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Durham University

Department of Physics

Staff profile

Publication details for Professor Ian Smail

Matthee, J., Sobral, D., Oteo, I., Best, P., Smail, I., Röttgering, H. & Paulino-Afonso, A. (2016). The CALYMHA survey: Lyα escape fraction and its dependence on galaxy properties at z = 2.23. Monthly Notices of the Royal Astronomical Society 458(1): 449-467.

Author(s) from Durham


We present the first results from our CAlibrating LYMan-α with Hα (CALYMHA) pilot survey at the Isaac Newton Telescope. We measure Lyα emission for 488 Hα selected galaxies at z = 2.23 from HiZELS in the COSMOS and UDS fields with a specially designed narrow-band filter (λc = 3918 Å, Δλ= 52 Å). We find 17 dual Hα-Lyα emitters (fLyα > 5 × 10−17 erg s−1 cm−2, of which 5 are X-ray AGN). For star-forming galaxies, we find a range of Lyα escape fractions (fesc, measured with 3″ apertures) from 2%−30%. These galaxies have masses from 3 × 108 M⊙ to 1011 M⊙ and dust attenuations E(B − V) = 0 − 0.5. Using stacking, we measure a median escape fraction of 1.6 ± 0.5% (4.0 ± 1.0% without correcting Hα for dust), but show that this depends on galaxy properties. The stacked fesc tends to decrease with increasing SFR and dust attenuation. However, at the highest masses and dust attenuations, we detect individual galaxies with fesc much higher than the typical values from stacking, indicating significant scatter in the values of fesc. Relations between fesc and UV slope are bimodal, with high fesc for either the bluest or reddest galaxies. We speculate that this bimodality and large scatter in the values of fesc is due to additional physical mechanisms such as outflows facilitating fesc for dusty/massive systems. Lyα is significantly more extended than Hα and the UV. fesc continues to increase up to at least 20 kpc (3σ, 40 kpc [2σ]) for typical SFGs and thus the aperture is the most important predictor of fesc.