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

Abstract

Numerous surveys are underway or planned that aim to exploit the Lyman-alpha emission line as a probe of the distant universe. We investigate the reliability of such high-z Lya studies and the validity of the conclusions that are based upon them. We examine whether reliable Lya fluxes (F_Lya) and equivalent widths (W_Lya) can be estimated from narrowband imaging surveys and whether any observational biases may be present. We have developed software to simulate the observed line and continuum properties of synthetic LAEs in the distant universe by adopting various typical observational techniques. This was used to investigate how detected F_Lya and W_Lya vary with properties of the host or IGM: internal dust reddening; intervening Lya absorption systems; the presence of underlying stellar populations. None of the techniques studied are greatly susceptible to underlying stellar populations or the contribution of nebular gas. We find that techniques that use one off-line filter on the red side of Lya result in highly inaccurate measurements of W_Lya. Adopting two off-line filters to estimate continuum at Lya 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 IGM Lya absorption can cause W_Lya to be overestimated by factors of up to 2: at z=6, the median W_Lya is overestimated by ~25%. The most robust approach is a SED fitting technique that fits E(B-V) and burst-age from synthetic models - broadband observations are needed that sample the UV continuum slope, 2175AA dust feature, and the 4000AA discontinuity. We also notice a redshift-dependent incompleteness that results from DLA systems close to the target LAEs, amounting to ~10% at z=6.