2,569 research outputs found
Radial Mixing in Galactic Discs
We show that spiral waves in galaxy discs churn the stars and gas in a manner
that largely preserves the overall angular momentum distribution and leads to
little increase in random motion. Changes in the angular momenta of individual
stars are typically as large as ~50% over the lifetime of the disk. The changes
are concentrated around the corotation radius for an individual spiral wave,
but since transient waves with a wide range of pattern speeds develop in rapid
succession, the entire disk is affected. This behaviour has profound
consequences for the metallicity gradients with radius in both stars and gas.
The ISM is also stirred by the same mechanism. We find observational support
for stirring, propose a simple model for the distribution of stars over
metallicity and age, and discuss other possible consequences.Comment: Accepted to appear in MNRAS, 13 pages, 16 figures, LaTeX uses
mn2e.cls Minor additions to text and one extra figur
The distribution of two-dimensional eccentricity of Sunyaev-Zeldovich Effect and X-ray surface brightness profiles
With the triaxial density profile of dark matter halos and the corresponding
equilibrium gas distribution, we derive two-dimensional Sunyaev-Zeldovich (SZ)
effect and X-ray surface brightness profiles for clusters of galaxies. It is
found that the contour map of these observables can be well approximated by a
series of concentric ellipses with scale-dependent eccentricities. The
statistical distribution of their eccentricities (or equivalently axial ratios)
is analyzed by taking into account the orientation of clusters with respect to
the line of sight and the distribution of the axial ratios and the
concentration parameters of dark matter halos. For clusters of mass
at redshift , the axial ratio is peaked at
for both SZ and X-ray profiles. For larger clusters, the
deviation from circular distributions is more apparent, with peaked at
for . To be more close to
observations, we further study the axial-ratio distribution for mass-limited
cluster samples with the number distribution of clusters at different redshifts
described by a modified Press-Schechter model. For a mass limit of value
, the average axial ratio is with a tail extended to . With fast advance of high
quality imaging observations of both SZ effect and X-ray emissions, our
analyses provide a useful way to probe cluster halo profiles and therefore to
test theoretical halo-formation models.Comment: 28 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128
The halos of elliptical galaxies are faint and difficult to explore, but they
contain vital clues to both structure and formation. We present the results of
an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby
elliptical NGC 5128. We extend the work of Hui et al.(1995) well into the halo
of the galaxy--out to distances of 100 and 50 kpc along the major and minor
axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these
780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe
exist at distances up to 80 kpc (~15 r_e), showing that the stellar halo
extends to the limit of our data. This study represents by far the largest
kinematic study of an elliptical galaxy to date, both in the number of velocity
tracers and in radial extent. We confirm the large rotation of the PNe along
the major axis, and show that it extends in a disk-like feature into the halo.
The rotation curve of the stars flattens at ~100 km/s with V/sigma between 1
and 1.5, and with the velocity dispersion of the PNe falling gradually at
larger radii. The two-dimensional velocity field exhibits a zero-velocity
contour with a pronounced twist, showing that the galaxy potential is likely
triaxial in shape, tending toward prolate. The total dynamical mass of the
galaxy within 80 kpc is ~5 x 10^{11} M_sun, with M/L_B ~ 13. This mass-to-light
ratio is much lower than what is typically expected for elliptical galaxies.Comment: 21 pages, 13 figures (figures 3-8 best viewed in color), accepted for
publication in the Astrophysical Journa
Getting the astrophysics and particle physics of dark matter out of next-generation direct detection experiments
The next decade will bring massive new data sets from experiments of the
direct detection of weakly interacting massive particle (WIMP) dark matter. The
primary goal of these experiments is to identify and characterize the
dark-matter particle species. However, mapping the data sets to the
particle-physics properties of dark matter is complicated not only by the
considerable uncertainties in the dark-matter model, but by its poorly
constrained local distribution function (the "astrophysics" of dark matter). In
this Letter, I propose a shift in how to do direct-detection data analysis. I
show that by treating the astrophysical and particle physics uncertainties of
dark matter on equal footing, and by incorporating a combination of data sets
into the analysis, one may recover both the particle physics and astrophysics
of dark matter. Not only does such an approach yield more accurate estimates of
dark-matter properties, but may illuminate how dark matter coevolves with
galaxies.Comment: 4 pages, 4 figures, replaced to match version accepted by Phys. Rev.
A New Estimate of the Hubble Time with Improved Modeling of Gravitational Lenses
This paper examines free-form modeling of gravitational lenses using Bayesian
ensembles of pixelated mass maps. The priors and algorithms from previous work
are clarified and significant technical improvements are made. Lens
reconstruction and Hubble Time recovery are tested using mock data from simple
analytic models and recent galaxy-formation simulations. Finally, using
published data, the Hubble Time is inferred through the simultaneous
reconstruction of eleven time-delay lenses. The result is
H_0^{-1}=13.7^{+1.8}_{-1.0} Gyr.Comment: 24 pages, 9 figures. Accepted to Ap
Black Hole Motion as Catalyst of Orbital Resonances
The motion of a black hole about the centre of gravity of its host galaxy
induces a strong response from the surrounding stellar population. We treat the
case of a harmonic potential analytically and show that half of the stars on
circular orbits in that potential shift to an orbit of lower energy, while the
other half receive a positive boost and recede to a larger radius. The black
hole itself remains on an orbit of fixed amplitude and merely acts as a
catalyst for the evolution of the stellar energy distribution function f(E). We
show that this effect is operative out to a radius of approx 3 to 4 times the
hole's influence radius, R_bh. We use numerical integration to explore more
fully the response of a stellar distribution to black hole motion. We consider
orbits in a logarithmic potential and compare the response of stars on circular
orbits, to the situation of a `warm' and `hot' (isotropic) stellar velocity
field. While features seen in density maps are now wiped out, the kinematic
signature of black hole motion still imprints the stellar line-of-sight mean
velocity to a magnitude ~18% the local root mean-square velocity dispersion
sigma.Comment: revised version, typos fixed, added references, 20 pages MN styl
The proper motion of the Arches cluster with Keck Laser-Guide Star Adaptive Optics
We present the first measurement of the proper motion of the young, compact
Arches cluster near the Galactic center from near-infrared adaptive optics (AO)
data taken with the recently commissioned laser-guide star (LGS) at the Keck
10-m telescope. The excellent astrometric accuracy achieved with LGS-AO
provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken
4.3 years earlier. Over the 4.3 year baseline, a spatial displacement of the
Arches cluster with respect to the field population is measured to be 24.0 +/-
2.2 mas, corresponding to a proper motion of 5.6 +/- 0.5 mas/yr or 212 +/- 29
km/s at a distance of 8 kpc. In combination with the known line-of-sight
velocity of the cluster, we derive a 3D space motion of 232 +/- 30 km/s of the
Arches relative to the field. The large proper motion of the Arches cannot be
explained with any of the closed orbital families observed in gas clouds in the
bar potential of the inner Galaxy, but would be consistent with the Arches
being on a transitional trajectory from x1 to x2 orbits. We investigate a
cloud-cloud collision as the possible origin for the Arches cluster. The
integration of the cluster orbit in the potential of the inner Galaxy suggests
that the cluster passes within 10 pc of the supermassive black hole only if its
true GC distance is very close to its projected distance. A contribution of
young stars from the Arches cluster to the young stellar population in the
inner few parsecs of the GC thus appears increasingly unlikely. The measurement
of the 3D velocity and orbital analysis provides the first observational
evidence that Arches-like clusters do not spiral into the GC. This confirms
that no progenitor clusters to the nuclear cluster are observed at the present
epoch.Comment: 22 pdflatex pages including 12 figures, reviewed version accepted by
Ap
What will Gaia tell us about the Galactic disk?
Gaia will provide parallaxes and proper motions with accuracy ranging from 10
to 1000 microarcsecond on up to one billion stars. Most of these will be disk
stars: for an unreddened K giant at 6 kpc, it will measure the distance
accurate to 15% and the transverse velocity to an accuracy of about 1 km/s.
Gaia will observe tracers of Galactic structure across the whole HR diagram,
including Cepheids, RR Lyrae, white dwarfs, F dwarfs and HB stars. Onboard low
resolution spectrophotometry will permit -- in addition to a Teff estimate --
dwarf/giant discrimination, metallicity measurement and extinction
determination. For the first time, then, Gaia will provide us with a 3D
spatial/properties map and at least a 2D velocity map of these tracers (RVs
will be obtained too for brighter stars.) This will be a goldmine of
information from which to learn about the origin and evolution of the Galactic
disk. I briefly review the Gaia mission, and then show how the expected
astrometric accuracies translate into distance and velocity accuracies and
statistics. I examine the impact Gaia should have on a few scientific areas
relevant to the Galactic disk. I discuss how a better determination of the
spiral arm locations and pattern speed, plus a better reconstruction of the
Sun's orbit over the past billion years (from integration through the
Gaia-measured gravitational potential) will allow us to assess the possible
role of spiral arm crossings in ice ages and mass extinctions on the Earth.Comment: Proceedings of IAU 254 "The Galaxy disk in a cosmological context",
Copenhagen, June 2008, invited talk, 8 pages. This version: corrected K giant
distance accurac
Accretion by the Galaxy
Cosmology requires at least half of the baryons in the Universe to be in the
intergalactic medium, much of which is believed to form hot coronae around
galaxies. Star-forming galaxies must be accreting from their coronae. HI
observations of external galaxies show that they have HI halos associated with
star formation. These halos are naturally modelled as ensembles of clouds
driven up by supernova bubbles. These models can fit the data successfully only
if clouds exchange mass and momentum with the corona. As a cloud orbits, it is
ablated and forms a turbulent wake where cold high-metallicity gas mixes with
hot coronal gas causing the prompt cooling of the latter. As a consequence the
total mass of HI increases. This model has recently been used to model the
Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's
parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a
remarkable degree of consistency, despite the very different natures of the two
external galaxies and the dramatic difference in the nature of the data for our
Galaxy and the external galaxies. The parameter values are also consistent with
hydrodynamical simulations of the ablation of individual clouds. The model
predicts that a galaxy that loses its cool-gas disc for instance through a
major merger cannot reform it from its corona; it can return to steady star
formation only if it can capture a large body of cool gas, for example by
accreting a gas-rich dwarf. Thus the model explains how major mergers can make
galaxies "red and dead."Comment: Invited review at "Assembling the Puzzle of the Milky Way", Grand
Bornand, April 2011; 6 page
Cooling flows and quasars: different aspects of the same phenomenon? I. Concepts
We present a new class of solutions for the gas flows in elliptical galaxies
containing massive central black holes (BH). Modified King model galaxies are
assumed. Two source terms operate: mass loss from evolving stars, and a
secularly declining heating by SNIa. Relevant atomic physical processes are
modeled in detail. Like the previous models investigated by Ciotti et al.
(1991), these new models first evolve through three consecutive evolutionary
stages: wind, outflow, and inflow. At this point the presence of the BH alters
dramatically the subsequent evolution, because the energy emitted by the BH can
heat the surrounding gas to above virial temperatures, causing the formation of
a hot expanding central bubble. Short and strong nuclear bursts of radiation
are followed by longer periods during which the X-ray galaxy emission comes
from the coronal gas (Lx). The range and approximate distribution spanned by Lx
are found to be in accordance with observations of X-ray early type galaxies.
Moreover, although high accretion rates occur during bursting phases when the
central BH has a luminosity characteristic of QSOs, the total mass accreted is
very small when compared to that predicted by stationary cooling-flow solutions
and computed masses are in accord with putative BH nuclear masses. In the
bursting phases Lx is low and the surface brightness profile is very low
compared to pre-burst or to cooling flow models. We propose that these new
models, while solving some long-standing problems of the cooling flow scenario,
can provide a unified description of QSO-like objects and X-ray emitting
elliptical galaxies, these being the same objects observed at two different
evolutionary phases.Comment: 10 pages, ApJ LaTeX, plus 5 .eps figures and TeX-macro aasms4.sty -
revised version - in press on ApJ Letter
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