New results from the Magellan IMACS spectroscopic Ly[alpha] survey: NICMOS observations of Ly[alpha] emitters at z = 5.7

Abstract

We present NICMOS J[subscript 110] (rest-frame 1200–2100 [Angstrom]) observations of the three z = 5.7 Ly[alpha] emitters discovered in the blind multislit spectroscopic survey by Martin et al. These images confirm the presence of the two sources that were previously only seen in spectroscopic observations. The third source, which is undetected in our J[subscript 110] observations, has been detected in narrowband imaging of the Cosmic Origins Survey, so our nondetection implies a rest-frame equivalent width [greater than] 146 [Angstrom] (3[sigma]). The two J[subscript 110]-detected sources have more modest rest-frame equivalent widths of 30–40 [Angstrom], but all three are typical of high-redshift Ly[alpha] emitters. In addition, the J[subscript 110]-detected sources have UV luminosities that are within a factor of 2 of L*[subscript UV], and sizes that appear compact (r[subscript hl] ∼ 0".15) in our NIC2 images—consistent with a redshift of 5.7. We use these UV-continuum and Ly[alpha] measurements to estimate the i[subscript 775]–z[subscript 850] colors of these galaxies and show that at least one and possibly all three would be missed by the i-dropout Lyman break galaxy selection. These observations help demonstrate the utility of multislit narrowband spectroscopy as a technique for finding faint emission-line galaxies.