2,475 research outputs found
Galaxy Light Concentration. I. Index stability and the connection with galaxy structure, dynamics, and supermassive black holes
We explore the stability of different galaxy light concentration indices as a
function of the outermost observed galaxy radius. With a series of analytical
light-profile models, we show mathematically how varying the radial extent to
which one measures a galaxy's light can strongly affect the derived galaxy
concentration. The "mean concentration index", often used for parameterizing
high-redshift galaxies, is shown to be horribly unstable, even when modeling
one-component systems such as elliptical, dwarf elliptical and pure exponential
disk galaxies. The C_31 concentration index performs considerably better but is
also heavily dependent on the radial extent, and hence exposure depth, of any
given galaxy. We show that the recently defined central concentration index is
remarkably stable against changes to the outer radius, providing a meaningful
and reliable estimate of galaxy concentration. The index n from the r^(1/n)
models is shown to be monotonically related with the central concentration of
light, giving the index n a second and perhaps more tangible meaning. With a
sample of elliptical and dwarf elliptical galaxies, we present correlations
between the central light concentration and the global parameters: luminosity
(Pearson's r = -0.82), effective radius (r = 0.67), central surface brightness
(r = -0.88), and velocity dispersion (r = 0.80). The more massive elliptical
galaxies are shown to be more centrally concentrated. We speculate that the
physical mechanism behind the recently observed correlation between the central
velocity dispersion (mass) of a galaxy and the mass of its central supermassive
black hole may be connected with the central galaxy concentration. That is, we
hypothesize that it may not simply be the amount of mass in a galaxy but rather
how that mass is distributed that controls the mass of the central black hole.Comment: (aastex, 18 pages including 13 figures
Influence of atomic polarization and horizontal illumination on the Stokes profiles of the He I 10830 multiplet
The polarization observed in the spectral lines of the He I 10830 multiplet
carries valuable information on the dynamical and magnetic properties of plasma
structures in the solar chromosphere and corona, such as spicules, prominences,
filaments, emerging magnetic flux regions, etc. Here we investigate the
influence of atomic level polarization on the emergent Stokes profiles for a
broad range of magnetic field strengths, in both 90 degree and forward
scattering geometry. We show that, contrary to a widespread belief, the
selective emission and absorption processes caused by the presence of atomic
level polarization may have an important influence on the emergent linear
polarization, even for magnetic field strengths as large as 1000 G.
Consequently, the modeling of the Stokes Q and U profiles should not be done by
taking only into account the contribution of the transverse Zeeman effect
within the framework of the Paschen-Back effect theory, unless the magnetic
field intensity of the observed plasma structure is sensibly larger than 1000
G. We point out also that in low-lying optically thick plasma structures, such
as those of active region filaments, the (horizontal) radiation field generated
by the structure itself may substantially reduce the positive contribution to
the anisotropy factor caused by the (vertical) radiation field coming from the
underlying solar photosphere, so that the amount of atomic level polarization
may turn out to be negligible. Only under such circumstances may the emergent
linear polarization of the He I 10830 multiplet in such regions of the solar
atmosphere be dominated by the contribution caused by the transverse Zeeman
effect.Comment: Accepted for publication in The Astrophysical Journal (It is
tentatively scheduled for the ApJ January 20, 2007 issue
Derechos y falsos derechos: derechos razonables y no razonables
Si esaminano le critiche che si rivolgono alla teoria della indivisibilit\ue0 del diritti, in particolare, quelle volte a distinguere tra i diritti di libert\ue0 e i diritti sociali. L'argomentazione serve a discutere alcuni elementi di una teoria dei diritti fondamentali
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