838 research outputs found
Vertical velocities from proper motions of red clump giants
We derive the vertical velocities of disk stars in the range of
Galactocentric radii of R=5-16 kpc within 2 kpc in height from the Galactic
plane. This kinematic information is connected to dynamical aspects in the
formation and evolution of the Milky Way, such as the passage of satellites and
vertical resonance and determines whether the warp is a long-lived or a
transient feature.
We used the proper motions of the PPMXL survey, correcting of systematic
errors with the reference of quasars. From the color-magnitude diagram K versus
(J-K) we selected the standard candles corresponding to red clump giants and
used the information of their proper motions to build a map of the vertical
motions of our Galaxy. We derived the kinematics of the warp both analytically
and through a particle simulation to fit these data. Complementarily, we also
carried out the same analysis with red clump giants spectroscopically selected
with APOGEE data, and we predict the improvements in accuracy that will be
reached with future Gaia data.
A simple model of warp with the height of the disk z_w(R,phi)=gamma (R-R_sun)
sin(phi-phi_w) fits the vertical motions if d(gamma)/dt/gamma=-34+/-17
Gyr^{-1}; the contribution to d(gamma)/dt comes from the southern warp and is
negligible in the north. The vertical motion in the warp apparently indicates
that the main S-shaped structure of the warp is a long-lived feature, whereas
the perturbation that produces an irregularity in the southern part is most
likely a transient phenomenon. With the use of the Gaia end-of-mission products
together with spectroscopically classified red clump giants, the precision in
vertical motions can be increased by an order of magnitude at least.Comment: Accepted for publication in A&A. arXiv admin note: text overlap with
arXiv:1402.355
Black hole evolution: II. Spinning black holes in a supernova-driven turbulent interstellar medium
Supermassive black holes (BH) accrete gas from their surroundings and
coalesce with companions during galaxy mergers, and both processes change the
BH mass and spin. By means of high-resolution hydrodynamical simulations of
galaxies, either idealised or embedded within the cosmic web, we explore the
effects of interstellar gas dynamics and external perturbations on BH spin
evolution. All these physical quantities were evolved on-the-fly in a
self-consistent manner. We use a `maximal' model to describe the turbulence
induced by stellar feedback to highlight its impact on the angular momentum of
the gas accreted by the BH. Periods of intense star formation are followed by
phases where stellar feedback drives large-scale outflows and hot bubbles. We
find that BH accretion is synchronised with star formation, as only when gas is
cold and dense do both processes take place. During such periods, gas motion is
dominated by consistent rotation. On the other hand, when stellar feedback
becomes substantial, turbulent motion randomises gas angular momentum. However
BH accretion is strongly suppressed in that case, as cold and dense gas is
lacking. In our cosmological simulation, at very early times (z>6), the
galactic disc has not yet settled and no preferred direction exists for the
angular momentum of the accreted gas, so the BH spin remains low. As the gas
settles into a disc (6>z>3), the BH spin then rapidly reaches its maximal
value. At lower redshifts (z<3), even when galaxy mergers flip the direction of
the angular momentum of the accreted gas, causing it to counter-rotate, the BH
spin magnitude only decreases modestly and temporarily. Should this be a
typical evolution scenario for BH, it potentially has dramatic consequences
regarding their origin and assembly, as accretion on maximally spinning BH
embedded in thin Shakura-Sunyaev disc is significantly reduced.Comment: 16 pages, 13 figures, MNRAS accepte
Why are active galactic nuclei and host galaxies misaligned?
It is well established observationally that the characteristic angular momentum axis on small scales around active galactic nuclei (AGN), traced by radio jets and the putative torus, is not well correlated with the large-scale angular momentum axis of the host galaxy. In this paper, we show that such misalignments arise naturally in high-resolution simulations in which we follow angular momentum transport and inflows from galaxy to sub-pc scales near AGN, triggered either during galaxy mergers or by instabilities in isolated discs. Sudden misalignments can sometimes be caused by single massive clumps falling into the centre slightly off-axis, but more generally, they arise even when the gas inflows are smooth and trace only global gravitational instabilities. When several nested, self-gravitating modes are present, the inner ones can precess and tumble in the potential of the outer modes. Resonant angular momentum exchange can flip or re-align the spin of an inner mode on a short time-scale, even without the presence of massive clumps. We therefore do not expect that AGN and their host galaxies will be preferentially aligned, nor should the relative alignment be an indicator of the AGN fuelling mechanism. We discuss implications of this conclusion for AGN feedback and black hole (BH) spin evolution. The misalignments may mean that even BHs accreting from smooth large-scale discs will not be spun up to maximal rotation and so have more modest radiative efficiencies and inefficient jet formation. Even more random orientations/lower spins are possible if there is further unresolved clumpiness in the gas, and more ordered accretion may occur if the inflow is slower and not self-gravitating
Galaxia: a code to generate a synthetic survey of the Milky Way
We present here a fast code for creating a synthetic survey of the Milky Way.
Given one or more color-magnitude bounds, a survey size and geometry, the code
returns a catalog of stars in accordance with a given model of the Milky Way.
The model can be specified by a set of density distributions or as an N-body
realization. We provide fast and efficient algorithms for sampling both types
of models. As compared to earlier sampling schemes which generate stars at
specified locations along a line of sight, our scheme can generate a continuous
and smooth distribution of stars over any given volume. The code is quite
general and flexible and can accept input in the form of a star formation rate,
age metallicity relation, age velocity dispersion relation and analytic density
distribution functions. Theoretical isochrones are then used to generate a
catalog of stars and support is available for a wide range of photometric
bands. As a concrete example we implement the Besancon Milky Way model for the
disc. For the stellar halo we employ the simulated stellar halo N-body models
of Bullock & Johnston (2005). In order to sample N-body models, we present a
scheme that disperses the stars spawned by an N-body particle, in such a way
that the phase space density of the spawned stars is consistent with that of
the N-body particles. The code is ideally suited to generating synthetic data
sets that mimic near future wide area surveys such as GAIA, LSST and HERMES. As
an application we study the prospect of identifying structures in the stellar
halo with a simulated GAIA survey. We plan to make the code publicly available
at http://galaxia.sourceforge.net.Comment: Accepted for publication in Ap
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
High-Velocity Clouds in the Nearby Spiral Galaxy M 83
We present deep HI 21-cm and optical observations of the face-on spiral
galaxy M 83 obtained as part of a project to search for high-velocity clouds
(HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M
83, with 5.6x10^7 Msolar of HI contained in a disk rotating 40-50 km/s more
slowly in projection than the bulk of the gas. We interpret this as a
vertically extended thick disk of neutral material, containing 5.5% of the
total HI within the central 8 kpc. Using an automated source detection
algorithm to search for small-scale HI emission features, we find eight
distinct, anomalous-velocity HI clouds with masses ranging from 7x10^5 to
1.5x10^7 Msolar and velocities differing by up to 200 km/s compared to the HI
disk. Large on-disk structures are coincident with the optical spiral arms,
while unresolved off-disk clouds contain no diffuse optical emission down to a
limit of 27 r' mag per square arcsec. The diversity of the thick HI disk and
larger clouds suggests the influence of multiple formation mechanisms, with a
galactic fountain responsible for the slowly-rotating disk and on-disk discrete
clouds, and tidal effects responsible for off-disk cloud production. The mass
and kinetic energy of the HI clouds are consistent with the mass exchange rate
predicted by the galactic fountain model. If the HVC population in M 83 is
similar to that in our own Galaxy, then the Galactic HVCs must be distributed
within a radius of less than 25 kpc.Comment: 30 pages, 23 figures; accepted for publication in ApJ. Some figures
have been altered to reduce their siz
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