894 research outputs found
HI studies of the Sculptor group galaxies. VIII. The background galaxies: NGC 24 and NGC 45
In order to complete our HI survey of galaxies in the Sculptor group area,
VLA observations of NGC 24 and NGC 45 are presented. These two galaxies of
similar magnitude M_B ~ -17.4 lie in the background of the Sculptor group and
are low surface brightness galaxies, especially NGC 45. The HI distribution and
kinematics are regular for NGC 24 while NGC 45 exhibits a kinematical twist of
its major axis. A tilted-ring model shows that the position angle of the major
axis changes by ~25 degrees. A best-fit model of their mass distribution gives
mass-to-light ratios for the stellar disk of 2.5 and 5.2 for NGC 24 and NGC 45
respectively. These values are higher than the ones expected from stellar
population synthesis models. Despite the large dark matter contribution, the
galaxy mass is still dominated by the stellar component in their very inner
regions. These high mass-to-light ratios are typical of what is seen in low
surface brightness galaxies and may indicate that, in those galaxies, disks are
far from the maximum disk case. The halo parameters derived from the best-fit
models are thus lower limits.Comment: 23 pages, 10 figures, accepted for publication in The Astronomical
Journa
Globular clusters and dwarf galaxies in Fornax - I. Kinematics in the cluster core from multi-object spectroscopy
We acquired radial velocities of a significant number of globular clusters
(GCs) on wide fields between galaxies in the nearby Fornax cluster of galaxies,
in order to derive their velocity dispersion radial profile and to probe the
dynamics of the cluster. We used FLAMES on the VLT to obtain accurate
velocities for 149 GCs, within a ~500x150 kpc strip centered on NGC 1399, the
Fornax central galaxy. These objects are at the very bright tail (M_V < -9.5)
of the GC luminosity function, overlapping the so-called ``ultra-compact
dwarfs'' magnitude range. Eight of the brightest FLAMES-confirmed members
indeed show hints of resolution in the subarcsecond pre-imaging data we used
for selecting the ~500 targets for FLAMES spectroscopy. Ignoring the GCs around
galaxies by applying 3d_25 diameter masks, we find 61 GCs of 20.0 < V < 22.2
lying in the intra-cluster (IC) medium. The velocity dispersion of the
population of ICGCs is 200 km/s at ~150 kpc from the central NGC 1399 and rises
to nearly 400 km/s at 200 kpc, a value which compares with the velocity
dispersion of the population of dwarf galaxies, thought to be infalling from
the surroundings of the cluster.Comment: To be published in A&A Letters. 4 pages, 3 figures, 3 table
Triangulum galaxy viewed by Planck
We used Planck data to study the M33 galaxy and find a substantial
temperature asymmetry with respect to its minor axis projected onto the sky
plane. This temperature asymmetry correlates well with the HI velocity field at
21 cm, at least within a galactocentric distance of 0.5 degree, and it is found
to extend up to about 3 degrees from the galaxy center. We conclude that the
revealed effect, that is, the temperature asymmetry and its extension, implies
that we detected the differential rotation of the M33 galaxy and of its
extended baryonic halo.Comment: 8 pages, 8 figures, in press on Astronomy and Astrophysics, main
journa
The catalog of radial velocity standard stars for the Gaia RVS: status and progress of the observations
A new full-sky catalog of Radial Velocity standard stars is being built for
the determination of the Radial Velocity Zero Point of the RVS on board of
Gaia. After a careful selection of 1420 candidates matching well defined
criteria, we are now observing all of them to verify that they are stable
enough over several years to be qualified as reference stars. We present the
status of this long-term observing programme on three spectrographs : SOPHIE,
NARVAL and CORALIE, complemented by the ELODIE and HARPS archives. Because each
instrument has its own zero-point, we observe intensively IAU RV standards and
asteroids to homogenize the radial velocity measurements. We can already
estimate that ~8% of the candidates have to be rejected because of variations
larger than the requested level of 300 m/s.Comment: Proceedings of SF2A2010, S. Boissier, M. Heydari-Malayeri, R. Samadi
and D. Valls-Gabaud (eds), 3 pages, 2 figure
Satellite Constellation Internet Affordability and Need
Large satellite constellations in low-Earth orbit seek to be the infrastructure for global broadband Internet and other telecommunication needs. We briefly review the impacts of satellite constellations on astronomy and show that the Internet service offered by these satellites will primarily target populations where it is unaffordable, not needed, or both. The harm done by tens to hundreds of thousands of low-Earth orbit satellites to astronomy, stargazers worldwide, and the environment is not acceptable
H-alpha Kinematics of the SINGS Nearby Galaxies Survey. II
This is the second part of an H-alpha kinematics follow-up survey of the
Spitzer Infrared Nearby Galaxies Survey (SINGS) sample. The aim of this program
is to shed new light on the role of baryons and their kinematics and on the
dark/luminous matter relation in the star forming regions of galaxies, in
relation with studies at other wavelengths. The data for 37 galaxies are
presented. The observations were made using Fabry-Perot interferometry with the
photon-counting camera FaNTOmM on 4 different telescopes, namely the
Canada-France-Hawaii 3.6m, the ESO La Silla 3.6m, the William Herschel 4.2m,
and the Observatoire du mont Megantic 1.6m telescopes. The velocity fields are
computed using custom IDL routines designed for an optimal use of the data. The
kinematical parameters and rotation curves are derived using the GIPSY
software. It is shown that non-circular motions associated with galactic bars
affect the kinematical parameters fitting and the velocity gradient of the
rotation curves. This leads to incorrect determinations of the baryonic and
dark matter distributions in the mass models derived from those rotation
curves.Comment: 18 pages, 5 figures, 4 tables. Accepted for publication in MNRAS. All
high-res. figures are available at
http://www.astro.umontreal.ca/fantomm/singsII
Asymmetries in random motions of neutral Hydrogen gas in spiral galaxies
(Abridged). It has been recently shown that random motions of the neutral
Hydrogen gas of the Triangulum galaxy (M33) exhibit a bisymmetric perturbation
which is aligned with the minor axis of the galaxy, suggesting a projection
effect. To investigate if perturbations in the velocity dispersion of nearby
discs are comparable to those of M33, the sample is extended to 32 galaxies
from The HI Nearby Galaxy Survey and the Westerbork HI Survey of Spiral and
Irregular Galaxies. We study velocity asymmetries in the disc planes by
performing Fourier transforms of high-resolution HI velocity dispersion maps
corrected for beam smearing effects, and measure the amplitudes and phase
angles of the Fourier harmonics. We find strong perturbations of first, second
and fourth orders. The strongest asymmetry is the bisymmetry, which is
predominantly associated with the presence of spiral arms. The first order
asymmetry is generally oriented close to the disc major axis, and the second
and fourth order asymmetries are preferentially oriented along intermediate
directions between the major and minor axes of the discs. These results are
evidence that strong projection effects shape the HI velocity dispersion maps.
The most likely source of systematic orientations is the anisotropy of
velocities, through the projection of streaming motions stronger along one of
the planar directions in the discs. Moreover, systematic phase angles of
asymmetries in the HI velocity dispersion could arise from tilted velocity
ellipsoids. We expect a larger incidence of correlation between the radial and
tangential velocities of HI gas. Our methodology is a powerful tool to
constrain the dominant direction of streaming motions and thus the shape of the
velocity ellipsoid of HI gas, which is de facto anisotropic at the angular
scales probed by the observations.Comment: 40 pages, 33 figures. Accepted for publication in Astronomy &
Astrophysics. Full resolution version available upon reques
On the flow map for 2D Euler equations with unbounded vorticity
In Part I, we construct a class of examples of initial velocities for which
the unique solution to the Euler equations in the plane has an associated flow
map that lies in no Holder space of positive exponent for any positive time. In
Part II, we explore inverse problems that arise in attempting to construct an
example of an initial velocity producing an arbitrarily poor modulus of
continuity of the flow map.Comment: http://iopscience.iop.org/0951-7715/24/9/013/ for published versio
Formation of singularities in solutions to ideal hydrodynamics of freely cooling inelastic gases
We consider solutions to the hyperbolic system of equations of ideal granular
hydrodynamics with conserved mass, total energy and finite momentum of inertia
and prove that these solutions generically lose the initial smoothness within a
finite time in any space dimension for the adiabatic index Further, in the one-dimensional case we introduce a solution
depending only on the spatial coordinate outside of a ball containing the
origin and prove that this solution under rather general assumptions on initial
data cannot be global in time too. Then we construct an exact axially symmetric
solution with separable time and space variables having a strong singularity in
the density component beginning from the initial moment of time, whereas other
components of solution are initially continuous.Comment: 13 pages, 3 figure
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