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 with associated HST imaging for
a luminous (m_uv=25) galaxy, `Himiko', at a redshift z=6.595. The galaxy is
remarkable for its high star formation rate, 100 Mo/yr, securely estimated from
our deep HST and Spitzer photometry, and the absence of any evidence for strong
AGN activity or gravitational lensing magnification. Our ALMA observations
probe an order of magnitude deeper than previous IRAM observations, yet fail to
detect a 1.2mm dust continuum, indicating a flux <52uJy comparable with 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 [CII] 158um,
L([CII]) < 5.4x10^7 Lo, a diagnostic of the hot interstellar gas often
described as a valuable probe for early galaxies. In fact, our observations
indicate Himiko lies off the local L([CII]) - star formation rate scaling
relation by a factor of more than 30. Both aspects of our ALMA observations
suggest Himiko is an 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 Lyman alpha
emitting gas, discovered in our earlier work with Subaru, is powered by star
formation and the dense circum-galactic 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.
