Alma Reveals Metals yet No Dust within Multiple Components in Cr7

We present spectroscopic follow-up observations of CR7 with ALMA, targeted at constraining the infrared (IR) continuum and [Cii]158μm line-emission at high spatial resolution matched to the HST/WFC3 imaging. CR7 is a luminous Lyα emitting galaxy at z = 6.6 that consists of three separated UV-continuum components. Our observations reveal several well-separated components of [Cii] emission. The two most luminous components in [Cii] coincide with the brightest UV components (A and B), blue-shifted by ≈ 150 km s−1 with respect to the peak of Lyα emission. Other [Cii] components are observed close to UV clumps B and C and are blue-shifted by ≈ 300 and ≈ 80 km s−1 with respect to the systemic redshift. We do not detect FIR continuum emission due to dust with a 3σ limiting luminosity LIR(Td = 35 K) < 3.1 × 10 L . This allows us to mitigate uncertainties in the dust-corrected SFR and derive SFRs for the three UV clumps A, B and C of 29, 6 and 7 M yr −1. All clumps have [Cii] luminosities consistent within the scatter observed in the local relation between SFR and L[CII], implying that strong Lyα emission does not necessarily anti-correlate with [Cii] luminosity. Combining our measurements with the literature, we show that galaxies with blue UV slopes have weaker [Cii] emission at fixed SFR, potentially due to their lower metallicities and/or higher photoionisation. Comparison with hydrodynamical simulations suggests that CR7’s clumps have metallicities of 0.1 < Z/Z < 0.2. The observed ISM structure of CR7 indicates that we are likely witnessing the build up of a central galaxy in the early Universe through complex accretion of satellites.