Active galactic nuclei (AGN) 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 light-year). Most astronomers now believe that the power for AGN comes from accretion of matter onto the central supermassive black hole located at the centre of every galaxy with a central, spheroidal stellar component. A major goal in the study of AGN has been to understand how the accretion onto the black hole is triggered, and why such activity in the nuclear region of galaxies is associated with strong bursts of star formation in much extended regions. Numerical simulations have shown that interactions between galaxies can bring gas from the disc to the central regions, leading to enhanced star formation and AGN activity. On the observational side, it has long been known that galaxy-galaxy interactions are associated with enhanced star formation. However, there has been little clear evidence in support of a similar interaction-induced enhancement of nuclear activity.

Galaxies are not isolated. In the generally accepted picture, most of them have undergone some form of gravitational interaction, or even merger, with another galaxy during the lifetime of the Universe (see Fig. 1 for an example of interacting galaxies). Indeed, it is believed that galaxy-galaxy interactions/mergers play a central role in at least four of the most important processes in galaxy formation and evolution. First, in the currently popular "hierarchical formation paradigm", galaxies are formed through merging of smaller galaxies. Second, gas-rich spiral galaxies transform to gas-poor elliptical galaxies through mergers, as also shown by numerical simulations (Research Highlight Februrary 2005). Third, interacting galaxies often exhibit high rates of star formation. This is explained by the fact that, tidal forces induced by interactions casue the gas in the galaxies to lose its angular momentum, and to fall to the centres of the galaxies, where it may ignite a firestorm of star birth. Finally, some of this gas may be accreted onto the central supermassive black hole, and this will trigger activity in the nucleus of the galaxy. (Such galaxies are called active galaxies and their nuclei are termed as Active Galactic Nuclei, AGN.) This conjecture sounds plausible given the observed tight correlation between the mass of the black hole and the total mass of the stars in the central, spheroidal component of galaxies (called bulge) (Research Highlight April 2005, Research Highlight August 2007), and has motivated many theoretical models in which accretion onto black holes and AGN activity are assumed to be closely linked to galaxy interactions and mergers (Research Highlight Februrary 2005). However, on the observational side, there has been little direct, clear evidence in support of this hyphothesis.

The large redshift surveys assembled in recent years, in particular the Sloan Digital Sky Survey (SDSS), have provided angular positions and redshifts for hundreds of thousounds of galaxies and have allowed astronomers to make detailed maps of how galaxies are distributed in space (Research Highlight Februrary 2006). The survey also provides an optical spectrum for each galaxy which allows to determine a variety of physical properties such as the total mass of their stars (stellar mass), the rate at which they form stars (star formation rate), and the rate at which matter is accreting on to the central supermassive black hole (accretion rate) (Research Highlight January 2003). The new data allows astronomers to obtain a much clearer picture of the connections between star formation, AGN activity and galaxy-galaxy interactions.

A team of astronomers at the Max Planck Institute for Astrophysics (Cheng Li, Guinevere Kauffmann and Simon White), in collaboration with colleagues from Johns Hopkins University (Timothy Heckman) and Shanghai Astronomical Observatory (Yipeng Jing), have been exploring these connections by applying a variety of statistics to SDSS data. It has been shown that all galaxies, regardless of whether they are active or inactive, form new stars at higher rates if they have close neighbours. In contrast, the accretion rate onto the black hole is not influenced by the presence or absence of close neighbours. Previous work has shown that strong AGN activity is often associated with strong bursts of recent star formation in the host galaxy, known as the starburst-AGN connection (Research Highlight January 2003). This leads the team to conclude that star formation induced by a close neighbour and star formation associated with black hole accretion are distinct events. However, these events may be part of the same long-term physical process, for example a merger, provided that they are separated in time. In this case, accretion on to the black hole and its associated star formation would occur only after the two interacting galaxies have merged.

Cheng Li

Publications

Cheng Li, Guinevere Kauffmann, Timothy M. Heckman, Simon D. M. White and Y. P. Jing, "Interactions, star formation and AGN activity", 2008, Monthly Notices of Royal Astronomical Society, in press arXiv:0712.0383

Cheng Li, Guinevere Kauffmann, Timothy M. Heckman, Y. P. Jing and Simon D. M. White, "Interaction-induced star formation in a complete sample of 10⁵ nearby star-forming galaxies", 2008, Monthly Notices of Royal Astronomical Society, in press arXiv:0711.3792

Cheng Li, Guinevere Kauffmann, Lan Wang, Simon D. M. White, Timothy M. Heckman and Y. P. Jing, "The clustering of narrow-line AGN in the local Universe", 2006, Monthly Notices of Royal Astronomical Society, 373, 457-468 astro-ph/0607492