A CLOSE COMPARISON BETWEEN OBSERVED AND MODELED Lyα LINES FOR z ∼ 2.2 Lyα EMITTERS

We present the results of an Ly alpha profile analysis of 12 Ly alpha emitters (LAEs) at z similar to 2.2 with high-resolution Ly alpha spectra. We find that all 12 objects have an Ly alpha 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 nu(Ly alpha,r) = 174 +/- 19 km s(-1) (Delta nu(Ly alpha,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 uniform expanding shell model constructed by Verhamme et al. reproduces not only the Ly alpha 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 Ly alpha photons produced by gravitational cooling. We show that the small D nu(Ly alpha,r) values of our sample can be explained by low neutral. hydrogen column densities of log(N-H (I)) = 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 H (I) gas masses. This result suggests that low N-H (I) values, giving reduced numbers of resonant scattering of Ly alpha photons, are the key to the strong Ly alpha emission of LAEs.