Jekyll & Hyde: quiescence and extreme obscuration in a pair of massive galaxies 1.5 Gyr after the Big Bang

Understand

We obtained ALMA spectroscopy and imaging to investigate the origin of the unexpected sub-mm emission toward the most distant quiescent galaxy known to date, ZF-COSMOS-20115 at z=3.717.

• We show here that this sub-mm emission is produced by another massive, compact and extremely obscured galaxy, located only 3.1 kpc away from the quiescent galaxy.
• We dub the quiescent and dusty galaxies Jekyll and Hyde, respectively.
• No dust emission is detected at the location of Jekyll, implying SFR < 13 Msun/yr, which is the most stringent upper limit ever obtained for a quiescent galaxy at these redshifts.