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The Connection Between Active Galactic Nuclei and Starbursts

Active galactic nuclei (hereafter abbreviated AGNs) are among the most spectacular objects in the Universe. They produce prodigious luminosities (in some cases as much as 10,000 times the luminosity of a typical galaxy) in tiny volumes (much less than one cubic parsec). Most astronomers now believe that the power for AGNs comes from accretion onto the supermassive black holes located at the centre of every galaxy with a bulge.

Seyfert galaxy NGC 7742

Figure 1: The core of the Seyfert 2 galaxy NGC 7742. The lumpy thick ring around the core is an area of active star formation. The ring is about 3000 light-years from the core.

Schematic sketch of an AGN

Figure 2: A torus of dusty material is thought to surround the accreting black holes in most AGN. For type 2 AGN, the observer's view of the light from the central source is blocked by this material. Radiation which escapes perpendicular to the torus is able to ionize gas clouds in the surrounding galaxy, which leads to characteristic emisson line signatures in the galaxy spectrum.

Starburst diagnostic diagram

Figure 3: A starburst "diagnostic" diagram for AGN in the Sloan Digital Sky Survey. The magenta and red points show the predicted location of galaxies that have had continuous star formation histories. Most normal galaxies (left) lie close to these predictions. A significant fraction of powerful AGN (right) are displaced away from the locus occupied by the continuous models. This shows that they must have experienced recent starbursts.

Active galaxies are also noteworthy in that they display very strong cosmological evolution. The most luminous active galaxies were a thousand times more numerous at redshift 2.5 than they are today. This strong evolution with redshift suggests that galaxy formation and the creation of active nuclei may have gone hand-in-hand in the early Universe.

Although they are numerous, it is not possible to study distant AGN in detail, because the light from the surrounding host galaxy is usually too faint to detect. In order to understand the physical processes that are responsible for fuelling the central black holes in galaxies, it is more useful to study AGN in the local Universe.

In collaboration with a group at Johns Hopkins University in Baltimore (T. Heckman, C. Tremonti and S. Ridgway), scientists at MPA are studying a very large sample of 22,000 AGN drawn from the Sloan Digital Sky Survey. The active galaxies under investigation are so-called Type 2 AGN. In these objects, most of the radiation emitted by matter very close to the black hole is obscured by a central dusty "torus". Radiation can, however, escape at right angles to the plane of the torus (see Fig. 2) and ionize gas further out in the galaxy. This causes characteristic emission lines to appear in the spectrum of the galaxy, which diagnose the presence of the active nucleus. Most of the luminosity that is produced as material falls onto the black hole is thus hidden from view in Type 2 AGN. As a result, it is possible to obtain a much clearer picture of the surrounding host galaxy. It is then possible to ask how galaxies that contain actively accreting black holes differ from galaxies in which the black hole lies dormant.

The main conclusion reached by the MPA/JHU team is that strong AGN activity is often associated with strong bursts of recent star formation in the host galaxy (see Fig. 3). Conversely, in bulge-dominated galaxies where black holes lie dormant, star formation has almost always stopped. This so-called starburst-AGN connection has been a controversial subject among AGN specialists for many years. The results of the MPA/JHU group establish that AGN activity and star formation are inextricably linked in massive, bulge-dominated systems.

Astronomers interested in galaxy formation must now turn their attention to undertanding how the formation and fuelling of central supermassive black holes influenced the evolution of their galactic hosts.

Guinevere Kauffmann

Further information:
Sloan Digital Sky Survey
Studying Galaxies in the Sloan Digital Sky Survey

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