A Close Relationship between Lyα and Mg ii in Green Pea Galaxies* †

Abstract

The Mg ii λλ 2796, 2803 doublet is often used to measure interstellar medium absorption in galaxies, thereby serving as a diagnostic for feedback and outflows. However, the interpretation of Mg ii remains confusing, due to resonant trapping and re-emission of the photons, analogous to Ly α . Therefore, in this paper, we present new MMT Blue Channel Spectrograph observations of Mg ii for a sample of 10 Green Pea galaxies at z  ∼ 0.2–0.3, where Ly α was previously observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope . With strong, (mostly) double-peaked Ly α profiles, these galaxies allow us to observe Mg ii in the limit of low H i column density. We find strong Mg ii emission and little-to-no absorption. We use photoionization models to show that nebular Mg ii from H ii regions is non-negligible, and the ratios of Mg ii λλ 2796, 2803/[O iii ] λ 5007 versus [O iii ] λ 5007/[O ii ] λ 3727 form a tight sequence. Using this relation, we predict intrinsic Mg ii flux, and show that Mg ii escape fractions range from 0 to 0.9. We find that the Mg ii escape fraction correlates tightly with the Ly α escape fraction, and the Mg ii line profiles show evidence for broader and more redshifted emission when the escape fractions are low. These trends are expected if the escape fractions and velocity profiles of Ly α and Mg ii are shaped by resonant scattering in the same low column density gas. As a consequence of a close relation with Ly α , Mg ii may serve as a useful diagnostic in the epoch of reionization, where Ly α and Lyman continuum photons are not easily observed.