Galaxies are thought to grow over cosmic time through repeated mergers of smaller systems. However, in particularly dense environments, this growth may have been greatly accelerated, allowing already mature galaxies with masses several times that of the Milky Way to exist when the universe was only about one billion years.
Our Ruby-Rush Project searches for these extremely rare galaxies in the early universe: massive, red, and already quenched systems that had stopped forming stars at a very early epoch. Using the Subaru Telescope and the University of Tokyo Atacama Observatory (TAO), together with our uniquely developed medium-band filters, we carry out observations designed to detect the Balmer break, a key signature of star-formation quenching, with high precision.
By surveying several square degrees of sky, we can identify even such rare objects in statistically meaningful numbers, from a few to several tens. This enables us to investigate when, where, and through what physical processes these “monster” galaxies formed in the early universe.