Star Formation Extinguished by Quasars

According to new research, a galaxy with a quasar in the middle is not a good place to grow up. As active galactic nuclei (AGN) evolve, they pass through a “quasar phase”, where the accretion disk surrounding the central black hole blasts intense radiation into space. The quasar far outshines the entire host galaxy. After the quasar phase, when the party is over, it is as if there is no energy left and star formation stops.

AGN are the compact, active and bright central cores to active galaxies. The intense brightness from these active galactic cores is produced by the gravitationally driven accretion disk of hot matter spinning and falling into a supermassive black hole at the centre. During the lifetime of an AGN, the black hole/accretion disk combo will undergo a “quasar phase” where intense radiation is blasted from the superheated gases surrounding the black hole. Typically quasars are formed in young galaxies.

Although the quasar phase is highly energetic and tied with young galaxy formation, according to new results from the Sloan Digital Sky Survey, it also marks the end for any further star birth in the galaxy. These findings will be presented today (Friday 4th April) at the RAS National Astronomy Meeting in Belfast, Northern Ireland, by Paul Westoby having just completed a study of 360 000 galaxies in the local Universe. He carried out this research with Carole Mundell and Ivan Baldry from the Astrophysics Research Institute of Liverpool, John Moores University, UK. This study was proposed to understand the relationship between accreting black holes, the birth of stars in galactic cores and the evolution of galaxies as a whole. The results are astonishingly detailed.

By analysing so many galaxies, quite a detailed picture emerges. The primary result to come from this shows that as a young galactic core is dominated by a highly energetic quasar, star formation stops. After this phase in a galaxy’s life, star formation is not possible; the remaining stars are left to evolve by themselves.

It is believed that all AGNs go through the quasar phase in their early galactic lives. It is also thought that most massive galaxies will have a supermassive black hole hiding inside their galactic cores passively, having already gone through the quasar phase. Westoby notes that some dormant supermassive black holes can be “reignited” into a secondary quasar phase, but the mechanisms behind this are sketchy.

“The starlight from the host galaxy can tell us much about how the galaxy has evolved […] Galaxies can be grouped into two simple colour families: the blue sequence, which are young, hotbeds of star-formation and the red sequence, which are massive, cool and passively evolving..” – Paul Westoby.

It is found there is a sudden cut-off point for star formation, and this occurs right after the quasar phase. After the quasar phase, the AGN relaxes into a quieter state, there is no star formation and gradual evolution of stars progresses into the “red sequence” of star evolution.

Other findings include the indication that regardless of the size of the galaxy, it is the shape of the galactic “bulge” that matters. Without a large classical bulge in the centre, supermassive black holes that drive the AGN are not possible. Therefore, only galaxies with a bulge have AGN at the core. Another factor affecting supermassive black hole formation is the density of galaxies in a volume of space. Should there be too many, supermassive black holes become a scarcity.

Source: The RAS National Astronomy Meeting 2008