2,284 research outputs found
HI study of the warped spiral galaxy NGC5055: a disk/dark matter halo offset?
We present a study of the HI distribution and dynamics of the nearby spiral
galaxy NGC5055 based on observations with the Westerbork Synthesis Radio
Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to
3.5 R_25, and shows a pronounced warp that starts at the end of the bright
optical disk (R_25= 11.6 kpc). This very extended warp has large-scale
symmetry, which along with the rotation period of its outer parts (~1.5 Gyr at
40 kpc), suggests a long-lived phenomenon. The rotation curve rises steeply in
the central parts up to the maximum velocity (v_max ~ 206 km/s). Beyond the
bright stellar disk (R_25), it shows a decline of about 25 km/s and then
remains flat out to the last measured point. The standard analysis with
luminous and dark matter components shows the dynamical importance of the disk.
The best fit to the rotation curve is obtained with a ``maximum disk''. Less
satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to
the mass-to-light ratio in F band of the disk. Such a ``minimum disk''
contributes about 60% of the observed maximum rotational velocity. NGC5055
shows remarkable overall regularity and symmetry. A mild lopsidedness is
noticeable, however, both in the distribution and kinematics of the gas. The
tilted ring analysis of the velocity field led us to adopt different values for
the kinematical centre and for the systemic velocity for the inner and the
outer parts of the system. This has produced a remarkable result: the
kinematical and geometrical asymmetries disappear, both at the same time. These
results point at two different dynamical regimes: an inner region dominated by
the stellar disk and an outer one, dominated by a dark matter halo offset with
respect to the disk.Comment: Accepted for publication in A&A. Minor correction
Hydrostatic models for the rotation of extra-planar gas in disk galaxies
We show that fluid stationary models are able to reproduce the observed,
negative vertical gradient of the rotation velocity of the extra-planar gas in
spiral galaxies. We have constructed models based on the simple condition that
the pressure of the medium does not depend on density alone (baroclinic instead
of barotropic solutions: isodensity and isothermal surfaces do not coincide).
As an illustration, we have successfully applied our method to reproduce the
observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid
stationary models discussed here can describe a hot homogeneous medium as well
as a "gas" made of discrete, cold HI clouds with an isotropic velocity
dispersion distribution. Although the method presented here generates a density
and velocity field consistent with observational constraints, the stability of
these configurations remains an open question.Comment: 12 pages, 9 figures. Accepted for publication in Astronomy and
Astrophysic
Extra-planar gas in the spiral galaxy NGC 4559
We present 21-cm line observations of the spiral galaxy NGC 4559, made with
the Westerbork Synthesis Radio Telescope. We have used them to study the HI
distribution and kinematics, the relative amount and distribution of luminous
and dark matter in this galaxy and, in particular, the presence of extra-planar
gas. Our data do reveal the presence of such a component, in the form of a
thick disk, with a mass of 5.9 x 10^8 Mo (one tenth of the total HI mass) and a
mean rotation velocity 25-50 km/s lower than that of the thin disk. The
extra-planar gas may be the result of galactic fountains but accretion from the
IGM cannot be ruled out. With this study we confirm that lagging, thick HI
layers are likely to be common in spiral galaxies.Comment: 17 pages, 10 figures. Accepted for publication in A&
Kinematics of the Ionised Gas in the Spiral Galaxy NGC 2403
We present a study of the kinematics of the ionised gas in the nearby spiral
galaxy NGC 2403 using deep long-slit spectra obtained with the 4.2-m William
Herschel Telescope. The data show the presence of a halo component of ionised
gas that is rotating more slowly than the gas in the disk. The kinematics of
this ionised halo gas is similar to that of the neutral halo gas. On small
scales, broad line profiles (up to 300 km/s wide) indicate regions of fast
outflows of ionised gas. We discuss these new results in the context of
galactic fountain models.Comment: 17 pages, 10 figures, accepted for publication in A&
Multiscale Mass-Spring Models of Carbon Nanotube Arrays Accounting for Mullins-like Behavior and Permanent Deformation
Based on a one-dimensional discrete system of bistable springs, a mechanical model is introduced to describe plasticity and damage in carbon nanotube (CNT) arrays. The energetics of the mechanical system are investigated analytically, the stress-strain law is derived, and the mechanical dissipation is computed, both for the discrete case as well as for the continuum limit. An information-passing approach is developed that permits the investigation of macroscopic portions of the material. As an application, the simulation of a cyclic compression experiment on real CNT foam is performed, considering both the material response during the primary loading path from the virgin state and the damaged response after preconditioning
On the additive manufacturing, post-tensioning and testing of bi-material tensegrity structures
An investigation on the additive manufacturing and the experimental testing of 3D models of tensegrity prisms and columns is presented. An Electron Beam Melting facility (Arcam EBM S12) is employed to 3D print structures composed of tensegrity prisms endowed with rigid bases and temporary supports, which are made out of the titanium alloy Ti6Al4V. The temporary supports are removed after the additive manufacturing phase, when Spectra cross-strings are added to the 3D printed models, and a suitable state of internal prestress is applied to the structure. The experimental part of the study shows that the examined structures feature stiffening-type elastic response under large or moderately large axial strains induced by compressive loading. Such a geometrically nonlinear behavior confirms previous theoretical results available in the literature, and paves the way to the use of tensegrity prisms and columns as innovative mechanical metamaterials and smart devices
The gaseous haloes of disc galaxies
The study of gas outside the plane of disc galaxies is crucial to
understanding the circulation of material within a galaxy and between galaxies
and the intergalactic environment. We present new HI observations of the
edge-on galaxy NGC891, which show an extended halo component lagging behind the
disc in rotation. We compare these results for NGC891 with other detections of
gaseous haloes. Finally, we present a dynamical model for the formation of
extra-planar gas.Comment: 6 pages, 2 figures; to appear in the proceedings of the Island
Universes conference held in Terschelling, Netherlands, July 2005, ed. R. de
Jon
Kinematics of diffuse ionized gas in the disk halo interface of NGC 891 from Fabry-P\'erot observations
The properties of the gas in halos of galaxies constrain global models of the
interstellar medium. Kinematical information is of particular interest since it
is a clue to the origin of the gas. Here we report observations of the
kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to
determine the rotation curve of the halo gas. We have obtained a Fabry-P\'erot
data cube in Halpha to measure the kinematics of the halo gas with angular
resolution much higher than obtained from HI 21 cm observations. The data cube
was obtained with the TAURUS II spectrograph at the WHT on La Palma. The
velocity information of the diffuse ionized gas extracted from the data cube is
compared to model distributions to constrain the distribution of the gas and in
particular the halo rotation curve. The best fit model has a central
attenuation tau_H-alpha=6, a dust scale length of 8.1 kpc, an ionized gas scale
length of 5.0 kpc. Above the plane the rotation curve lags with a vertical
gradient of -18.8 km/s/kpc. We find that the scale length of the H-alpha must
be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation
curve above the plane rises less steeply than in the plane. This is all in
agreement with the velocities measured in the HI.Comment: A&A, in press. 13 pages, 19 figure
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