Quasars are basically massive black holes surrounded by large accretion disks of matter and can be mostly found in active galactic nuclei. As they swallow large quantities of matter, quasars may eject gas into the interstellar space, so that star formation processes are stopped and the galaxy housing it evolves passively in time. By studying the light of about 360,000 galaxies in the local universe, researchers from the Astrophysics Research Institute of Liverpool John Moore University revealed several relations between quasars and star formation and the evolution of the host galaxy as a whole.

It is widely believed that most of - if not all - the active galactic nuclei go through a quasar stage, when the supermassive black holes at the center swallow large quantities of matter and emit radiation into the interstellar space. Some quasars may become so bright that they could outshine all stars within the parent galaxy. Most of these supermassive black holes experienced quasars phases during the early life of the galaxy, but some may restart without any clear reason.

"The starlight from the host galaxy can tell us much about how the galaxy has evolved. Galaxies can be grouped into two simple color families: the blue sequence, which are young, hotbeds of star-formation, and the red sequence, which are massive, cool and passively evolving. Scientists have thought for some time that AGN host galaxies might be a stepping stone between the two families and therefore represent a critical point in the lifetime of a galaxy, but our study has been able to rule this out," said Paul Westoby, presenting the results of the study at the Royal Astronomical Society's National Astronomy Meeting.

However, during their study, the researching team discovered that some galaxies present red sequence overlaps, meaning that the star formation process could have been stopped at some point in time by the influence of a quasar.

The vast majority of the galaxies in the local universe seem to present bulges at their centers, which formed through merges, while a very little number of galaxies present a disk-only configuration, which suggests that the supermassive black hole formation influences only the evolution of the active galactic nuclei, not that of the whole galaxy.

Westoby also reveals that his researching team discovered a type of galaxies that are composed by a supermassive black hole and a population of young stars surrounding it and seem to be influenced by active galactic nuclei feedback, which is only significant during the quasar stage.