Lyα equivalent width distribution of Lyα emitting galaxies at redshift z ~ 4.5

Lyα line equivalent widths (EWs) provide important clues to the physical nature of highredshift Lyman alpha emitters (LAEs). However, measuring the Lyα EWdistribution 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 Lya 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 EWdistribution. 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 broadband- to-narrow-band ratio, which then gives a larger EW distribution scale length. Assuming an exponential form of the intrinsic Lyα EWdistribution dN/dEW = N exp^-EW/W0, we obtain W₀ = 167⁺⁴⁴ _-19 Å (uncorrected for IGM absorption of Lya, and s_g = 160⁺⁴³ _-12 Å 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 narrowband images for an effective and reasonably complete LAE survey. Comparing with consistent measurements at other redshifts, we see a strong evolution in Lyα EWdistributionwith redshift which goes as a power-law form of W0 ξ (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.