Tracing the Reionization Epoch with ALMA: [C II] Emission in z ~ 7 Galaxies

Abstract

We present new results on [C II]158 μ {{m}} emission from four galaxies in the reionization epoch. These galaxies were previously confirmed to be at redshifts between 6.6 and 7.15 from the presence of the Lyα emission line in their spectra. The Lyα emission line is redshifted by 100-200 km s<SUP>-1</SUP> compared to the systemic redshift given by the [C II] line. These velocity offsets are smaller than what is observed in z̃ 3 Lyman break galaxies (LBGs) with similar UV luminosities and emission line properties. Smaller velocity shifts reduce the visibility of Lyα and hence somewhat alleviate the need for a very neutral intergalactic medium at z̃ 7 to explain the drop in the fraction of Lyα emitters observed at this epoch. The galaxies show [C II] emission with L[C II] = 0.6 - 1.6× {10}<SUP>8</SUP>{L}<SUB>☉ </SUB>: these luminosities place them consistently below the star formation rate (SFR)-L[C II] relation observed for low-redshift star-forming and metal-poor galaxies and also below z = 5.5 LBGs with similar SFRs. We argue that previous undetections of [C II] in z̃ 7 galaxies with similar or smaller SFRs are due to selection effects: previous targets were mostly strong Lyα emitters and therefore probably metal-poor systems, while our galaxies are more representative of the general high-redshift star-forming population.