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 [CII]${}_{158\mu \mathrm{m}}$ line-emission at high spatial resolution matched to the HST/WFC3 imaging. CR7 is a luminous Ly$\alpha$ 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 $\approx 150$ km s${}^{-1}$ with respect to the peak of Ly$\alpha$ emission. Other [CII] components are observed close to UV clumps B and C and are blue-shifted by $\approx 300$ and $\approx 80$ km s${}^{-1}$ with respect to the systemic redshift. We do not detect FIR continuum emission due to dust with a 3$\sigma$ limiting luminosity L${}_{\mathrm{IR}}(T_d=35\mathrm{K})<3.1\times 10^{10}$ L${}_{\odot }$. 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${}_{\odot }$ yr${}^{-1}$. All clumps have [CII] luminosities consistent within the scatter observed in the local relation between SFR and L${}_{[\mathrm{CII}]}$, implying that strong Ly$\alpha$ 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 $0.1<Z/{\mathrm{Z}}_{\odot }<0.2$. 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.

17 pages, accepted for publication in ApJ

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