Determining Lya properties of distant galaxies is of great interest for various astrophysical studies. We examine how the strength of Lya emission can be constrained from broad-band SED fits instead of relying on spectroscopy. We use our SED fitting tool including the effects of nebular emission, considering in particular Lya emission as a free parameter, and we demonstrate our method with simulations of mock galaxies. Using this tool we analyse a large sample of U, B, V, and i dropout galaxies with multi-band photometry. We find significant trends of the fraction of galaxies with Lya emission increasing both with redshift z and towards fainter magnitude (at fixed z), and similar trends for the Lya equivalent width. Our inferred Lya properties are in good agreement with the available spectroscopic observations and other data. These results demonstrate that the strength of Lya emission in distant star-forming galaxies can be inferred quantitatively from broad-band SED fits, at least statistically for sufficiently large samples with a good photometric coverage.
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