FILTER-INDUCED BIAS IN Lyα EMITTER SURVEYS: A COMPARISON BETWEEN STANDARD AND TUNABLE FILTERS. GRAN TELESCOPIO CANARIAS PRELIMINARY RESULTS

J A De Diego,A Bongiovanni,T Verdugo,M Sánchez-Portal,J Cepa,J I González-Serrano,M A De Leo

doi:10.1088/0004-6256/146/4/96

Abstract

Lyman-alpha emitter (LAE) surveys have successfully used the excess in a narrow-band filter compared to a nearby broad-band image to find candidates. However, the odd spectral energy distribution (SED) of LAEs combined with the instrumental profile have important effects on the properties of the candidate samples extracted from these surveys. We investigate the effect of the bandpass width and the transmission profile of the narrow-band filters used for extracting LAE candidates at redshifts z ~ 6.5 through Monte Carlo simulations, and we present pilot observations to test the performance of tunable filters to find LAEs and other emission-line candidates. We compare the samples obtained using a narrow ideal-rectangular-filter, the Subaru NB921 narrow-band filter, and sweeping across a wavelength range using the ultra-narrow-band tunable filters of the instrument OSIRIS, installed at the 10.4m Gran Telescopio Canarias. We use this instrument for extracting LAE candidates from a small set of real observations. Broad-band data from the Subaru, HST and Spitzer databases were used for fiting SEDs to calculate photometric redshifts and to identify interlopers. Narrow-band surveys are very efficient to find LAEs in large sky areas, but the samples obtained are not evenly distributed in redshift along the filter bandpass, and the number of LAEs with equivalent widths < 60 angstroms can be underestimated. These biased results do not appear in samples obtained using ultra-narrow-band tunable filters. However, the field size of tunable filters is restricted because of the variation of the effective wavelength across the image. Thus narrow-band and ultra-narrow-band surveys are complementary strategies to investigate high-redshift LAEs.

Highlights

The odd spectral energy distribution (SED) of Lyα emitter galaxies (LAEs) convolved with the instrumental profile can introduce unsought biases on the characteristics of the candidate samples extracted from photometric surveys
Note that the detection of these objects will depend on the line parameters, and that the recorded signal will be diminished by the loss of the line flux outside the filter window, and these objects tend to have larger equivalent width (EW) that make detections easier
We have shown that the asymmetrical profile of the continuum around the Lyα line in LAEs yields a detection bias that affects the ability to find objects at redshifts where the Lyα line lies near the long-wavelength edge of narrowband photometric filters

Summary

Introduction

The odd spectral energy distribution (SED) of Lyα emitter galaxies (LAEs) convolved with the instrumental profile can introduce unsought biases on the characteristics of the candidate samples extracted from photometric surveys. LAE candidates are usually selected by an excess in a narrowband filter compared to a nearby broadband image (Cowie & Hu 1998) The latter technique is routinely used to find LAE candidates in the Subaru Deep Field (Taniguchi et al 2005; Kashikawa et al 2006, 2011) and the Subaru/ XMM-Newton Deep Survey Field (Ota et al 2010; Ouchi et al 2010; Shibuya et al 2012). An important result of the narrowband surveys is that the luminosity function (LF) of LAEs remains constant between redshifts 3.0 z 5.7, but evolves dramatically between 5.7 z 6.5, and maybe beyond (Pentericci et al 2011; Hibon et al 2012; Ota & Iye 2012)

Results

Discussion

Conclusion