-correlation between ionisation
    efficiency (ξion) and dust extinction in LAEs. Observed Lyα luminosities and EW0
    are easy measurable quantities at high redshift, thus making our relation a practical
    tool to estimate intrinsic Lyα and LyC luminosities under well controlled and
    simple assumptions. Our results allow observed Lyα luminosities to be used to
    compute SFRs for LAEs at z ∼ 0−2.6 within ±0.2 dex of the Hα dust corrected SFRs.
    We apply our empirical SFR(Lyα,EW0) calibration to several sources at z ≥ 2.6
    to find that star-forming LAEs have SFRs typically ranging from 0.1 to 20 M⊙ yr−1
    and that our calibration might be even applicable for the most luminous LAEs within
    the epoch of re-ionisation. Our results imply high ionisation efficiencies (log10[ξion/Hz erg−1]
    = 25.4−25.6) and low dust content in LAEs across cosmic time, and will be easily
    tested with future observations with JWST which can obtain Hα and Hβ measurements
    for high-redshift LAEs.'
acknowledgement: We thank the anonymous referees for multiple comments and suggestions
  which have improved the manuscript. JM acknowledges the support of a Huygens PhD
  fellowship from Leiden University. We have benefited greatly from the publicly available
  programming language PYTHON, including the NUMPY & SCIPY (Van Der Walt et al. 2011;
  Jones et al. 2001), MATPLOTLIB (Hunter 2007) and ASTROPY (Astropy Collaboration
  2013) packages, and the TOPCAT analysis program (Taylor 2013). The results and samples
  of LAEs used for this paper are publicly available (see e.g. Sobral et al. 2017,
  2018a) and we also provide the toy model used as a PYTHON script.
article_number: A157
article_processing_charge: No
article_type: original
arxiv: 1
author:
- first_name: David
  full_name: Sobral, David
  last_name: Sobral
- first_name: Jorryt J
  full_name: Matthee, Jorryt J
  id: 7439a258-f3c0-11ec-9501-9df22fe06720
  last_name: Matthee
  orcid: 0000-0003-2871-127X
citation:
  ama: 'Sobral D, Matthee JJ. Predicting Lyα escape fractions with a simple observable:
    Lyα in emission as an empirically calibrated star formation rate indicator. <i>Astronomy
    &#38; Astrophysics</i>. 2019;623. doi:<a href="https://doi.org/10.1051/0004-6361/201833075">10.1051/0004-6361/201833075</a>'
  apa: 'Sobral, D., &#38; Matthee, J. J. (2019). Predicting Lyα escape fractions with
    a simple observable: Lyα in emission as an empirically calibrated star formation
    rate indicator. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/201833075">https://doi.org/10.1051/0004-6361/201833075</a>'
  chicago: 'Sobral, David, and Jorryt J Matthee. “Predicting Lyα Escape Fractions
    with a Simple Observable: Lyα in Emission as an Empirically Calibrated Star Formation
    Rate Indicator.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences, 2019. <a href="https://doi.org/10.1051/0004-6361/201833075">https://doi.org/10.1051/0004-6361/201833075</a>.'
  ieee: 'D. Sobral and J. J. Matthee, “Predicting Lyα escape fractions with a simple
    observable: Lyα in emission as an empirically calibrated star formation rate indicator,”
    <i>Astronomy &#38; Astrophysics</i>, vol. 623. EDP Sciences, 2019.'
  ista: 'Sobral D, Matthee JJ. 2019. Predicting Lyα escape fractions with a simple
    observable: Lyα in emission as an empirically calibrated star formation rate indicator.
    Astronomy &#38; Astrophysics. 623, A157.'
  mla: 'Sobral, David, and Jorryt J. Matthee. “Predicting Lyα Escape Fractions with
    a Simple Observable: Lyα in Emission as an Empirically Calibrated Star Formation
    Rate Indicator.” <i>Astronomy &#38; Astrophysics</i>, vol. 623, A157, EDP Sciences,
    2019, doi:<a href="https://doi.org/10.1051/0004-6361/201833075">10.1051/0004-6361/201833075</a>.'
  short: D. Sobral, J.J. Matthee, Astronomy &#38; Astrophysics 623 (2019).
date_created: 2022-07-06T11:08:16Z
date_published: 2019-03-26T00:00:00Z
date_updated: 2022-07-19T09:37:20Z
day: '26'
doi: 10.1051/0004-6361/201833075
extern: '1'
external_id:
  arxiv:
  - '1803.08923'
intvolume: '       623'
keyword:
- Space and Planetary Science
- Astronomy and Astrophysics
- 'galaxies: high-redshift / galaxies: star formation / galaxies: statistics / galaxies:
  evolution / galaxies: formation / galaxies: ISM'
language:
- iso: eng
main_file_link:
- open_access: '1'
  url: https://arxiv.org/abs/1803.08923
month: '03'
oa: 1
oa_version: Published Version
publication: Astronomy & Astrophysics
publication_identifier:
  eissn:
  - 1432-0746
  issn:
  - 0004-6361
publication_status: published
publisher: EDP Sciences
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'Predicting Lyα escape fractions with a simple observable: Lyα in emission
  as an empirically calibrated star formation rate indicator'
type: journal_article
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
volume: 623
year: '2019'
...