Studying [CII] Emission in Low-mass Galaxies at z ~ 7

Abstract

We report on a ${[\mathrm{CII}]}_{158\hat{\mathrm{I}}¼\mathrm{m}}$ search using the Atacama Large Millimeter/submillimeter Array (ALMA) on three lensed, confirmed {\lya} emitting galaxies at $z\sim 7$. Our targets are ultra-violet (UV) faint systems with stellar masses on the order of $M_{\ast }\sim 10^9{M}_{\odot }$. We detect a single [CII] line emission ($4\ddot{I}ƒ$) from the brightest ($L\sim 2.4\times 10^{10}{L}_{\odot }$) galaxy in our sample, MACS0454-1251. We determine a systemic redshift ($z_{[\mathrm{CII}]}=6.3151\pm 0.0005$) for MACS0454-1251 and measure a {\lya} velocity offset of $\hat{I}”v\approx 300\pm 70{\mathrm{km}\mathrm{s}}^{-1}$. The remaining two galaxies we detect no {\ct} but provide $3\ddot{I}ƒ$ upper limits on their {\ct} line luminosities which we use to investigate the $L_{\text{[CII]}}-\mathrm{SFR}$ relation. Overall our single {\ct} detection shows agreement with the relation for dwarf and local starburst galaxies. Our [CII] deficient galaxies could potentially be exhibiting low metallicities ($Z<Z_{\odot }$). 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 $T_d=35\mathrm{K}$ dust masses ($M_{\mathrm{dust}}$) range from $<4.8\times 10^7{M}_{\odot }$ to $2.3\times 10^8{M}_{\odot }$. Collectively, our results suggest faint reionization era sources could be metal poor and/or could have strong feedback suppressing [CII] emission.

7 pages, 4 figures accepted for publication to MNRAS