A Close Comparison between Observed and Modeled Ly{\alpha} Lines for z ~ 2.2 Lyman Alpha Emitters

TL;DR

This study analyzes high-resolution Lya spectra of 12 z=2.2 Lyman Alpha Emitters, comparing observed profiles with models to understand their gas properties and outflows, revealing low neutral hydrogen columns as key to their strong emission.

Contribution

The paper provides the first detailed comparison of observed and modeled Lya profiles for z~2.2 LAEs, highlighting the role of low NHI in their emission.

Findings

All LAEs show redward main peaks with some having blue bumps.

Average velocity offset of main peak is ~174 km/s, smaller than LBGs.

Low NHI (~10^18.9 cm^-2) explains strong Lya emission.

Abstract

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.