105 research outputs found
Pattern speed of main spiral arms in NGC 2997: Estimate based on very young stellar complexes
Deep JHK-Brg photometry of the southern arm of the grand-design spiral galaxy
NGC 2997 was obtained by ISAAC/VLT. All sources in the field brighter than K=19
mag were located. Color-color diagrams were used to identify young stellar
complexes among the extended sources. Ages can be estimated for the youngest
complexes and correlated with azimuthal distances from the spiral arms defined
by the K-band intensity variation. The extended sources with Mk <-12 mag
display a diffuse appearance and are more concentrated inside the arm region
than fainter ones, which are compact and uniformly distributed in the disk. The
NIR colors of the bright diffuse objects are consistent with them being young
starforming complexes with ages <10 Myr and reddened by up to 8 mag of visual
extinction. They show a color gradient as a function of their azimuthal
distance from the spiral arms. Interpreting this gradient as an age variation,
the pattern speed Op = 16 km/s/kpc of the main spiral was derived assuming
circular motion. The alignment and color gradient of the bright, diffuse
complexes strongly support a density wave scenario for NGC 2997. Only the
brightest complexes with Mk <-12 mag show a well aligned structure along the
arm, suggesting that a strong compression in the gas due to the spiral
potential is required to form these most massive aggregates, while smaller
starforming regions are formed more randomly in the disk. The sharp transition
between the two groups at Mk = -12 mag may be associated with expulsion of gas
when the first supernovae explode in the complex.Comment: 4 pages, 5 figures. Accepted for publication as A&A Lette
NGC 1300 Dynamics: I. The gravitational potential as a tool for detailed stellar dynamics
In a series of papers we study the stellar dynamics of the grand design
barred-spiral galaxy NGC~1300. In the first paper of this series we estimate
the gravitational potential and we give it in a form suitable to be used in
dynamical studies. The estimation is done directly from near-infrared
observations. Since the 3D distribution of the luminous matter is unknown, we
construct three different general models for the potential corresponding to
three different assumptions for the geometry of the system, representing
limiting cases. A pure 2D disc, a cylindrical geometry (thick disc) and a third
case, where a spherical geometry is assumed to apply for the major part of the
bar. For the potential of the disc component on the galactic plane a Fourier
decomposition method is used, that allows us to express it as a sum of
trigonometric terms. Both even and odd components are considered, so that the
estimated potential accounts also for the observed asymmetries in the
morphology. For the amplitudes of the trigonometric terms a smoothed cubic
interpolation scheme is used. The total potential in each model may include two
additional terms (Plummer spheres) representing a central mass concentration
and a dark halo component, respectively. In all examined models, the relative
force perturbation points to a strongly nonlinear gravitational field, which
ranges from 0.45 to 0.8 of the axisymmetric background with the pure 2D being
the most nonlinear one. We present the topological distributions of the stable
and unstable Lagrangian points as a function of the pattern speed .
In all three models there is a range of values, where we find
multiple stationary points whose stability affects the overall dynamics of the
system.Comment: 14 pages, 11 figures, published in MNRA
NGC 1300 Dynamics: III. Orbital analysis
We present the orbital analysis of four response models, that succeed in
reproducing morphological features of NGC 1300. Two of them assume a planar
(2D) geometry with =22 and 16 \ksk respectively. The two others
assume a cylindrical (thick) disc and rotate with the same pattern speeds as
the 2D models. These response models reproduce most successfully main
morphological features of NGC 1300 among a large number of models, as became
evident in a previous study. Our main result is the discovery of three new
dynamical mechanisms that can support structures in a barred-spiral grand
design system. These mechanisms are presented in characteristic cases, where
these dynamical phenomena take place. They refer firstly to the support of a
strong bar, of ansae type, almost solely by chaotic orbits, then to the support
of spirals by chaotic orbits that for a certain number of pat tern revolutions
follow an n:1 (n=7,8) morphology, and finally to the support of spiral arms by
a combination of orbits trapped around L and sticky chaotic orbits with
the same Jacobi constant. We have encountered these dynamical phenomena in a
large fraction of the cases we studied as we varied the parameters of our
general models, without forcing in some way their appearance. This suggests
that they could be responsible for the observed morphologies of many
barred-spiral galaxies. Comparing our response models among themselves we find
that the NGC 130 0 morphology is best described by a thick disc model for the
bar region and a 2D disc model for the spirals, with both components rotating
with the same pattern speed =16 \ksk !. In such a case, the whole
structure is included inside the corotation of the system. The bar is supported
mainly by regular orbits, while the spirals are supported by chaotic orbits.Comment: 18 pages, 32 figures, accepted for publication in MNRA
Representations of world coordinates in FITS
The initial descriptions of the FITS format provided a simplified method for
describing the physical coordinate values of the image pixels, but deliberately
did not specify any of the detailed conventions required to convey the
complexities of actual image coordinates. Building on conventions in wide use
within astronomy, this paper proposes general extensions to the original
methods for describing the world coordinates of FITS data. In subsequent
papers, we apply these general conventions to the methods by which spherical
coordinates may be projected onto a two-dimensional plane and to
frequency/wavelength/velocity coordinates.Comment: 15 Pages, 1 figure, LaTex with Astronomy & Astrophysics macro
package, submitted to A&A, related papers at
http://www.aoc.nrao.edu/~egreise
Properties of young star cluster systems: the age signature from near-infrared integrated colours
A recent JHKs study of several grand-design spiral galaxies shows a bimodal
distribution of their system of star clusters and star forming complexes in
colour-magnitude and colour-colour diagrams. In a comparison with stellar
population models including gas, the (J-H) vs (H-Ks) diagram reveals that
embedded clusters, still immersed in their parental clouds of gas and dust,
generally have a redder (H-Ks) colour than older clusters, whose gas and dust
have already been ejected. This bimodal behaviour is also evident in the
colour-magnitude diagram MK vs (J-Ks), where the brightest clusters split into
two sequences separating younger from older clusters. In addition, the
reddening-free index Qd = (H-Ks) - 0.884 (J-H) has been shown to correlate with
age for the young clusters and thus provided an effective way to differentiate
the embedded clusters from the older ones. We aim to study the behaviour of
these photometric indices for star cluster systems in the Local Group. We
investigate the effectiveness of the Qd index in sorting out clusters of
different ages at their early evolutionary stages. Surface photometry was
carried out for 2MASS images of populous clusters younger than ~100Myr whose
ages were available. Some clusters, particularly the embedded ones, were
studied for the first time using this method. The integrated magnitudes and
colours extracted from the surface photometry of the most populous
clusters/complexes in the Local Group shows the expected bimodal distribution
in the colour-colour and colour-magnitude diagrams. In particular, we confirm
the index Qd as a powerful tool for distinguishing clusters younger than about
7Myr from older clusters. (abridged)Comment: 10 pages, 11 figures; accepted for publication in A&
The evolution of chaos in active galaxy models with an oblate or a prolate dark halo component
The evolution of chaotic motion in a galactic dynamical model with a disk, a
dense nucleus and a flat biaxial dark halo component is investigated. Two cases
are studied: (i) the case where the halo component is oblate and (ii) the case
where a prolate halo is present. In both cases, numerical calculations show
that the extent of the chaotic regions decreases exponentially as the
scale-length of the dark halo increases. On the other hand, a linear
relationship exists between the extent of the chaotic regions and the flatness
parameter of the halo component. A linear relationship between the critical
value of the angular momentum and the flatness parameter is also found. Some
theoretical arguments to support the numerical outcomes are presented. An
estimation of the degree of chaos is made by computing the Lyapunov
Characteristic Exponents. Comparison with earlier work is also made.Comment: Published in Astronomische Nachrichten journa
The Integrated Polarization of Spiral Galaxy Disks
We present integrated polarization properties of nearby spiral galaxies at
4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a
function of inclination. Spiral galaxies in our sample have observed integrated
fractional polarization in the range < 1% to 17.6%. At inclinations less than
50 degrees, the fractional polarization depends mostly on the ratio of random
to regular magnetic field strength. At higher inclinations, Faraday
depolarization associated with the regular magnetic field becomes more
important. The observed degree of polarization is lower (<4%) for more luminous
galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization
angle of the integrated emission is aligned with the apparent minor axis of the
disk for galaxies without a bar. In our axially symmetric models, the
polarization angle of the integrated emission is independent of wavelength.
Simulated distributions of fractional polarization for randomly oriented spiral
galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization
measurements, e.g. with the SKA, of unresolved spiral galaxies allow
statistical studies of the magnetic field in disk galaxies using large samples
in the local universe and at high redshift. As these galaxies behave as
idealized background sources without internal Faraday rotation, they can be
used to detect large-scale magnetic fields in the intergalactic medium.Comment: 13 pages, 6 figures; Accepted for publication in The Astrophysical
Journa
Are external perturbations responsible for chaotic motion in galaxies?
We study the nature of motion in a logarithmic galactic dynamical model, with
an additional external perturbation. Two different cases are investigated. In
the first case the external perturbation is fixed, while in the second case it
is varying with the time. Numerical experiments suggest, that responsible for
the chaotic phenomena is the external perturbation, combined with the dense
nucleus. Linear relationships are found to exist, between the critical value of
the angular momentum and the dynamical parameters of the galactic system that
is, the strength of the external perturbation, the flattening parameter and the
radius of the nucleus. Moreover, the extent of the chaotic regions in the phase
plane, increases linearly as the strength of the external perturbation and the
flattening parameter increases. On the contrary, we observe that the percentage
covered by chaotic orbits in the phase plane, decreases linearly, as the scale
length of the nucleus increases, becoming less dense. Theoretical arguments are
used to support and explain the numerically obtained outcomes. A comparison of
the present outcomes with earlier results is also presented.Comment: Published in Chaos, Solitons & Fractals journa
NGC 1300 Dynamics: II. The response models
We study the stellar response in a spectrum of potentials describing the
barred spiral galaxy NGC 1300. These potentials have been presented in a
previous paper and correspond to three different assumptions as regards the
geometry of the galaxy. For each potential we consider a wide range of
pattern speed values. Our goal is to discover the geometries and the
supporting specific morphological features of NGC 1300. For this
purpose we use the method of response models. In order to compare the images of
NGC 1300 with the density maps of our models, we define a new index which is a
generalization of the Hausdorff distance. This index helps us to find out
quantitatively which cases reproduce specific features of NGC 1300 in an
objective way. Furthermore, we construct alternative models following a
Schwarzschild type technique. By this method we vary the weights of the various
energy levels, and thus the orbital contribution of each energy, in order to
minimize the differences between the response density and that deduced from the
surface density of the galaxy, under certain assumptions. We find that the
models corresponding to \ksk and \ksk are
able to reproduce efficiently certain morphological features of NGC 1300, with
each one having its advantages and drawbacks.Comment: 13 pages, 10 figures, accepted for publication in MNRA
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