Gas Motion Study of Lya Emitters at z~2 Using UV and Optical Spectral Lines
Abstract
We present the results of Magellan/MMIRS and Keck/NIRSPEC spectroscopy for five Lya emitters (LAEs) at z=2.2 for which high-resolution FUV spectra from Magellan/MagE are available. We detect nebular emission lines including Ha on the individual basis and low-ionization interstellar (LIS) absorption lines in a stacked FUV spectrum, and measure average offset velocities of the Lya line, DeltavLya, and LIS absorption lines, Deltavabs, with respect to the systemic velocity defined by the nebular lines. For a sample of eight z~2-3 LAEs without AGN from our study and the literature, we obtain DeltavLya = 175+/-35 km s-1, which is significantly smaller than that of Lyman-break Galaxies (LBGs), DeltavLya=400 km s-1. The stacked FUV spectrum gives Deltavabs = -179 +/- 73 km s-1, comparable to that of LBGs. These positive DeltavLya and negative Deltavabs suggest that LAEs also have outflows. In contrast to LBGs, however, the LAEs' DeltavLya is as small as |Deltavabs|, suggesting low neutral hydrogen column densities. Such a low column density with a small number of resonant scattering may cause the observed strong Lya emission of LAEs. We find an anti-correlation between Lya equivalent width (EW) and DeltavLya 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 DeltavLya values, constraints on the reionization history derived from the Lya transmissivity may need to be revised.
Turn this paper into a lesson
ArcXiv compiles a structured reading guide from this paper's metadata: plain-English importance, contributions, prerequisite concepts, which sections to read first, flashcards, and a quiz. Grounded in the abstract, never invented.