The utility of Ly α emission lines as a probe of interactions between high redshift galaxies and their environments

Abstract

The Lyman-alpha emission line is the strongest feature in the spectrum of most high redshift galaxies, and is typically observed as being highly asymmetric due to galactic outflows. Quantifying this asymmetry is challenging. Here, we explore how measurements of one parameterisation, Lyman-alpha skewness, are affected by instrumental resolution and detection signal-to-noise and thus whether this can be extended throughout the archive. We model these effects through simulated lines and apply our derived corrections to existing archival data sets (including sources observed with FORS2 and DEIMOS) to reconstruct the intrinsic line emission parameters. We find a large uncertainty in parameter reconstruction at low resolutions (R<3000) and high skew values, as well as substantial random errors resulting from the masking of sky lines. We suggest that interpretations of spectral line asymmetry should be made with caution, while a simpler parametrization, like B/R (blue-red flux asymmetry), is likely to yield more robust results. We see a possible weak trend in velocity width with mass, although there is no evidence in the data for a reliable correlation of skew with galaxy mass, star formation rate or age at z=4-5. Using our results, we investigate the possibilities of recovering emission line asymmetry with current and future instruments.