New Results from the Magellan IMACS Spectroscopic Lyman Alpha Survey: NICMOS Observations of Lyman Alpha emitters at z=5.7

TL;DR

This paper presents NICMOS observations of three z=5.7 Lyman Alpha emitters, confirming two detections, analyzing their properties, and discussing implications for high-redshift galaxy surveys and selection techniques.

Contribution

First NICMOS imaging of z=5.7 LAEs confirms detections and characterizes their properties, highlighting the effectiveness of multislit narrowband spectroscopy for faint galaxy detection.

Findings

Two sources confirmed with modest equivalent widths

All three sources are typical of high-redshift LAEs

Some LAEs may be missed by i-dropout selection

Abstract

We present NICMOS J110 (rest-frame 1200-2100 A) observations of the three z=5.7 Lyman Alpha emitters discovered in the blind multislit spectroscopic survey by Martin et al. (2008). These images confirm the presence of the two sources which were previously only seen in spectroscopic observations. The third source, which is undetected in our J110 observations has been detected in narrowband imaging of the Cosmic Origins Survey (COSMOS), so our nondetection implies a rest frame equivalent width >146 Angstroms (3 sigma). The two J110-- detected sources have more modest rest frame equivalent widths of 30-40 Angstroms, but all three are typical of high-redshift LAEs. In addition, the J110- detected sources have UV luminosities that are within a factor of two of L*_{UV}, and sizes that appear compact (r_{hl} ~ 0."15) in our NIC2 images -- consistent with a redshift of 5.7. We use these UV-continuum and Lyman Alpha measurements to estimate the i-z colors of these galaxies, and show that at least one, and possibly all three would be missed by the i-dropout LBG selection. These observations help demonstrate the utility of multislit narrowband spectroscopy as a technique for finding faint emission line galaxies.