Tuorla Observatory News on 2 February 2004

One hundred BL Lac galaxies

This montage shows one hundred images of so-called BL Lac galaxies. The images were obtained by VISPA researcher Dr. Kari Nilsson and collaborators at the Nordic Optical Telescope. BL Lacs are galaxies, which harbour a very bright, central source of emission. The central source is thought to be a black hole, which is active, releasing huge amounts of energy as gas and stars fall into it; so much so that it may greatly outshine the underlying galaxy.

100 BL Lacs

The glare from the central source may be so strong that detecting the background galaxy is a considerable challenge; nevertheless, researchers are interested to do so because the type of galaxy, and the environment in which it finds itself, are vital clues in tracking down the nature of these objects.

The research team, consisting of Drs. Kari Nilsson, Leo Takalo and Aimo Sillanpää at Tuorla Observatory, Tapio Pursimo (Nordic Optical Telescope and Jochen Heidt (Landessternwarte Heidelberg) and W. Brinkmann ( Max-Planck-Institut für Extraterrestrische Physik, Garching) has used the very high imaging qualities of the Nordic Optical Telescope, and made a very careful subtraction of the central source in order to reveal the underlying galaxy or galaxies in each target. They were able to see the underlying galaxy in 62 of the 100 targets, 37 of them never seen before.

A particular type of AGN, called BL Lacs, was examined by the team. This type of AGN lacks the strong emission lines seen in most other types of AGN; they also change brightness, and time-scales as short as minutes and as long as decades.

The hosts were found to be quite normal luminous, large elliptical galaxies, just like galaxies of the same type, which do not harbour an active, central black hole. This indicates that the galactic environment has little effect on the nuclear activity and possibly every galaxy is capable of entering an active phase at some stage during their evolution. Contrary to some previous studies, not a single clear example of a disk-dominated (i.e. spiral) galaxy was found, although in a few cases the galaxies clearly possessed embedded disk components. Also a dozen optically weak nuclei were found supporting the view that the BL Lac sequence continues to lower luminosities than previously thought.

The research has been published in the journal Astronomy and Astrophysics, vol 400, page 95 in 2003.

NOT Images of the RGB BL Lacs in the Tuorla catalogue

This page was last modified by  Rami Rekola  on  31/05/2006 16:48  astroweb@utu.fi