What is the physical origin of strong Lya emission? II. Gas Kinematics
  and Distribution of Lya Emitters
  We present a statistical study of velocities of Lya, interstellar (IS)
absorption, and nebular lines and gas covering fraction for Lya emitters (LAEs)
at z~2. We make a sample of 22 LAEs with a large Lya equivalent width (EW) of >
50A based on our deep Keck/LRIS observations, in conjunction with spectroscopic
data from the Subaru/FMOS program and the literature. We estimate the average
velocity offset of Lya from a systemic redshift determined with nebular lines
to be dv_Lya=234+-9 km s-1. Using a Kolmogorv-Smirnov test, we confirm the
previous claim of Hashimoto et al. (2013) that the average dv_Lya of LAEs is
smaller than that of LBGs. Our LRIS data successfully identify blue-shifted
multiple IS absorption lines in the UV continua of four LAEs on an individual
basis. The average velocity offset of IS absorption lines from a systemic
redshift is dv_IS=204+-27 km s-1, indicating LAE's gas outflow with a velocity
comparable to typical LBGs. Thus, the ratio, R^Lya_ IS = dv_Lya/dv_IS of LAEs,
is around unity, suggestive of low impacts on Lya transmission by resonant
scattering of neutral hydrogen in the IS medium. We find an anti-correlation
between Lya EW and the covering fraction, f_c, estimated from the depth of
absorption lines, where f_c is an indicator of average neutral hydrogen column
density, N_HI. The results of our study support the idea that N_HI is a key
quantity determining Lya emissivity.
