A Close Comparison between Observed and Modeled Ly{\alpha} Lines for z ~
  2.2 Lyman Alpha Emitters
  We present the results of a Lya profile analysis of 12 Lya emitters (LAEs) at
z = 2.2 with high-resolution Lya spectra. We find that all 12 objects have a
Lya profile with the main peak redward of the systemic redshift defined by
nebular lines, and five have a weak, secondary peak blueward of the systemic
redshift (blue bump). The average velocity offset of the red main peak (the
blue bump, if any) with respect to the systemic redshift is Delta_v_Lya,r =
174+/- 19 km s-1 (Delta_v_Lya,b = -316+/-45 km s-1), which is smaller than
(comparable to) that of Lyman-break galaxies (LBGs). The outflow velocities
inferred from metal absorption lines in three individual and one stacked
spectra are comparable to those of LBGs. The expanding shell model constructed
by Verhamme et al. (2006) reproduces not only the Lya profiles but also other
observed quantities including the outflow velocity and the FWHM of nebular
lines for the non-blue bump objects. On the other hand, the model predicts too
high FWHMs of nebular lines for the blue bump objects, although this
discrepancy may disappear if we introduce additional Lya photons produced by
gravitational cooling. We show that the small Delta_v_Lya,r values of our
sample can be explained by low neutral-hydrogen column densities of log(NHI) =
18.9 cm-2 on average. This value is more than one order of magnitude lower than
those of LBGs but is consistent with recent findings that LAEs have high
ionization parameters and low Hi gas masses. This result suggests that low NHI
values, giving reduced numbers of resonant scattering of Lya photons, are the
key to the strong Lya emission of LAEs.
