A quiet old galaxy has been found to have a younger, more active alter ego lurking nearby, prompting the discoverers to dub the pair Jekyll and Hyde.
The slightly older Jekyll — officially called ZF-COSMOS-20115 and described in detail in 2017 — was born in the early Universe. Many galaxies of the same vintage and mass churned out stars for billions of years, but Jekyll stopped by about 1.5 billion years after the Big Bang, making it the earliest known ‘quiescent’ galaxy.
Studying data from the Atacama Large Millimeter/submillimeter Array telescope in Chile, Corentin Schreiber at Leiden University in the Netherlands and his colleagues found a dust-shrouded galaxy — Hyde — lurking right next to the staid record-setter.
Both galaxies are compact and massive, but Hyde probably maintains a modest rate of star formation. Jekyll and Hyde could represent two stages of the process that winds down galactic star production, making the pair a natural laboratory, the authors write.