The most luminous galaxies are not particularly bright in the visible light region. Most of their light — which can be hundreds or thousands times greater than the light from our Milky Way — is emitted in infrared wavelengths. The power source of these galaxies is star formation and other activity around a supermassive black hole at the galaxies’ heart, the so-called active galactic nucleus (AGN).
The radiation from the physical processes around AGN is absorbed by dust that is re-emited in the infrared. The manner and degree to which AGN affect their surroundings are a critical part of each galaxy’s evolution. Astronomers working on the EU-funded ‘AGN as probes of galaxy evolution’ (AGNPROBES) project have been successful in quantifying related processes.
The AGNPROBES scientists focused on two different types of dust-obscured quasars — the compact region in the centre of galaxies that includes a black hole. They selected galaxies where stars are still being born around their centre or were being formed until recently. The mass of their AGN was estimated and compared with the statistical dispersion of stars’ velocities around their mean value.
For the sample of galaxies where there is ongoing star formation, the astronomers found more massive black holes at their centre and lower stellar velocity dispersion. Together with the shift of emission lines towards longer wavelengths which is caused by the atmosphere of stars, these findings indicated an unusually rapid growth of these galaxies.
The same trend was observed in the sample of post-starburst quasars. The spectra of these AGNs displayed both broad emission lines as well as absorption lines by the host galaxy. These were used to estimate for the first time the velocity dispersion of stars around the galaxies’ centre, which had been born before star formation activity was temporarily stopped.
The AGNPROBES project was a systematic attempt to analyse galaxies with higher redshift than those analysed until today. Through the empirical M-sigma relation between black holes’ mass and stellar velocity dispersion, the causal link between galaxies and supermassive black holes that reside within them was better established. However, the question of the interdependence and its origin remain open.
Provided by CORDIS