Resolved UV and [CII] structures of luminous galaxies within the epoch of reionisation

Abstract

We present new deep ALMA and HST/WFC3 observations of MASOSA and VR7, two luminous Ly$\hat{I}\pm$ emitters (LAEs) at $z=6.5$, for which the UV continuum level differ by a factor four. No IR dust continuum emission is detected in either, indicating little amounts of obscured star formation and/or high dust temperatures. MASOSA, with a UV luminosity M${}_{1500}=-20.9$, compact size and very high Ly$\hat{I}\pm$ EW${}_0\approx 145$ A, is undetected in [CII] to a limit of L${}_{[\mathrm{CII}]}<2.2\times 10^7$ L${}_{\odot }$ implying a metallicity $Z\lesssim 0.07Z_{\odot }$. Intriguingly, our HST data indicates a red UV slope ${\hat{I}}^2=-1.1\pm 0.7$, at odds with the low dust content. VR7, which is a bright (M${}_{1500}=-22.4$) galaxy with moderate color (${\hat{I}}^2=-1.4\pm 0.3$) and Ly$\hat{I}\pm$ EW${}_0=34$ A, is clearly detected in [CII] emission (S/N=15). VR7's rest-frame UV morphology can be described by two components separated by $\approx 1.5$ kpc and is globally more compact than the [CII] emission. The global [CII]-UV ratio indicates $Z\approx 0.2Z_{\odot }$, but there are large variations in the UV-[CII] ratio on kpc scales. We also identify diffuse, possibly outflowing, [CII]-emitting gas at $\approx 100$ km s${}^{-1}$ with respect to the peak. VR7 appears assembling its components at a slightly more evolved stage than other luminous LAEs, with outflows already shaping its direct environment at $z\sim 7$. Our results further indicate that the global [CII]-UV relation steepens at SFR $<30$ M${}_{\odot }$ yr${}^{-1}$, naturally explaining why the [CII]-UV ratio is anti-correlated with Ly$\hat{I}\pm$ EW in many, but not all, observed LAEs.

Main text 19 pages, 16 figures. Appendix tables available in digital format on request. Accepted for publication by ApJ. v3: updated typo in appendix table