57 research outputs found
Stellar Orbital Studies in Normal Spiral Galaxies II: Restrictions to Structural and Dynamical parameters on Spiral Arms
Making use of a set of detailed potential models for normal spiral galaxies,
we analyze the disk stellar orbital dynamics as the structural and dynamical
parameters of the spiral arms (mass, pattern speed and pitch angle) are
gradually modified. With this comprehensive study of ordered and chaotic
behavior, we constructed an assemblage of orbitally supported galactic models
and plausible parameters for orbitally self-consistent spiral arms models. We
find that, to maintain orbital support for the spiral arms, the spiral arm
mass, M, must decrease with the increase of the pitch angle, ; if
is smaller than , M can be as large as , ,
of the disk mass, for Sa, Sb, and Sc galaxies, respectively. If
increases up to , the maximum M is of the disk
mass independently in this case of morphological type. For values larger than
these limits, spiral arms would likely act as transient features. Regarding the
limits posed by extreme chaotic behavior, we find a strong restriction on the
maximum plausible values of spiral arms parameters on disk galaxies beyond
which, chaotic behavior becomes pervasive. We find that for smaller than
, , , for Sa, Sb, and Sc galaxies,
respectively, M can go up to , of the mass of the disk. If the
corresponding is around , , , M
is , , of the mass of the disk. Beyond these
values, chaos dominates phase space, destroying the main periodic and the
neighboring quasi-periodic orbits.Comment: 51 pages in preprint format, 30 figures, Accepted for publication in
Ap
The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics
We have used N-body simulations for the Milky Way to investigate the
kinematic and structural properties of the old metal-poor stellar halo in the
barred inner region of the Galaxy. We find that the extrapolation of the
density distribution for bulge RR Lyrae stars, , approximately
matches the number density of RR Lyrae in the nearby stellar halo. We follow
the evolution of such a tracer population through the formation and evolution
of the bar and box/peanut bulge in the N-body model. We find that its density
distribution changes from oblate to triaxial, and that it acquires slow
rotation in agreement with recent measurements. The maximum radial velocity is
km/s at , and the velocity dispersion is
km/s. Even though the simulated metal-poor halo in the bulge has a
barred shape, just of the orbits follow the bar, and it does not trace
the peanut/X structure. With these properties, the RR Lyrae population in the
Galactic bulge is consistent with being the inward extension of the Galactic
metal-poor stellar halo.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
The Galactic Branches as a Possible Evidence for Transient Spiral Arms
With the use of a background Milky-Way-like potential model, we performed
stellar orbital and magnetohydrodynamic (MHD) simulations. As a first
experiment, we studied the gaseous response to a bisymmetric spiral arm
potential: the widely employed cosine potential model and a self-gravitating
tridimensional density distribution based model called PERLAS. Important
differences are noticeable in these simulations, while the simplified cosine
potential produces two spiral arms for all cases, the more realistic density
based model produces a response of four spiral arms on the gaseous disk, except
for weak arms -i.e. close to the linear regime- where a two-armed structure is
formed. In order to compare the stellar and gas response to the spiral arms, we
have also included a detailed periodic orbit study and explored different
structural parameters within observational uncertainties. The four armed
response has been explained as the result of ultra harmonic resonances, or as
shocks with the massive bisymmetric spiral structure, among other. From the
results of this work, and comparing the stellar and gaseous responses, we
tracked down an alternative explanation to the formation of branches, based
only on the orbital response to a self-gravitating spiral arms model. The
presence of features such as branches, might be an indication of transiency of
the arms.Comment: 17 pages, 9 figures. Accepted for publication in MNRA
Revisiting the Tale of Hercules: how stars orbiting the Lagrange points visit the Sun
We propose a novel explanation for the Hercules stream consistent with recent
measurements of the extent and pattern speed of the Galactic bar. We have
adapted a made-to-measure dynamical model tailored for the Milky Way to
investigate the kinematics of the solar neighborhood (SNd). The model matches
the 3D density of the red clump giant stars (RCGs) in the bulge and bar as well
as stellar kinematics in the inner Galaxy, with a pattern speed of 39 km
s kpc. Cross-matching this model with the DR1 TGAS data
combined with RAVE and LAMOST radial velocities, we find that the model
naturally predicts a bimodality in the -velocity distribution for
nearby stars which is in good agreement with the Hercules stream. In the model,
the Hercules stream is made of stars orbiting the Lagrange points of the bar
which move outward from the bar's corotation radius to visit the SNd. While the
model is not yet a quantitative fit of the velocity distribution, the new
picture naturally predicts that the Hercules stream is more prominent inward
from the Sun and nearly absent only a few pc outward of the Sun, and
plausibly explains that Hercules is prominent in old and metal-rich stars.Comment: 7 pages, 5 figures. ApJ Letters, in pres
Halo intruders in the Galactic bulge revealed by HST and Gaia : the globular clusters Terzan 10 and Djorgovski 1
Context. The low-latitude globular clusters Terzan 10 and Djorgovski 1 are projected in the Galactic bulge, in a Galactic region highly affected by extinction. A discrepancy of a factor of ∼2 exists in the literature in regards to the distance determination of these clusters. Aims. We revisit the colour-magnitude diagrams (CMDs) of these two globular clusters with the purpose of disentangling their distance determination ambiguity and, for the first time, of determining their orbits to identify whether or not they are part of the bulge/bar region. Methods. We use Hubble Space Telescope CMDs, with the filters F606W from ACS and F160W from WFC3 for Terzan 10, and F606W and F814W from ACS for Djorgosvski 1, and combine them with the proper motions from Gaia Data Release 2. For the orbit integrations, we employed a steady Galactic model with bar. Results. For the first time the blue horizontal branch of these clusters is clearly resolved. We obtain reliable distances of d⊙ = 10.3 ± 1.0 kpc and 9.3 ± 0.5 kpc for Terzan 10, and Djorgovski 1 respectively, indicating that they are both currently located in the bulge volume. From Gaia DR2 proper motions, together with our new distance determination and recent literature radial velocities, we are able to show that the two sample clusters have typical halo orbits that are passing by the bulge/bar region, but that they are not part of this component. For the first time, halo intruders are identified in the bulge
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