ALMA Reveals Metals yet No Dust within Multiple Components in CR7

Abstract

We present spectroscopic follow-up observations of CR7 with ALMA, targeted at constraining the infrared (IR) continuum and [C II]_(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 [C II] emission. The two most luminous components in [C II] coincide with the brightest UV components (A and B), blueshifted by ≈150 km s^(−1) with respect to the peak of Lyα emission. Other [C II] components are observed close to UV clumps B and C and are blueshifted 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 L_(IR)(T_d = 35 K) < 3.1 x 10^(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 28, 5, and 7 M⊙ yr^(−1). All clumps have [C II] luminosities consistent within the scatter observed in the local relation between SFR and L_([C II]), implying that strong Lyα emission does not necessarily anti-correlate with [C II] luminosity. Combining our measurements with the literature, we show that galaxies with blue UV slopes have weaker [C II] emission at fixed SFR, potentially due to their lower metallicities and/or higher photoionization. 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.

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