97 research outputs found
The Galactic bar and the large scale velocity gradients in the Galactic disk
Aims: We investigate whether the cylindrical (galactocentric) radial velocity
gradient of ~ -3 km/s/kpc, directed radially from the Galactic center and
recently observed in the stars of the solar neighborhood with the RAVE survey,
can be explained by the resonant effects of the bar near the solar
neighborhood.
Methods: We compared the results of test particle simulations of the Milky
Way with a potential that includes a rotating bar with observations from the
RAVE survey. To this end we applied the RAVE selection function to the
simulations and convolved these with the characteristic RAVE errors. We
explored different "solar neighborhoods" in the simulations, as well as
different bar models
Results: We find that the bar induces a negative radial velocity gradient at
every height from the Galactic plane, outside the outer Lindblad resonance and
for angles from the long axis of the bar compatible with the current estimates.
The selection function and errors do not wash away the gradient, but often make
it steeper, especially near the Galactic plane, because this is where the RAVE
survey is less radially extended. No gradient in the vertical velocity
ispresent in our simulations, from which we may conclude that this cannot be
induced by the bar.Comment: 9 pages, 11 figures, v2. Accepted for publication on A&
Constraining the Milky Way potential using the dynamical kinematic substructures
We present a method to constrain the potential of the non-axisymmetric
components of the Galaxy using the kinematics of stars in the solar
neighborhood. The basic premise is that dynamical substructures in phase-space
(i.e. due to the bar and/or spiral arms) are associated with families of
periodic or irregular orbits, which may be easily identified in orbital
frequency space. We use the "observed" positions and velocities of stars as
initial conditions for orbital integrations in a variety of gravitational
potentials. We then compute their characteristic frequencies, and study the
structure present in the frequency maps. We find that the distribution of
dynamical substructures in velocity- and frequency-space is best preserved when
the integrations are performed in the "true" gravitational potential.Comment: 2 pages, 4 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (Apr. 17-22, 2011
Kinematics of symmetric Galactic longitudes to probe the spiral arms of the Milky Way with Gaia
Aims. We model the effects of the spiral arms of the Milky Way on the disk stellar kinematics in the Gaia observable space. We also estimate the Gaia capabilities of detecting the predicted signatures. Methods. We use both controlled orbital integrations in analytic potentials and self-consistent simulations. We introduce a new strategy to investigate the effects of spiral arms, which consists of comparing the stellar kinematics of symmetric Galactic longitudes (+l and −l), in particular the median transverse velocity as determined from parallaxes and proper motions. This approach does not require the assumption of an axisymmetric model because it involves an internal comparison of the data. Results. The typical differences between the transverse velocity in symmetric longitudes in the models are of the order of ~2 km s-1, but can be larger than 10 km s-1 for certain longitudes and distances. The longitudes close to the Galactic centre and to the anti-centre are those with larger and smaller differences, respectively. The differences between the kinematics for +l and −l show clear trends that depend strongly on the properties of spiral arms. Thus, this method can be used to quantify the importance of the effects of spiral arms on the orbits of stars in the different regions of the disk, and to constrain the location of the arms, main resonances and, thus, pattern speed. Moreover, the method allows us to test different origin scenarios of spiral arms and the dynamical nature of the spiral structure (e.g. grand design versus transient multiple arms). We estimate the number of stars of each spectral type that Gaia will observe in certain representative Galactic longitudes, their characteristic errors in distance and transverse velocity, and the error in computing the median velocity as a function of distance. We will be able to measure the median transverse velocity exclusively with Gaia data, with precision smaller than ~1 km s-1 up to distances of ~4-6 kpc for certain giant stars, and up to ~2-4 kpc and better kinematic precision (≲0.5 km s-1) for certain sub-giants and dwarfs. These are enough to measure the typical signatures seen in the models. Conclusions. The Gaia catalogue will allow us to use the presented approach successfully and improve significantly upon current studies of the dynamics of the spiral arms of our Galaxy. We also show that a similar strategy can be used with line-of-sight velocities, which could be applied to Gaia data and to upcoming spectroscopic surveys
Detection of satellite remnants in the Galactic Halo with Gaia III. Detection limits for Ultra Faint Dwarf Galaxies
We present a method to identify Ultra Faint Dwarf Galaxy (UFDG) candidates in
the halo of the Milky Way using the future Gaia catalogue and we explore its
detection limits and completeness. The method is based on the Wavelet Transform
and searches for over-densities in the combined space of sky coordinates and
proper motions, using kinematics in the search for the first time. We test the
method with a Gaia mock catalogue that has the Gaia Universe Model Snapshot
(GUMS) as a background, and use a library of around 30 000 UFDGs simulated as
Plummer spheres with a single stellar population. For the UFDGs we use a wide
range of structural and orbital parameters that go beyond the range spanned by
real systems, where some UFDGs may remain undetected. We characterize the
detection limits as function of the number of observable stars by Gaia in the
UFDGs with respect to that of the background and their apparent sizes in the
sky and proper motion planes. We find that the addition of proper motions in
the search improves considerably the detections compared to a photometric
survey at the same magnitude limit. Our experiments suggest that Gaia will be
able to detect UFDGs that are similar to some of the known UFDGs even if the
limit of Gaia is around 2 magnitudes brighter than that of SDSS, with the
advantage of having a full-sky catalogue. We also see that Gaia could even find
some UFDGs that have lower surface brightness than the SDSS limit.Comment: Accepted for publication in MNRA
The imprints of the Galactic Bar on the Thick Disk with RAVE
We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius centered on the Sun, in the RAVE data set. We find clear asymmetries in the v R v∞ velocity distributions of thin and thick disk stars: there are more stars moving radially outward for low azimuthal velocities and more radially inward for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick-disk stars should no longer be described by Schwarzschilds, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully
Tails and streams around the Galactic globular clusters NGC 1851, NGC 1904, NGC 2298 and NGC 2808
We present Dark Energy Camera imaging for the peculiar Galactic globular clusters NGC 1851, NGC 1904 (M 79), NGC 2298 and NGC 2808. Our deep photometry reveals that all the clusters have an important contribution of stars beyond their King tidal radii and present tails with different morphologies. We have also explored the surroundings of the clusters where the presence of the Canis Major overdensity and/or the low Galactic latitude Monoceros ring at d⊙ ∼ 8 kpc is evident. A second stellar system is found at d⊙ ∼ 17 kpc and spans at least 18 deg × 15 deg in the sky. As one of the possible scenarios to explain that feature, we propose that the unveiled system is part of Monoceros explained as a density wave moving towards the outer Milky Way. Alternatively, the unveiled system might be connected with other known halo substructures or associated with the progenitor dwarf galaxy of NGC 1851 and NGC 1904, which are widely considered accreted globular clusters
A novel method to bracket the corotation radius in galaxy discs:vertex deviation maps
We map the kinematics of stars in simulated galaxy disks with spiral arms
using the velocity ellipsoid vertex deviation (l). We use test particle
simulations, and for the first time, fully self-consistent high resolution
N-body models. We compare our maps with the Tight Winding Approximation model
analytical predictions. We see that for all barred models spiral arms rotate
closely to a rigid body manner and the vertex deviation values correlate with
the density peaks position bounded by overdense and underdense regions. In such
cases, vertex deviation sign changes from negative to positive when crossing
the spiral arms in the direction of disk rotation, in regions where the spiral
arms are in between corotation (CR) and the Outer Lindblad Resonance (OLR). By
contrast, when the arm sections are inside the CR and outside the OLR, l
changes from negative to positive.We propose that measurements of the vertex
deviations pattern can be used to trace the position of the main resonances of
the spiral arms. We propose that this technique might exploit future data from
Gaia and APOGEE surveys. For unbarred N-body simulations with spiral arms
corotating with disk material at all radii, our analysis suggests that no clear
correlation exists between l and density structures
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