The CALYMHA survey: Lyα escape fraction and its dependence on galaxy properties at z = 2.23

Jorryt Matthee, David Sobral, Iván Oteo, Philip Best, Ian Smail, Huub Röttgering, Ana Paulino-Afonso

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We present the first results from our CAlibrating LYMan α withHα (CALYMHA) pilot survey at the Isaac Newton Telescope. Wemeasure Lyα emission for 488 Hα selected galaxies at z =2.23 from High-z Emission Line Survey in the COSMOS and UDS fields witha specially designed narrow-band filter (λc = 3918Å, Δλ = 52 Å). We find 17 dualHα-Lyα emitters [fLyα > 5 ×10-17 erg s-1 cm-2, of which five areX-ray active galactic nuclei (AGN)]. For star-forming galaxies, we finda range of Lyα escape fractions (fesc, measured with 3arcsec apertures) from 2 to 30 per cent. These galaxies have masses from3 × 108 M⊙ to 1011M⊙ and dust attenuations E(B - V) = 0-0.5. Usingstacking, we measure a median escape fraction of 1.6 ± 0.5 percent (4.0 ± 1.0 per cent without correcting Hα for dust),but show that this depends on galaxy properties. The stackedfesc tends to decrease with increasing star formation rateand dust attenuation. However, at the highest masses and dustattenuations, we detect individual galaxies with fesc muchhigher than the typical values from stacking, indicating significantscatter in the values of fesc. Relations betweenfesc and UV slope are bimodal, with high fesc foreither the bluest or reddest galaxies. We speculate that this bimodalityand large scatter in the values of fesc is due to additionalphysical mechanisms such as outflows facilitating fesc fordusty/massive systems. Lyα is significantly more extended thanHα and the UV. fesc continues to increase up to atleast 20 kpc (3σ, 40 kpc [2σ]) for typical star-forminggalaxies and thus the aperture is the most important predictor of fesc.
Original languageEnglish
Pages (from-to)449-467
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date10 Feb 2016
Publication statusPublished - 1 May 2016


  • galaxies: evolution
  • galaxies: high-redshift
  • galaxies: ISM


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