2019ApJ...881..124M

We present new deep ALMA and Hubble Space Telescope (HST)/WFC3 observations of MASOSA and VR7, two luminous Lyα emitters (LAEs) at z = 6.5, for which the UV continuum levels differ by a factor of 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 M₁₅₀₀ = -20.9, compact size, and very high Lyα EW₀~145Å, is undetected in [C II] to a limit of L_[C II]_ < 2.2 x 10⁷ L_☉, implying a metallicity Z <= 0.07 Z_☉. Intriguingly, our HST data indicate a red UV slope β = -1.1 ± 0.7, at odds with the low dust content. VR7, which is a bright (M₁₅₀₀ = -22.4) galaxy with moderate color (β = -1.4 ± 0.3) and Lyα EW₀ = 34 Å, is clearly detected in [C II] emission (S/N = 15). VR7's rest-frame UV morphology can be described by two components separated by ≃1.5 kpc and is globally more compact than the [C II] emission. The global [C II]/UV ratio indicates Z ≃ 0.2 Z_☉, but there are large variations in the UV/[C II] ratio on kiloparsec scales. We also identify diffuse, possibly outflowing, [C II]-emitting gas at ≃100 km s^–1 with respect to the peak. VR7 appears to be assembling its components at a slightly more evolved stage than other luminous LAEs, with outflows already shaping its direct environment at z ∼ 7. Our results further indicate that the global [C II]-UV relation steepens at SFR < 30 M_☉ yr^–1, naturally explaining why the [C II]/UV ratio is anticorrelated with Lyα EW in many, but not all, observed LAEs.