Studying [CII] emission in low-mass galaxies at z ∼ 7

Abstract We report on a $\rm {[CII]}_{158\mu \rm {m}}$ search using the Atacama Large Millimeter/submillimeter Array (ALMA) on three lensed, confirmed Lyα emitting galaxies at z ∼ 7. Our targets are ultra-violet (UV) faint systems with stellar masses on the order of M* ∼ 109M⊙. We detect a single [CII] line emission (4σ) from the brightest (L ∼ 2.4 × 1010L⊙) galaxy in our sample, MACS0454-1251. We determine a systemic redshift (z[CII] = 6.3151 ± 0.0005) for MACS0454-1251 and measure a Lyα velocity offset of $\Delta v \approx 300 \pm 70 \rm {km\, s}^{-1}$. The remaining two galaxies we detect no [CII] but provide 3σ upper limits on their [CII] line luminosities which we use to investigate the $L_{\textrm {[CII]}} - \rm {SFR}$ relation. Overall our single [CII] detection shows agreement with the relation for local dwarf galaxies. Our [CII] deficient galaxies could potentially be exhibiting low metallicities (Z < Z⊙). Another possible explanation for weaker [CII] emission could be strong feedback from star formation disrupting molecular clouds. We do not detect continuum emission in any of the sources, placing upper limits on their dust masses. Assuming a single dust temperature of $T_{d}=35 \rm {K}$ dust masses (Mdust) range from <4.8 × 107M⊙ to 2.3 × 108M⊙. Collectively, our results suggest faint reionization era sources could be metal poor and/or could have strong feedback suppressing [CII] emission.