Lyα Equivalent Width Distribution at Redshift z $\sim$ 4.5 [CEA]

Ly{\alpha} line equivalent widths (EWs) provide important clues to the physical nature of high redshift Lyman alpha emitters (LAEs). However, measuring the Ly{\alpha} EW distribution of high-z narrowband selected LAEs can be hard because many sources do not have broadband photometry. We investigate the possible biases in measuring the intrinsic Ly{\alpha} EW distribution for a LAE sample at z $\sim$ 4.5 in the Extended Chandra Deep Field South (ECDFS). Only weak Malmquist-type bias in both the intrinsic Ly{\alpha} luminosity function and the Ly{\alpha} EW distribution were found. However, the observed EW distribution is severely biased if one only considers LAEs with detections in the continuum. Taking the broadband non-detections into account requires fitting the distribution of the broadband-to-narrowband ratio, which then gives a larger EW distribution scale length. Assuming an exponential form of the intrinsic Ly{\alpha} EW distribution dN/dEW = N exp(-EW/W$_0$), we obtain W$_0$ = 167+/-44{\AA} (uncorrected for IGM absorption of Ly{\alpha}). We discuss the likely range of IGM absorption effects in light of recent measurements of Ly{\alpha} line profiles and velocity offsets. Our data are consistent with Ly{\alpha} 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 magnitude 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 Ly{\alpha} EW distribution with redshift which goes as a power-law form of W$_0$ $\prop$(1+z)$^${\xi}, with {\xi} = 1.7$\pm$0.1 if no IGM corrections are applied to the Ly{\alpha} line; or {\xi} = 2.2$\pm$0.1 after applying a maximal IGM-absorption correction to Ly{\alpha} line) from z = 0.3 to 6.5.

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Date added: Mon, 21 Oct 13