Studying [C II] 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 Ly α emitting galaxies at z ~ 7. Our targets are ultraviolet (UV) faint systems with stellar masses on the order of M_* ~ 10⁹ M_⊙. We detect a single [C II] line emission (4σ) from the brightest (L ~ 2.4 × 10¹⁰L_⊙) galaxy in our sample, MACS0454-1251. We determine a systemic redshift (z_{[C II]} = 6.3151 ± 0.0005) for MACS0454-1251 and measure a Ly α velocity offset of $\Delta v \approx 300 \pm 70 \rm {km\, s}^{-1}$. The remaining two galaxies we detect no [C II] but provide 3σ upper limits on their [C II] line luminosities which we use to investigate the $L_{\textrm {[CII]}} - \rm {SFR}$ relation. Overall our single [C II] detection shows agreement with the relation for local dwarf galaxies. Our [C II] deficient galaxies could potentially be exhibiting low metallicities (Z < Z_⊙). Another possible explanation for weaker [C II] 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 (M_dust) range from <4.8 × 10⁷ M_⊙ to 2.3 × 10⁸ M_⊙. Collectively, our results suggest faint reionization era sources could be metal poor and/or could have strong feedback suppressing [C II] emission.