An Empirical Study of the Relationship between Ly{\alpha} and UV selected Galaxies: Do Theorists and Observers `Select' the Same Objects?

TL;DR

This study examines how observational and theoretical methods of selecting Lyman Alpha Emitters (LAEs) differ, revealing that simple luminosity and equivalent width cuts may not accurately replicate observational selection, affecting predicted LAE densities.

Contribution

The paper develops an empirical model linking UV and Ly{ extalpha} luminosities to better understand LAE selection and highlights discrepancies between models and observations.

Findings

Model overpredicts LAE luminosity functions by 2-3 times.

Simple selection cuts do not accurately mimic observational criteria.

Predicted LAE evolution at high redshift remains consistent.

Abstract

Lyman Alpha Emitters (LAEs) are galaxies that have been selected on the basis of a strong Ly{\alpha} emission line in their spectra. Observational campaigns have dramatically increased the sample of known LAEs, which now extends out to z=7. These discoveries have motivated numerous theoretical studies on the subject, which usually define LAEs in their models based on sharp Ly{\alpha} luminosity and equivalent width (EW) cuts. While broadly representative, this procedure does not mimic the selection from observational programs in detail, which instead use cuts in various colour-spaces. We investigate what implications this disjoint may have for studies that aim to model LAEs. We construct an empirical model for the number density of star forming galaxies as a function of their UV and Ly{\alpha} luminosity, utilising measured constraints on the luminosity functions (LFs) of drop-out galaxies, and their luminosity dependent probability distribution function of Ly{\alpha} EW. In particular, we investigate whether the LAE LFs can be reproduced by defining LAEs using a (z-dependent) Ly{\alpha} luminosity and EW threshold. While we are able to reproduce the observed distribution of Ly{\alpha} EW among LAEs out to restframe EW 200 A, we find that our formalism over-predicts both the UV and Ly{\alpha} LFs of LAEs by a factor of 2-3, and is inconsistent with observations at the ~95% level. This tension is partially resolved if we assume the Ly{\alpha} EW-distribution of drop-out galaxies to be truncated at restframe EW>150 A. However the overprediction indicates that modeling LAEs with simple REW and luminosity cuts does not accurately mimic observed selection criteria, and can lead to uncertainties in the predicted number density of LAEs. On the other hand, the predicted z-evolution is not affected. We apply our formalism to drop-out galaxies at z>6, and predict the LFs of LAEs at z=7-9.