42 research outputs found
On the vertical equilibrium of the local Galactic disk and the search for disk dark matter
Estimates of the dynamical surface mass density at the solar Galactocentric
distance are commonly derived assuming that the disk is in vertical equilibrium
with the Galactic potential. This assumption has recently been called into
question, based on the claim that the ratio between the kinetic and the
gravitational energy in such solutions is a factor of 3 larger than required if
Virial equilibrium is to hold. Here we show that this ratio between energies
was overerestimated and that the disk solutions are likely to be in Virial
equilibrium after all. We additionally demonstrate, using one-dimensional
numerical simulations, that the disks are indeed in equilibrium. Hence, given
the uncertainties, we find no reason to cast doubt on the steady-state
solutions which are traditionally used to measure the matter density of the
disk.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
The shape of "dark matter" halos in disc galaxies according to the modified dynamics
Analyses of halo shapes for disc galaxies are said to give incongruous
results. I point out that the modified dynamics (MOND) predicts for disc
galaxies a distribution of fictitious dark matter that comprises two
components: a pure disc and a rounder halo. The former dominates the true disc
in regions of small accelerations, where it controls the z-dynamics in the disc
(disc flaring etc.); it has a finite total mass. It also dominates the round
component near the centre where the geometry is nearly planar. The second
component controls motions far from the plane, has a total enclosed mass that
diverges linearly with radius, and determines the rotation curve at large
radii. Its ellipticity may be appreciable at small radii but vanishes
asymptotically. This prediction of MOND differs from what one expects from
galaxy-formation scenarios with dark matter.
Analyses to date, which, as they do, assume one component--usually with a
constant ellipticity, perforce give conflicting results for the best-value
ellipticity, depending on whether they probe the disc or the sphere, small
radii or large ones.Comment: 8 page
The line-of-sight warp of the spiral galaxy ESO 123-G23
We present 3-D modelling of the distribution and kinematics of the neutral
hydrogen in the spiral galaxy ESO 123-G23. The optical appearance of this
galaxy is an almost perfectly edge-on disk, while the neutral hydrogen is found
to extend vertically out to about 15 kpc on either side of the galactic plane.
The HI layer and the major features of the HI data cube can be successfully
explained by a model dominated by a strong (about 30 degrees) line-of-sight
warp. Other models were tried, including a flare model and a two-component
model, but they clearly do not reproduce the data. This is the first
unambiguous detection of a galactic warp that has the maximum deviation from
the central plane almost along the line-of-sight. No evidence for the presence
of any companion galaxy is found in the HI data cube. Line-of-sight warps in
edge-on galaxies are probably frequent, but escape detection as they are too
weak. Moreover they may easily be mistaken as flares or 'thick disks'. A 3-D
modelling of the HI layer as the one presented here is needed in order to
distinguish between these possibilities.Comment: 5 pages, 5 figures, accepted by A&
The Flattened Dark Matter Halo of M31 as Deduced from the Observed HI Scale Heights
In this paper, we use the outer-galactic HI scale height data as well as the
observed rotation curve as constraints to determine the halo density
distribution of the Andromeda galaxy (M31). We model the galaxy as a
gravitationally-coupled system of stars and gas, responding to the external
force-field of a known Hernquist bulge and the dark matter halo, the density
profile of the latter being characterized by four free parameters. The
parameter space of the halo is optimized so as to match the observed HI
thickness distribution as well as the rotation curve on an equal footing,
unlike the previous studies of M31 which were based on rotation curves alone.
We show that an oblate halo, with an isothermal density profile, provides the
best fit to the observed data. This gives a central density of 0.011 M_sun
/pc^3, a core radius of 21 kpc, and an axis ratio of 0.4. The main result from
this work is the flattened dark matter halo for M31, which is required to match
the outer galactic HI scale height data. Interestingly, such flattened halos
lie at the most oblate end of the distribution of halo shapes found in recent
cosmological simulations.Comment: 21 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Formation of polar ring galaxies
Polar ring galaxies are peculiar systems in which a gas rich, nearly polar
ring surrounds an early-type or elliptical host galaxy. Two formation scenarios
for these objects have been proposed: they are thought to form either in major
galaxy mergers or by tidal accretion of the polar material from a gas rich
donor galaxy. Both scenarios are studied through N-body simulations including
gas dynamics and star formation. Constraints on physical parameters are drawn
out, in order to determine which scenario is the most likely to occur. Polar
ring galaxies from each scenario are compared with observations and we discuss
whether the accretion scenario and the merging scenario account for
observational properties of polar ring galaxies. The conclusion of this study
is that the accretion scenario is both the most likely and the most supported
by observations. Even if the merging scenario is rather robust, most polar ring
galaxies are shown to be the result of tidal gas accretion events.Comment: Accepted for publication in A&A. 20 pages, 14 figures. The resolution
of several figures has been reduced in this versio
The cold gaseous halo of NGC 891
We present HI observations of the edge-on galaxy NGC 891. These are among the
deepest ever performed on an external galaxy. They reveal a huge gaseous halo,
much more extended than seen previously and containing almost 30 % of the HI.
This HI halo shows structures on various scales. On one side, there is a
filament extending (in projection) up to 22 kpc vertically from the disk. Small
(M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently
counter-rotating) velocities, are also detected. The overall kinematics of the
halo gas is characterized by differential rotation lagging with respect to that
of the disk. The lag, more pronounced at small radii, increases with height
from the plane. There is evidence that a significant fraction of the halo is
due to a galactic fountain. Accretion from intergalactic space may also play a
role in building up the halo and providing low angular momentum material needed
to account for the observed rotation lag. The long HI filament and the
counter-rotating clouds may be direct evidence of such accretion.Comment: Accepted for publication in The Astronomical Journal. High-resolution
version available at http://www.astron.nl/~oosterlo/n891Pape
Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way
An all high-latitude sky survey for cool carbon giant stars in the Galactic
halo has revealed 75 such stars, of which the majority are new detections. Of
these, more than half are clustered on a Great Circle on the sky which
intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its
proper motion vector, while many of the remainder are outlying Magellanic Cloud
C-stars. A pole-count analysis of the carbon star distribution clearly
indicates that the Great Circle stream we have isolated is statistically
significant, being a 5-6 sigma over-density. These two arguments strongly
support our conclusion that a large fraction of the Halo carbon stars
originated in Sgr. The stream orbits the Galaxy between the present location of
Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star,
at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a
large range in both Galactic R and z distances are probed. That the stream is
observed as a Great Circle indicates that the Galaxy does not exert a
significant torque upon the stream, so the Galactic potential must be nearly
spherical in the regions probed by the stream. We present N-body experiments
simulating this disruption process as a function of the distribution of mass in
the Galactic halo. A likelihood analysis shows that, in the Galactocentric
distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost
spherical. We rule out, at high confidence levels, the possibility that the
Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7.
This result is quite unexpected and contests currently popular galaxy formation
models. (Abridged)Comment: 26 pages, 13 figures (6 in color, 8 chunky due to PS compression),
minor revisions, accepted by Ap
Periodic orbits in warped disk
It is often assumed that a warped galaxy can be modeled by a set of rings.
This paper verifies numerically the validity of this assumption by the study of
periodic orbits populating a heavy self-gravitating warped disk. The phase
space structure of a warped model reveals that the circular periodic orbits of
a flat disk are transformed in quasi annular periodic orbits which conserve
their stability. This lets us also explore the problem of the persistence of a
large outer warp. In particular, the consistency of its orbits with the density
distribution is checked as a function of the pattern speed.Comment: 9 pages, including 11 figures. Accepted for publication in A&