513 research outputs found
The Highly Flattened Dark Matter Halo of NGC 4244
In a previous paper (Olling 1995, \aj, 110, 591; astro-ph/9505002) a method
was developed to determine the shapes of dark matter halos of spiral galaxies
from an accurate determination of the rotation curve, the flaring of the gas
layer and the velocity dispersion in the HI. Here this method is applied to the
almost edge-on Scd galaxy NGC 4244 for which the necessary parameters are
determined in the accompanying paper (AJ, Aug. 1996; astro-ph/9605110).
The observed flaring of the HI beyond the optical disk puts significant
constraints on the shape of the dark matter halo, which are almost independent
of the stellar mass-to-light ratio. NGC 4244's dark matter halo is found to be
highly flattened with a shortest-to-longest axis ratio of 0.2 (-0.1)(+0.3). If
the dark matter is disk-like, the data presented in this paper imply that the
vertical velocity dispersion of the dark matter must be 10% - 30% larger than
the measured tangential dispersion in the HI. Alternatively, the measured
flaring curve is consistent with a round halo if the gaseous velocity
dispersion ellipsoid is anisotropic. In that case the vertical dispersion of
the gas is 50 - 70% of the measured tangential velocity dispersion.Comment: 16 pages LaTeX, uses aaspptwo style (357kByte). Includes 3 figures.
Complete paper, is also available at
http://www.astro.soton.ac.uk/~olling/PrePrints/Paper_03/ or via anonymous ftp
at ftp.astro.soton.ac.uk, cd pub/olling/Paper_03 To be published in the Aug.
1996 issue of the Astronomical Journa
CERTCOST News
The EU research project, CERTCOST, has issued the first digital CERTCOST newsletter. Despite the fact that organic
certification and its costs might be a somewhat dry subject from a consumer’s point of view, we intend to present the results in a eatable manner. The International Center for Research in Organic Food Systems, ICROFS, is responsible for dissemination of the CERTCOST project
Organic for the future
Organic research and innovation moves the organic food sector from a niche to a general reference model for sustainable agriculture.
Europe faces major challenges to conserve biodiversity, secure soil, and promote animal welfare and there is an urgent need to ensure global food security. Organic food and farming paves the way to meet these challenges; but to
succeed a further development of organic food and farming is key
Accurate Extra-Galactic Distances and Dark Energy: Anchoring the Distance Scale with Rotational Parallaxes
We investigate how the uncertainty on the Hubble constant (H_0) affects the
uncertainty in the Equation of State (EOS) of Dark Energy and the total density
of the Universe (Omega_tot). We use the approximate relations between the
cosmological parameters [Spergel etal (2007)] and use error-propagation to
estimate the effects of improving the CMB parameters and H_0 on the EOS of Dark
Energy (DE). First we assume that the additional data does not improve
significantly, but decrease the error on H_0 by a factor <~10. Second, we allow
improved additional data but current H_0 errors (i.e., the DE Task Force case).
In the 1st scenario, improvements of the CMB parameters hardly change the
accuracy of the EOS and Omega_tot, unless H_0 can be measured with an accuracy
of a few %. We find that a combination of moderate improvements for both H_0
and other data significantly constrains the evolution of dark energy, but at a
reduced cost. We review several methods (and their strengths and weaknesses)
that might yield extra-galactic distances with errors of about 1%. We review:
the Velocity Field method, two Maser methods, two Light Echo techniques, the
Binary Star method, and the Rotational Parallax (RP) technique. Because these
methods substantially rely on geometry rather than astrophysics or cosmology,
their results are quite robust. We focus on the advantages of the RP technique
which can provide single-step, bias-free distances to nearby spirals. These
distances can be used to improve the zero-point for other methods which in turn
allow for a much improved H_0 errors. Achieving an accuracy of ~2% in the
distances to M31, M33 and the LMC by the RP method requires proper motions from
future astrometric missions (SIM, GAIA and OBSS, or the SKA).Comment: 20 pages, 2 figures. Minor changes. Now includes a review of methods
capable of achieving unbiased 1% extra-galactic distances. MNRAS: Accepted
for publicatio
Luminous and Dark Matter in the Milky Way
(Abridged) Axisymmetric models of the Milky Way exhibit strong interrelations
between the Galactic constants (R_0 and T_0), the stellar columndensity (S_*)
and the shape of the dark matter (DM) halo. Here we present analytical
relations that can be used to investigate the effects of the uncertain gaseous
velocity dispersion on the HI flaring constraints. The contribution of cosmic
rays and magnetic fields to the pressure gradients is small. A significantly
flattened dark matter halo is only possible if R_0 <~ 6.8 kpc.
If R_0 is larger than ~7 kpc, or T_0 >~ 170 km/s, we can rule out two DM
candidates that require a highly flattened DM halo: 1) decaying massive
neutrinos; and 2) a disk of cold molecular hydrogen.
It is only possible to construct self-consistent models of the Galaxy based
on the IAU-recommended values for the Galactic constants in the unlikely case
that the the stellar columndensity is smaller than ~18 M_sun/pc^2. If we assume
that the halo is oblate and S_* = 35 +/- 5 M_sun/pc^2, R_0 <~ 8 kpc and T_0 <~
200 km/s.
Combining the best kinematical and star-count estimates of S_*, we conclude
that: 25 <~ S_* <~ 45 M_sun/pc^2. Kuijken & Gilmore's (1991) determination of
the columndensity of matter with |z|<=1.1 kpc is robust and valid over a wide
range of Galactic constants.
Our mass models show that the DM density in the Galactic centre is uncertain
by a factor 1000. In the Solar neighbourhood we find: rho_DM ~0.42 GeV/c^2/cm^3
or (11 +/- 5) mM_sun/pc^3 -- roughly 15% of rho_tot.Comment: Accepted for publication in MNRA
Refining the Oort and Galactic constants
The local stellar kinematics of the Milky Way offer a useful tool for
studying the rotation curve of the Galaxy. These kinematics -- usually
parameterized by the Oort constants A and B -- depend on the local gradient of
the rotation curve as well as its absolute value (Theta_0), and the Sun's
distance to the Galactic center (R_0). The density of interstellar gas in the
Milky Way is shown to vary non-monotonically with radius, and so contributes
significantly to the local gradient of the rotation curve. We have therefore
calculated mass models for the Milky Way that include this component, and have
derived the corresponding radial variation in the Oort constants. Between
0.9R_0 and 1.2R_0, the Oort functions, A(R) and B(R), differ significantly from
the general Theta/R dependence. Various previously-inexplicable observations
are shown to be consistent with these predictions. These models can explain the
40% difference between the values for 2 A R_0 derived from radial velocity data
originating in the inner and outer Galaxy. They also go some way toward
explaining the different shapes of the velocity ellipsoids of giant and dwarf
stars in the solar neighbourhood. However, a consistent picture only emerges if
one adopts small values of R_0 = 7.1 +/- 0.4 kpc and Theta_0 = 184 +/- 8 km/s.
With these Galactic constants, the Milky Way's rotation curve declines slowly
in the outer Galaxy; V_rot(20 kpc) = 166 kms. Our low value for R_0 agrees well
with the only direct determination (7.2 +/- 0.7 kpc, Reid 1993). Using these
Galactic constants, we find that the proper motion of Sgr A^* is consistent
with the observational constraints. The radial velocities and proper motions of
our best fit model are entirely consistent with the radial velocities of
Cepheids and the Hipparcos measurements of their proper motions.Comment: 11 pages, LaTeX, including 5 figures, using mn and epsf style files.
Accepted for publication in MNRA
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