GAS MOTION STUDY OF Lyα EMITTERS AT z ∼ 2 USING FUV AND OPTICAL SPECTRAL LINES

We present the results of Magellan/MMIRS and Keck/NIRSPEC spectroscopy for five Ly alpha emitters (LAEs) at z similar or equal to 2.2 for which high-resolution FUV spectra from Magellan/MagE are available. We detect nebular emission lines including H alpha on the individual basis and low-ionization interstellar (LIS) absorption lines in a stacked FUV spectrum, and measure average offset velocities of the Ly alpha line, Delta v(Ly alpha), and LIS absorption lines, Delta v(abs), with respect to the systemic velocity defined by the nebular lines. For a sample of eight z similar to 2-3 LAEs without active galactic nucleus from our study and the literature, we obtain Delta v(Ly alpha) = 175 +/- 35 km s(-1), which is significantly smaller than that of Lyman-break Galaxies (LBGs), Delta v(Ly alpha) similar or equal to 400 km s(-1). The stacked FUV spectrum gives Delta v(abs) = -179 +/- 73 km s(-1), comparable to that of LBGs. These positive Delta v(Ly alpha) and negative Delta v(abs) suggest that LAEs also have outflows. In contrast to LBGs, however, the LAEs' Delta v(Ly alpha) is as small as |Delta v(abs)|, suggesting low neutral hydrogen column densities. Such a low column density with a small number of resonant scattering may cause the observed strong Ly alpha emission of LAEs. We find an anti-correlation between Ly alpha equivalent width (EW) and Delta v(Ly alpha) in a compilation of LAE and LBG samples. Although its physical origin is not clear, this anti-correlation result appears to challenge the hypothesis that a strong outflow, by means of a reduced number of resonant scattering, produces a large EW. If LAEs at z > 6 have similarly small Delta v(Ly alpha) values, constraints on the reionization history derived from the Ly alpha transmissivity may need to be revised.