118 research outputs found
Deep wide field HI imaging of Messier 31
We report on preliminary results from a new deep 21-cm survey of the
Andromeda galaxy, based on observations performed with the Synthesis Telescope
and the 26-m antenna at DRAO. The HI distribution and kinematics of the disc
are analyzed and basic dynamical properties are derived. New HI structures are
discovered, like thin HI spur-like structures and an external arm in the disc
outskirts. The HI spurs are related to perturbed stellar clumps outside the
main disc of M31. The external arm lies on the far, receding side of the galaxy
and has no obvious counterpart in the opposite side. These HI perturbations
probably result from tidal interactions with companions. It is found a
dynamical mass of 4.7 +/- 0.5 x10^11 Msol enclosed within a radius R = 38 kpc
and a total mass of ~1 x10^12 Msol inside the virial radius.Comment: 4 pages, 1 figure, proceeding of the conference "Panoramic Radio
Astronomy: Wide-field 1-2 GHz research on galaxy evolution", June 02 - 05
2009, Groninge
The incorrect rotation curve of the Milky Way
In the fundamental quest of the rotation curve of the Milky Way, the
tangent-point (TP) method has long been the simplest way to infer velocities
for the inner, low latitude regions of the Galactic disk from observations of
the gas component. We test the validity of the method on realistic gas
distribution and kinematics of the Milky Way, using a numerical simulation of
the Galaxy. We show that the resulting velocity profile strongly deviates from
the true rotation curve of the simulation, as it overstimates it in the central
regions, and underestimates it around the bar corotation. Also, its shape
strongly depends on the orientation of the stellar bar. The discrepancies are
caused by highly non-uniform azimuthal velocities, and the systematic selection
by the TP method of high-velocity gas along the bar and spiral arms, or
low-velocity gas in less dense regions. The velocity profile is in good
agreement with the rotation curve only beyond corotation, far from massive
asymmetric structures. Therefore the observed velocity profile of the Milky Way
inferred by the TP method is expected to be very close to the true Galactic
rotation curve for 4.5<R<8 kpc. Another consequence is that the Galactic
velocity profile for R<4-4.5 kpc is very likely flawed by the non-uniform
azimuthal velocities, and does not represent the true Galactic rotation curve,
but instead local motions. The real shape of the innermost rotation curve is
probably shallower than previously thought. Using a wrong rotation curve has a
dramatic impact on the modelling of the mass distribution, in particular for
the bulge component of which derived enclosed mass within the central kpc and
scale radius are, respectively, twice and half of the actual values. We thus
strongly argue against using terminal velocities or the velocity curve from the
TP method for modelling the mass distribution of the Milky Way. (abridged)Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 10
figures, revised version after A&A language editin
Dark matter in low mass surface density galaxies
Low mass surface density spiral and irregular galaxies like low surface
brightness (LSB) and dwarf galaxies are unique laboratories to study the
dynamical properties of Dark Matter halos because their mass is generally
dominated by dark matter at all galactocentric radii. We present results from
the largest sample ever assembled of high resolution Halpha velocity fields of
LSB and dwarf galaxies in order to study their mass distributions.Comment: 2 pages, 1 figure, proceedings of the conference "Pathways Through an
Eclectic Universe", Johan Knapen, Terry Mahoney, and Alexandre Vazdekis ed
A mass-velocity anisotropy relation in galactic stellar disks
The ellipsoid of stellar random motions is a fundamental ingredient of galaxy
dynamics. Yet it has long been difficult to constrain this component in disks
others than the Milky Way. This article presents the modeling of the
azimuthal-to-radial axis ratio of the velocity ellipsoid of galactic disks from
stellar dispersion maps using integral field spectroscopy data of the CALIFA
survey. The measured azimuthal anisotropy is shown to be not strongly dependent
on the assumed vertical-to-radial dispersion ratio of the ellipsoid. The
anisotropy distribution shows a large diversity in the orbital structure of
disk galaxies from tangential to radial stellar orbits. Globally, the orbits
are isotropic in inner disk regions and become more radial as a function of
radius, although this picture tends to depend on galaxy morphology and
luminosity. The Milky Way orbital anisotropy profile measured from the Second
Gaia Data Release is consistent with those of CALIFA galaxies. A new
correlation is evidenced, linking the absolute magnitude or stellar mass of the
disks to the azimuthal anisotropy. More luminous disks have more radial orbits
and less luminous disks have isotropic and somewhat tangential orbits. This
correlation is consistent with the picture in galaxy evolution in which orbits
become more radial as the mass grows and is redistributed as a function of
time. With the help of circular velocity curves, it is also shown that the
epicycle theory fails to reproduce the diversity of the azimuthal anisotropy of
stellar random motions, as it predicts only nearly radial orbits in the
presence of flat curves. The origin of this conflict is yet to be identified.
It also questions the validity of the vertical-to-radial axis ratio of the
velocity ellipsoid derived by many studies in the framework of the epicyclic
approximation.Comment: Accepted for publication in Astronomy & Astrophysics on 18/07/2018,
final version after language editing, 19 pages, 24 figures, one long table
available online onl
Harmonic analysis of the Ha velocity field of NGC 4254
The ionized gas kinematics of the Virgo Cluster galaxy NGC 4254 (Messier 99)
is analyzed by an harmonic decomposition of the velocity field into Fourier
coefficients. The aims of this study are to measure the kinematical asymmetries
of Virgo cluster galaxies and to connect them to the environment. The analysis
reveals significant terms which origins are discussed.Comment: 3 pages, 2 figures, to appear in "Science Perspectives for 3D
Spectroscopy", ESO Astrophysics Symposia, M. Kissler-Patig, M.M. Roth & J.R.
Walsh ed
Kinematics and Mass Modeling of Messier 33: Halpha observations
As part of a long-term project to revisit the kinematics and dynamics of the
large disc galaxies of the Local Group, we present the first deep, wide-field
(42' x 56') 3D-spectroscopic survey of the ionized gas disc of Messier 33.
Fabry-Perot interferometry has been used to map its Ha distribution and
kinematics at unprecedented angular resolution (<3'') and resolving power
(12600), with the 1.6m telescope at the Observatoire du Mont Megantic. The
ionized gas distribution follows a complex, large-scale spiral structure,
unsurprisingly coincident with the already-known spiral structures of the
neutral and molecular gas discs. The kinematical analysis of the velocity field
shows that the rotation center of the Ha disc is distant from the photometric
center by 170 pc (sky projected distance) and that the kinematical major-axis
position angle and disc inclination are in excellent agreement with photometric
values. The Ha rotation curve agrees very well with the HI rotation curves for
0 6.5 kpc.
The reason for this discrepancy is not well understood. The velocity dispersion
profile is relatively flat around 16 km/s, which is at the low end of velocity
dispersions of nearby star-forming galactic discs. A strong relation is also
found between the Ha velocity dispersion and the Ha intensity. Mass models were
obtained using the Ha rotation curve but, as expected, the dark matter halo's
parameters are not very well constrained since the optical rotation curve only
extends out to 8 kpc.Comment: 26 pages, 18 figures, accepted for publication in MNRA
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