On the narrowband detection properties of high-redshift Lyman-alpha emitters

Context. Numerous surveys are currently underway or planned that aim to exploit the strengths of the Lyman-alpha emission line for cosmological purposes. Today, narrowband imaging surveys are frequently used as a probe of the distant universe. Aims. To investigate the reliability of the results of such high-z Ly alpha studies, and the validity of the conclusions that are based upon them. To determine whether reliable Ly alpha fluxes (F-Ly alpha) and equivalent widths (W-Ly alpha) can be estimated from narrowband imaging surveys and whether any observational biases may be present. Methods. We have developed software to simulate the observed line and continuum properties of synthetic Ly alpha galaxies in the distant universe by adopting various typical observational survey techniques. This was used to investigate how detected F-Ly alpha and W-Ly alpha vary with properties of the host galaxy or intergalactic medium: internal dust reddening; intervening Ly alpha absorption systems; the presence of underlying stellar populations. Results. None of the techniques studied are greatly susceptible to underlying stellar populations or the relative contribution of nebular gas. We find that techniques that use one off-line filter on the red side of Ly alpha result in highly inaccurate measurements of W-Ly alpha under all tests. Adopting two off-line filters to estimate continuum at Ly alpha is an improvement but is still unreliable when dust extinction is considered. Techniques that employ single narrow- and broad-band filters with the same central wavelength are not susceptible to internal dust, but Ly alpha absorption in the IGM can cause W-Ly alpha to be overestimated by factors of up to 2: at z = 6, the median W-Ly alpha is overestimated by similar to 25%. The most robust approach is a SED fitting technique that fits EB-V and burst-age from synthetic models broad-band observations are needed that sample the UV continuum slope, 2175 angstrom dust feature, and the 4000 angstrom discontinuity. We also notice a redshift-dependent incompleteness that results from DLA systems close to the target LAEs, amounting to similar to 10% at z = 6.