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

Free access article

Issue		A&A Volume 460, Number 3, December IV 2006


Page(s)		681 - 694
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20065493

A&A 460, 681-694 (2006)
DOI: 10.1051/0004-6361:20065493

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

M. Hayes and G. Östlin

Stockholm Observatory, AlbaNova University Centre, 106 91 Stockholm, Sweden
e-mail: matthew@astro.su.se

(Received 25 April 2006 / Accepted 5 September 2006)

Abstract
Context.Numerous surveys are currently underway or planned that aim to exploit the strengths of the Lyman-alpha emission line for cosmological purposes. Today, narrowband imaging surveys are frequently used as a probe of the distant universe.
Aims.To investigate the reliability of the results of such high-z Ly $\alpha$ studies, and the validity of the conclusions that are based upon them. To determine whether reliable Ly $\alpha$ fluxes ( $F_{{\rm Ly}\alpha}$ ) and equivalent widths ( $W_{{\rm Ly}\alpha}$ ) can be estimated from narrowband imaging surveys and whether any observational biases may be present.
Methods.We have developed software to simulate the observed line and continuum properties of synthetic Ly $\alpha$ galaxies in the distant universe by adopting various typical observational survey techniques. This was used to investigate how detected $F_{{\rm Ly}\alpha}$ and $W_{{\rm Ly}\alpha}$ vary with properties of the host galaxy or intergalactic medium: internal dust reddening; intervening Ly $\alpha$ absorption systems; the presence of underlying stellar populations.
Results.None of the techniques studied are greatly susceptible to underlying stellar populations or the relative contribution of nebular gas. We find that techniques that use one off-line filter on the red side of Ly $\alpha$ result in highly inaccurate measurements of $W_{{\rm Ly}\alpha}$ under all tests. Adopting two off-line filters to estimate continuum at Ly $\alpha$ 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 Ly $\alpha$ absorption in the IGM can cause $W_{{\rm Ly}\alpha}$ to be overestimated by factors of up to 2: at z=6, the median $W_{{\rm Ly}\alpha}$ 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, 2175 Å dust feature, and the 4000 Å discontinuity. We also notice a redshift-dependent incompleteness that results from DLA systems close to the target LAEs, amounting to $\sim$ 10% at z=6.

Key words: methods: observational -- cosmology: observations -- galaxies: high-redshift -- galaxies: starburst

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