NOEMA probes the [CII] and dust content in a 2175Å UV Bump Galaxy at $z=7.1$

The detection of the $2175$Å UV bump at $z>6$ challenges existing models of dust formation, suggesting rapid formation of small carbonaceous dust grains within the first billion years of cosmic time. We present the results of the first direct attempt at linking far-infrared (FIR) observations to the UV bump within the Epoch of Reionisation (EoR), through NOEMA observations of GNWY-7379420231 at $z=7.108$, a galaxy exhibiting the strongest known UV bump feature at $z>4$. We detect the [CII] 158$\mu$m emission line at 5.1$\sigma$ at $z_{[\mathrm{C}\mathrm{I}\mathrm{I}]}=7.1078\pm 0.0005$, in excellent agreement with the redshift derived from the [OIII] $\lambda 5007$Å line. The {\cii} luminosity implies ${\mathrm{S}\mathrm{F}\mathrm{R}}_{[\mathrm{C}\mathrm{I}\mathrm{I}]}=16.2_{-5.5}^{+5.8}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$, consistent with short timescale SFR tracers such as dust corrected ${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha }=18.0\pm 3.9M_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$ and ${\mathrm{S}\mathrm{F}\mathrm{R}}_{10\mathrm{M}\mathrm{y}\mathrm{r}}=20.5_{-5.0}^{+3.8}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$ from SED fitting. The dust continuum is not detected suggesting an obscured SFR of ${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{I}\mathrm{R}}<32M_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$ and a dust mass of $M_{\mathrm{d}}<5.3\times {10}^6{M}_{\odot }$ $(M_{\mathrm{d}}\mathrm{/}{M}_{\star }<2\mathrm{\%})$. Finally, the [CII]-derived dynamical mass of ${\log }_{10}(M_{\mathrm{d}\mathrm{y}\mathrm{n}}\mathrm{/}{M}_{\odot })=8.95_{-0.66}^{+0.51}$ and stellar mass of ${\log }_{10}\left({\mathrm{M}}_{\star }\mathrm{/}{\mathrm{M}}_{\odot }\right)=8.39_{-0.09}^{+0.13}$ suggest a gas-rich moderately massive galaxy. Taken together these results rule out GNWY-7379420231 being a heavily dust obscured or massive galaxy, but rather a `normal’ EoR galaxy with a recent upturn in star-formation. Our results suggest efficient shattering of larger dust grains in the diffuse, turbulent ISM and/or fortunate line of sight alignment are needed to explain the UV bump properties of GNWY-7379420231.