2014MNRAS.439.1101Z

Lyα line equivalent widths (EWs) provide important clues to the physical nature of high-redshift Lyman alpha emitters (LAEs). However, measuring the Lyα EW distribution of high-z narrow-band-selected LAEs can be hard because many sources do not have well-measured broad-band photometry. We investigate the possible biases in measuring the intrinsic Lyα EW distribution for a LAE sample at z ∼ 4.5 in the Extended Chandra Deep Field South (ECDFS). We show that our source selection procedures produce only weak Eddington type bias in both the intrinsic Lyα luminosity function and the Lyα EW distribution. However, the observed EW distribution is severely biased if one only considers LAEs with detections in the continuum. Taking the broad-band non-detections into account requires fitting the distribution of the broad-band-to-narrow-band ratio, which then gives a larger EW distribution scale length. Assuming an exponential form of the intrinsic Lyα EW distribution dN/dEW = N exp^-EW/W_0, we obtain W0 = 167^+44_-19Å (uncorrected for IGM absorption of Lyα, and σg = 160^+43_-12Å for a Gaussian EW distribution). We discuss the likely range of IGM absorption effects in light of recent measurements of Lyα line profiles and velocity offsets. Our data are consistent with Lyα EW being independent of UV luminosity (i.e. we do not see evidence for the `Ando' effect). Our simulations also imply that broad-band images should be 0.5-1 mag deeper than narrow-band images for an effective and reasonably complete LAE survey. Comparing with consistent measurements at other redshifts, we see a strong evolution in Lyα EW distribution with redshift which goes as a power-law form of W0 ∝ (1 + z)ξ, with ξ = 1.1±0.1 (0.6±0.1) if no IGM corrections are applied to the Lyα line; or ξ = 1.7±0.1 (1.2±0.1) after applying a maximal IGM-absorption correction to Lyα line for an exponential (a Gaussian) EW distribution from z = 0.3 to 6.5.