Tracing the reionization epoch with ALMA: [CII] emission in z~7 galaxies

TL;DR

This study reports new [CII] emission measurements from four z~7 galaxies, revealing smaller Ly$ ext{α}$ velocity offsets and providing insights into the epoch of reionization and galaxy properties at high redshift.

Contribution

First detection of [CII] emission in a representative sample of z~7 galaxies, clarifying the relationship between [CII] luminosity, star formation, and selection effects at high redshift.

Findings

Smaller Ly$ ext{α}$ velocity offsets at z~7 compared to z~3.

[CII] luminosities below low-redshift relations, indicating different ISM conditions.

Selection effects influence previous non-detections of [CII] in high-redshift galaxies.

Abstract

We present new results on [CII]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 compared to the systemic redshift given by the [CII] line. These velocity offsets are smaller than what is observed in z~3 Lyman break galaxies 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 IGM at z~7 to explain the drop in the fraction of Ly$\alpha$ emitters observed at this epoch. The galaxies show [CII] emission with L[CII]=0.6-1.6 x10$^8 L_\odot$: these luminosities place them consistently below the SFR-L[CII] relation observed for low redshift star forming and metal poor galaxies and also below z =5.5 Lyman break galaxies with similar star formation rates. We argue that previous undetections of [CII] in z~7 galaxies with similar or smaller star formation rates 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 .