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Recovering the systemic redshift of galaxies from their Lyman alpha line profile

A. Verhamme, T. Garel, E. Ventou, N. Bouché, E. C. Herenz, J. Richard, R. Bacon, K. B. Schmidt, M. V. Maseda, R. A. Marino, J. Brinchmann, S. Cantalupo, J. Caruana, B. Clément, C. Diener, A. Drake, T. Hashimoto, H. Inami, J. Kerutt, W. Kollatschny, F. Leclercq, V. Patrício, J. Schaye, L. Wisotzki, J. Zabl

Abstract

The Lyman alpha (Ly α) line of Hydrogen is a prominent feature in the spectra of star-forming galaxies, usually redshifted by a few hundreds of km s−1 compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics, and correlations with quasar absorption lines when only Ly α is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Ly α line profile. We use spectroscopic observations of Ly α emitters for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≈ 3 and z ≈ 6 as part of various multi-unit spectroscopic explorer guaranteed time observations. We also include a compilation of spectroscopic Ly α data from the literature spanning a wide redshift range (z ≈ 0–8). First, restricting our analysis to double-peaked Ly α spectra, we find a tight correlation between the velocity offset of the red peak with respect to the systemic redshift, Vredpeak⁠, and the separation of the peaks. Secondly, we find a correlation between Vredpeak and the full width at half-maximum of the Ly α line. Fitting formulas to estimate systemic redshifts of galaxies with an accuracy of ≤100 km s−1, when only the Ly α emission line is available, are given for the two methods.

Keywords
galaxies: high-redshift; galaxies: starburst; galaxies: statistics; ultraviolet: galaxies

Monthly Notices of the Royal Astronomical Society: Letters
Volume 478, Number 1, Page L60
2018 April

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia