High-redshift Ly$\mathsf{\alpha}$ emitters with a large equivalent width

Abstract

We report new follow-up spectroscopy of i -dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed i -dropout selection method is expected to select galaxies with large equivalent-width Lyα emission over a wide redshift range, 6.0 < ∼ z < ∼ 6.5. Two of four observed targets show a strong emission line with a clear asymmetric profile, identified as Lyα emitters at z = 6.11 and 6.00. Their rest-frame equivalent widths are 153 Å and 114 Å, which are lower limits on the intrinsic equivalent widths. Through our spectroscopic observations (including previous ones) of NB921-depressed i -dropout galaxies, we identified 5 galaxies in total with a rest-frame equivalent width larger than 100 Å at 6.0 < ∼ z < ∼ 6.5 out of 8 photometric candidates, which suggests that the NB921-depressed i -dropout selection method is possibly an efficient way to search for Lyα emitters with a large Lyα equivalent width, in a wider redshift range than with usual narrow-band excess techniques. By combining these findings with our previous observational results, we infer that the fraction of broad-band selected galaxies having a rest-frame equivalent width larger than 100 Å is significantly higher at z ∼ 6 (the cosmic age of ∼1 Gyr) than that at z ∼ 3 (∼2 Gyr), being consistent with the idea that the typical stellar population of galaxies is significantly younger at z ∼ 6 than that at z ∼ 3. The NB921-depressed i -dropout galaxies may be interesting candidates for hosts of massive, zero-metallicity Population III stars.

KECK SPECTROSCOPY OF LYMAN-BREAK GALAXIES AND ITS IMPLICATIONS FOR THE UV-CONTINUUM AND Lyα LUMINOSITY FUNCTIONS ATz> 6

“…The median value of the EWs is 50 Å, with 4 galaxies having EWs higher than 100 Å. They are on average smaller than those in the bright sample of Nagao et al (2004Nagao et al ( , 2005Nagao et al ( , 2007, whose Lyα EWs are in the range of 90-240 Å.…”

“…The SDF public data have a depth of B = 28.45, V = 27.74, R = 27.80, i = 27.43, NB921 = 26.54, and z = 26.62 (3σ detection for point sources), covering an effective area of ∼876 arcmin 2 . Nagao et al (2004Nagao et al ( , 2005Nagao et al ( , 2007 have used the public data to find bright LBGs down to z ∼ 26.1. Recently the SDF team has obtained a much deeper z -band image with a total integration time of ∼29 hr and a depth of ∼27.5 mag (Poznanski et al 2007;Richmond et al 2009).…”

“…The color cut is more stringent than i − z > 1.5 used by Nagao et al (2004Nagao et al ( , 2005Nagao et al ( , 2007. This is to reduce the number of contaminants scattered into the selection region due to large i -band photometric errors.…”

“…SDF has also found a sample of bright LBG candidates at z > 6 (Shimasaku et al 2005) up to z > 7 (Ouchi et al 2009). Five strong LAEs from a list of i -dropout objects (or LBG candidates) have already been spectroscopically confirmed at 6 < z < 6.4 (Nagao et al 2004(Nagao et al , 2007. Note that the difference between LAEs and LBGs is somewhat arbitrary and there is no clear separation line between them, so we simply call the galaxies found by the narrowband technique LAEs and those found by the dropout technique LBGs.…”

“…This image allows us to select candidates down to z = 27 mag, roughly 1 mag deeper than the LBGs found by Nagao et al (2004Nagao et al ( , 2005Nagao et al ( , 2007. The structure of the paper is as follows.…”