Astronomers at Tuorla Observatory
and Metsähovi Radio
Observatory, have made a study of over 300 so-called "BL Lac"
galaxies, concluding that one of the standard explanations for
these energetic emittors, which divides BL Lacs into various
subtypes, may be flawed.
BL Lacs are galaxies which host a black hole at their
core. Many, indeed almost all galaxies, are now thought to host
central black holes --- our own Milky Way galaxy is a good
example. Many of these black holes are surrounded by disks of
matter, slowly working its way inwards to be swallowed, as well as
having one or two bright 'jets', probably aligned perpendicularly
to the disk. When one of these jets happens to be pointed pretty
much toward us, observing at a safe distance on the Earth,
spectacular effects can occur. Firstly, viewing the jet like this
makes the central core of the galaxy look extremely bright -- as
bright as the entire galaxy in which the black hole is located.
Secondly, the spectrum of energy emitted can have very marked
changes in its character --- emissions from the core, normally seen
at radio and optical wavelengths, can emerge 'boosted' in energy
all the way to the X-ray or gamma ray regions.
Now researchers Elina Nieppola, Merja Tornikoski
and Esko Valtaoja have collected data for more than 300 BL
Lacs, compiling their spectral properties from the radio through
optical, infra-red, and if available X-ray and gamma ray regions.
A dedicated observational program at Metsähovi Radio Telescope
running from 2001-2004, at 37 GHz, formed the major fraction data
collected.
The researchers measured the amount of energy emitted by the
objects in the form of 'synchrotron' radiation, a result of the
boosting provided because we happen to be looking directly down the
jet coming from very close to the black hole. A prediction of the
model for BL Lacs is that the synchrotron spectral energy
distribution should have a peak in energy at a higher wavelength
for more energetic (i.e. luminous) sources. The researchers found
no such trend. Their sample is for the first time large enough to
confirm this quite clearly.
Until now, two clear types of BL Lac have been recognised, those
which peak in energy emission at high energies, and those at low
energies. This distinction, while useful, is emphasised because BL
Lacs are most easily detected in two distinct wave bands --- by
their radio and X-ray emissions. Based on the smooth distribution
of many properties of BL Lacs right across the electromagnetic
spectrum, the researchers conclude that the "underlying BL Lac
population as a whole is continuous and undivided",
Like to a pair of loving turtle-doves That could not live
asunder day or night.
King Henry VI, Part I, Act 2, Scene 2.
The study has been published in Astronomy and Astrophysics and
is available here.
Related Links
One
hundred BL Lac galaxies
Posted March 30th 2006.
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Image of a BL Lac. The object is RGB1532+302 (the large
elliptical galaxy in the upper right corner). The host galaxy of
this nearby BL Lac (redshift, z = 0.064) is surrounded by a small
group of spiral galaxies. The nucleus is relatively weak, only ~ 5%
of the total light of the system comes from the core. Credit: Kari
Nilsson. |