An Intensely Star-forming Galaxy at z ~ 7 with Low Dust and Metal Content Revealed by Deep ALMA and HST Observations

We report deep ALMA observations complemented by associated Hubble Space Telescope (HST) imaging for a luminous (m_(UV) = 25) galaxy, "Himiko," at a redshift of z = 6.595. The galaxy is remarkable for its high star formation rate, 100 M_☉ yr^(−1), which has been securely estimated from our deep HST and Spitzer photometry, and the absence of any evidence for strong active galactic nucleus activity or gravitational lensing magnification. Our ALMA observations probe an order of magnitude deeper than previous IRAM observations, yet fail to detect a 1.2 mm dust continuum, indicating a flux of <52 μJy, which is comparable to or weaker than that of local dwarf irregulars with much lower star formation rates. We likewise provide a strong upper limit for the flux of [C II] 158 μm, L_([C II]) < 5.4 x 10^7 L_☉, which is a diagnostic of the hot interstellar gas that is often described as a valuable probe for early galaxies. In fact, our observations indicate that Himiko lies off the local L_([C II])–star formation rate scaling relation by a factor of more than 30. Both aspects of our ALMA observations suggest that Himiko is a unique object with a very low dust content and perhaps nearly primordial interstellar gas. Our HST images provide unique insight into the morphology of this remarkable source, highlighting an extremely blue core of activity and two less extreme associated clumps. Himiko is undergoing a triple major merger event whose extensive ionized nebula of Lyα emitting gas, discovered in our earlier work with Subaru, is powered by star formation and the dense circumgalactic gas. We are likely witnessing an early massive galaxy during a key period of its mass assembly close to the end of the reionization era.