144 research outputs found
Modelling the Galactic disc: perturbed distribution functions in the presence of spiral arms
Starting from an axisymmetric equilibrium distribution function (DF) in
action space, representing a Milky Way thin disc stellar population, we use the
linearized Boltzmann equation to explicitly compute the response to a
three-dimensional spiral potential in terms of the perturbed DF. This DF, valid
away from the main resonances, allows us to investigate a snapshot of the
velocity distribution at any given point in three-dimensional configuration
space. Moreover, the first order moments of the DF give rise to non-zero radial
and vertical bulk flows -- namely breathing modes -- qualitatively similar to
those recently observed in the extended Solar neighbourhood. We show that these
analytically predicted mean stellar motions are in agreement with the outcome
of test-particle simulations. Moreover, we estimate for the first time the
reduction factor for the vertical bulk motions of a stellar population compared
to the case of a cold fluid. Such an explicit expression for the full perturbed
DF of a thin disc stellar population in the presence of spiral arms will be
helpful in order to dynamically interpret the detailed information on the Milky
Way disc stellar kinematics that will be provided by upcoming large astrometric
and spectroscopic surveys of the Galaxy.Comment: 15 pages. Submitted on 2 December 2015 to MNRAS. Accepted for
publication. Some typos corrected in v
The effects of bar-spiral coupling on stellar kinematics in the Galaxy
We investigate models of the Milky Way disc taking into account
simultaneously the bar and a two-armed quasi-static spiral pattern. Away from
major resonance overlaps, the mean stellar radial motions in the plane are
essentially a linear superposition of the isolated effects of the bar and
spirals. Thus, provided the bar is strong enough, even in the presence of
spiral arms, these mean radial motions are predominantly affected by the
Galactic bar for large scale velocity fluctuations. This is evident when
comparing the peculiar line-of-sight velocity power spectrum of our coupled
models with bar-only models. However, we show how forthcoming spectroscopic
surveys could disentangle bar-only non-axisymmetric models of the Galaxy from
models in which spiral arms have a significant amplitude. We also point out
that overlaps of low-order resonances are sufficient to enhance stellar
churning within the disc, even when the spirals amplitude is kept constant.
Nevertheless, for churning to be truly non-local, stronger or (more likely)
transient amplitudes would be needed: otherwise the disc is actually mostly
unaffected by churning in the present models. Finally, regarding vertical
breathing modes, the combined effect of the bar and spirals on vertical motions
is a clear non-linear superposition of the isolated effects of both components,
significantly superseding the linear superposition of modes produced by each
perturber separately, thereby providing an additional effect to consider when
analysing the observed breathing mode of the Galactic disc in the extended
Solar neighbourhood.Comment: 13 pages, 12 figures. MNRAS. Accepted for publication. v2 is the
published versio
Spiral and bar driven peculiar velocities in Milky Way sized galaxy simulations
We investigate the kinematic signatures induced by spiral and bar structure
in a set of simulations of Milky Way-sized spiral disc galaxies. The set
includes test particle simulations that follow a quasi-stationary density
wave-like scenario with rigidly rotating spiral arms, and -body simulations
that host a bar and transient, co-rotating spiral arms. From a location similar
to that of the Sun, we calculate the radial, tangential and line-of-sight
peculiar velocity fields of a patch of the disc and quantify the fluctuations
by computing the power spectrum from a two-dimensional Fourier transform. We
find that the peculiar velocity power spectrum of the simulation with a bar and
transient, co-rotating spiral arms fits very well to that of APOGEE red clump
star data, while the quasi-stationary density wave spiral model without a bar
does not. We determine that the power spectrum is sensitive to the number of
spiral arms, spiral arm pitch angle and position with respect to the spiral
arm. However, it is necessary to go beyond the line of sight velocity field in
order to distinguish fully between the various spiral models with this method.
We compute the power spectrum for different regions of the spiral discs, and
discuss the application of this analysis technique to external galaxies.Comment: 14 pages, 11 figures. Improved and MNRAS Accepte
Comment on "Evidence for dark matter in the inner Milky Way"
This is a brief rebuttal to arXiv:1502.03821, which claims to provide the
first observational proof of dark matter interior to the solar circle. We point
out that this result is not new, and can be traced back at least a quarter
century.Comment: 3 pages, 2 figures. In this version we add a figure from a 1998 paper
that shows the same result that arXiv:1502.03821 claims to be novel. We also
add a short note rebutting arXiv:1503.08784 which was written in response to
the first versio
A panchromatic view of PKS 0558-504: an ideal laboratory to study the disk-jet link
PKS 0558-504 is the brightest radio-loud Narrow-Line Seyfert 1 galaxy at
X-ray energies. Here we present results from the radio, optical, UV, and X-ray
bands obtained with Swift, XMM, and ATCA during a 10-day monitoring campaign in
September 2008. The simultaneous coverage at several wavelengths makes it
possible to investigate in detail the broadband spectral energy distribution
(SED) and the energetic of this source. The main results can be summarized as
follows. The ATCA reveals the presence of an extended radio emission in PKS
0558-504 with two lobe-like structures at ~7" from the bright central source.
The extended radio structure and the low value of the radio-loudness similar to
radio-quiet Seyfert galaxies coupled with constraints from higher energy bands
argue against a jet-dominated emission. The study of the SED, which is
dominated by a nearly constant optical-UV emission, supports the conclusion
that PKS 0558-504 is accreting at super-Eddington rate. This conclusion was
reached assuming M_BH=2.5e8 M_sun, which was obtained with a new scaling method
based on X-ray spectral variability results. A comparison between the accretion
luminosity and the kinetic power associated with the jet suggests that in this
source the accretion power dominates in agreement with the results obtained
from Radiation-MHD simulations of Galactic black holes (GBHs) accreting at the
Eddington rate. The combined findings from this panchromatic investigation
strongly suggest that PKS 0558-504 is a large-scale analog of GBHs in their
highly accreting intermediate state. Importantly, PKS 0558-504 may also be the
prototype of the parent population of the very radio-loud NLS1s recently
detected at gamma-ray energies.Comment: 11 pages, 5 figures, 3 tables, accepted for publication in Ap
Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters
Testing Newtonian gravity in the weak-acceleration regime is vital to our
understanding of the nature of the gravitational interaction. It has recently
been claimed that the velocity dispersion profiles of several globular clusters
flatten out at large radii, reminiscent of galaxy rotation curves, even though
globular clusters are thought to contain little or no dark matter. We
investigate this claim, using AAOmega observations of four globular clusters,
namely M22, M30, M53 and M68. M30, one such cluster that has had this claim
made for its velocity dispersion, was included for comparison with previous
studies. We find no statistically significant flattening of the velocity
dispersion at large radii for any of our target clusters and therefore we infer
the observed dynamics do not require that globular clusters are dark matter
dominated, or a modification of gravity. Furthermore, by applying a simple
dynamical model we determine the radial mass-to-light profiles for each
cluster. The isothermal rotations of each cluster are also measured, with M22
exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any
rotation, within the uncertainties.Comment: 7 pages, 4 figures and two tables. Accepted by MNRA
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