In section 4. Extragalactic astronomy
Jekyll & Hyde: quiescence and extreme obscuration in a pair of massive galaxies 1.5 Gyr after the Big Bang
It is possible to observe galaxies at very high redshifts (more than seven) with ALMA, but these are bright star forming galaxies; here the authors have searched for the most remote quiescent galaxy, at z=3.717. They discovered that the sub-millimeter emission of this object comes in fact from a companion galaxy, 3.2kpc away, which is dusty and star forming. The quiescent galaxy (Jekyll) has another face (Hyde), which has a strong [C II]158micron line, confirming the physical association of the two objects (their redshift difference is 550 km/s). Only Jekyll is detected in the Hubble images, meaning that Hyde is completely obscured by dust. SED fitting of multi-wavelength emission of both galaxies shows that Jekyll was fully quenched at least 200 Myr ago while Hyde harbors moderate star formation with an SFR ~120Msun/yr, or has also been quenched, as Jekyll has. The two objects have similar mass, compactness, environment, and star formation history, so the authors argue that Jekyll and Hyde can be seen as two stages of the same quenching process, and provide a unique laboratory to study this poorly understood phenomenon.