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 contain 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.
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, were 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 a 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 catalog
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