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

Understand

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.