TRACING THE REIONIZATION EPOCH WITH ALMA: [C II] EMISSION IN z similar to 7 GALAXIES
Résumé
We present new results on [C II] 158 mu 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 Ly alpha emission line in their spectra. The Ly alpha emission line is redshifted by 100-200 km s(-1) compared to the systemic redshift given by the [C II] line. These velocity offsets are smaller than what is observed in z similar to 3 Lyman break galaxies (LBGs) with similar UV luminosities and emission line properties. Smaller velocity shifts reduce the visibility of Ly alpha and hence somewhat alleviate the need for a very neutral intergalactic medium at z similar to 7 to explain the drop in the fraction of Ly alpha emitters observed at this epoch. The galaxies show [C II] emission with L[C II] = 0.6-1.6 x 10(8)L(circle dot): 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 z similar to 7 galaxies with similar or smaller SFRs are due to selection effects: previous targets were mostly strong Ly alpha emitters and therefore probably metal-poor systems, while our galaxies are more representative of the general high-redshift star-forming population.