Are galaxies with active galactic nuclei a transition population?
Version of Record online: 5 DEC 2007
Monthly Notices of the Royal Astronomical Society
Volume 382, Issue 4, pages 1541–1551, December 2007
How to Cite
Westoby, P. B., Mundell, C. G. and Baldry, I. K. (2007), Are galaxies with active galactic nuclei a transition population?. Monthly Notices of the Royal Astronomical Society, 382: 1541–1551. doi: 10.1111/j.1365-2966.2007.12553.x
- Issue online: 5 DEC 2007
- Version of Record online: 5 DEC 2007
- Accepted 2007 September 22. Received 2007 August 28; in original form 2007 June 28
- galaxies: active;
- galaxies: evolution;
- galaxies: Seyfert
We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range 0.01 < z < 0.16. The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of Hα equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour–magnitude space or colour–concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced Hα emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a ‘transition’ population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys.