Lorentz Center - The Triggering Mechanisms for Active Galactic Nuclei from 22 Jul 2013 through 26 Jul 2013
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    The Triggering Mechanisms for Active Galactic Nuclei
    from 22 Jul 2013 through 26 Jul 2013

 
Report on the Lorentz Centre Meeting on The Triggering of AGN 23 – 28th July 2013

It is increasingly recognized that the feedback effect associated with active galactic nuclei (AGN) plays a vital role in the evolution of galaxies. However, considerable uncertainties remain about how, when and where AGN are triggered. A challenge with making progress in this field is that different communities of astronomers working on the triggering problem (e.g. theorists, low-redshift observers, high-redshift observers) rarely meet to discuss their results and ideas collectively. Therefore the main aim of this Lorentz workshop was to provide a forum for discussion between the disparate communities interested in AGN triggering. The workshop was highly successful in this goal, and was marked by an unusually high level of free discussion of the key issues. Although this not the type of field in which there is likely to be a single “eureka moment” that leads to a stepwise change in perception, a general consensus was reached around the following issues.

 

  • Variability. It is important to take into account AGN variability when interpreting the results of survey data relevant to the triggering problem, because the duty cycle of the AGN activity is short relative to the timescales of the major events that define the evolution of galaxies.
  • The triggering mechanism is likely to change with redshift. Although there is now plenty of evidence that galaxy mergers are an important, perhaps dominant, mechanism for triggering the most luminous, quasar-like AGN in the local Universe, the substantially different conditions in the disks of forming galaxies at earlier epochs mean that other mechanisms, such as disk instabilities, are likely to become more important at high redshifts. 
  • The timing of the AGN activity. More theoretical work needs to be done on the timing of the AGN activity in galaxy mergers. Most theoretical models predict that the main phase of observed AGN activity will be triggered close to the peak, or shortly after, the merger-induced starburst that occurs around the time of coalescence of the black holes of the merging systems. However, this is in conflict with the detection of luminous AGN in systems that are observed at a much earlier merger stage, well before the nuclei have coalesced.
  • The last 100 pc. Regardless of the mechanism that delivers the gas into the central, kpc-scale regions of the host galaxies, little is known about how the gas loses sufficient angular momentum to move from the 100pc scale to the sub-pc scale required to fuel/trigger the AGN. While facilities such as ALMA are likely to make considerable progress in the next few years in determining the distribution and kinematics of the cool gas on scales smaller than 100pc, theoretical progress in this area will require a better understanding of the detailed physical mechanisms, in particular the complex interplay between the gas flows, star formation and the feedback effects associated with both the AGN and regions of rapid star formation.

 

The excellent facilities of the Lorentz Center, and the scheduling of a large fraction of the total time of the workshop to discussion, were key elements in the success of the workshop. In addition, the relatively young age profile of the participants helped to create a vibrant and open atmosphere, and the involvement of early career researchers in running some of the discussion sessions and delivering the summary talks at the end of the workshop also proved highly successful.

 

Cristina Ramos Almeida (La Laguna, Spain)

Clive Tadhunter (Sheffield, United Kingdom)



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