1,515 research outputs found
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
Two measures of the shape of the Milky Way's dark halo
In order to test the reliability of determinations of the shapes of galaxies'
dark matter halos, we have made such measurements for the Milky Way by two
independent methods, which make use of the stellar kinematics in the solar
neighbourhood and the observed flaring of the Galactic HI layer to estimate the
flattening of the Galactic dark halo. These techniques are found to produce a
consistent estimate for the halo shape, with a shortest-to-longest axis ratio
of q ~ 0.8, but only if one adopts somewhat non-standard values for the
distance to the Galactic centre, R_0, and the local Galactic rotation speed,
Theta_0. For consistency, one requires values of R_0 < 7.6 kpc and Theta_0 <
190 km/s. Although differing significantly from the current IAU-sanctioned
values, these upper limits are consistent with all existing observational
constraints. If future measurements confirm these lower values for the Galactic
constants, then the validity of the gas layer flaring method will be confirmed.
Further, dark matter candidates such as cold molecular gas and massive decaying
neutrinos, which predict very flat dark halos with q < 0.2, will be ruled out.
Conversely, if the Galactic constants were found to be close to the more
conventional values, then there would have to be some systematic error in the
methods for measuring dark halo shapes, so the existing modeling techniques
would have to be viewed with some scepticism.Comment: Accepted for publication in MNRAS. 10 pages, 6 figures, uses mn.sty
and epsf.st
Halo Geometry and Dark Matter Annihilation Signal
We study the impact of the halo shape and geometry on the expected weakly
interacting massive particle (WIMP) dark matter annihilation signal from the
galactic center. As the halo profile in the innermost region is still poorly
constrained, we consider different density behaviors like flat cores, cusps and
spikes, as well as geometrical distortions. We show that asphericity has a
strong impact on the annihilation signal when the halo profile near the
galactic center is flat, but becomes gradually less significant for cuspy
profiles, and negligible in the presence of a central spike. However, the
astrophysical factor is strongly dependent on the WIMP mass and annihilation
cross-section in the latter case.Comment: 5 pages, 4 figures, PR
Radial Dependence of the Pattern Speed of M51
The grand-design spiral galaxy M51 has long been a crucial target for
theories of spiral structure. Studies of this iconic spiral can address the
question of whether strong spiral structure is transient (e.g.
interaction-driven) or long-lasting. As a clue to the origin of the structure
in M51, we investigate evidence for radial variation in the spiral pattern
speed using the radial Tremaine-Weinberg (TWR) method. We implement the method
on CO observations tracing the ISM-dominant molecular component. Results from
the method's numerical implementation--combined with regularization, which
smooths intrinsically noisy solutions--indicate two distinct patterns speeds
inside 4 kpc at our derived major axis PA=170 deg., both ending at corotation
and both significantly higher than the conventionally adopted global value.
Inspection of the rotation curve suggests that the pattern speed interior to 2
kpc lacks an ILR, consistent with the leading structure seen in HST near-IR
observations. We also find tentative evidence for a lower pattern speed between
4 and 5.3 kpc measured by extending the regularized zone. As with the original
TW method, uncertainty in major axis position angle (PA) is the largest source
of error in the calculation; in this study, where \delta PA=+/-5 deg. a ~20%
error is introduced to the parameters of the speeds at PA=170 deg. Accessory to
this standard uncertainty, solutions with PA=175 deg. (also admitted by the
data) exhibit only one pattern speed inside 4 kpc, and we consider this
circumstance under the semblance of a radially varying PA.Comment: 14 pages in emulateapj format, 12 figures, accepted for publication
in Ap
The origin of polar ring galaxies: evidence for galaxy formation by cold accretion
Polar ring galaxies are flattened stellar systems with an extended ring of
gas and stars rotating in a plane almost perpendicular to the central galaxy.
We show that their formation can occur naturally in a hierarchical universe
where most low mass galaxies are assembled through the accretion of cold gas
infalling along megaparsec scale filamentary structures. Within a large
cosmological hydrodynamical simulation we find a system that closely resembles
the classic polar ring galaxy NGC 4650A. How galaxies acquire their gas is a
major uncertainty in models of galaxy formation and recent theoretical work has
argued that cold accretion plays a major role. This idea is supported by our
numerical simulations and the fact that polar ring galaxies are typically low
mass systems.Comment: 4 pages, 5 figures, stability of the ring discussed, minor changes to
match the accepted version by ApJL. A preprint with high-resolution figures
is available at http://krone.physik.unizh.ch/~andrea/PolarRing/PolarRing.p
The Kinematically Measured Pattern Speeds of NGC 2523 and NGC 4245
We have applied the Tremaine-Weinberg continuity equation method to derive
the bar pattern speed in the SB(r)b galaxy NGC 2523 and the SB(r)0/a galaxy NGC
4245 using the Calcium Triplet absorption lines. These galaxies were selected
because they have strong inner rings which can be used as independent tracers
of the pattern speed. The pattern speed of NGC 2523 is 26.4 6.1 km
s kpc, assuming an inclination of 49.7 and a distance
of 51.0 Mpc. The pattern speed of NGC 4245 is 75.5 31.3 km s
kpc, assuming an inclination of 35.4 and a distance of 12.6
Mpc. The ratio of the corotation radius to the bar radius of NGC 2523 and NGC
4245 is 1.4 0.3 and 1.1 0.5, respectively. These values place the
bright inner rings near and slightly inside the corotation radius, as predicted
by barred galaxy theory. Within the uncertainties, both galaxies are found to
have fast bars that likely indicate dark halos of low central concentration.
The photometric properties, bar strengths, and disk stabilities of both
galaxies are also discussed.Comment: Accepted for publication in The Astronomical Journal, 11 figures, 2
table
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
Dark-Matter Content of Early-Type Galaxies with Planetary Nebulae
We examine the dark matter properties of nearby early-type galaxies using
planetary nebulae (PNe) as mass probes. We have designed a specialised
instrument, the Planetary Nebula Spectrograph (PN.S) operating at the William
Herschel telescope, with the purpose of measuring PN velocities with best
efficiency. The primary scientific objective of this custom-built instrument is
the study of the PN kinematics in 12 ordinary round galaxies. Preliminary
results showing a dearth of dark matter in ordinary galaxies (Romanowsky et al.
2003) are now confirmed by the first complete PN.S datasets. On the other hand
early-type galaxies with a "regular" dark matter content are starting to be
observed among the brighter PN.S target sample, thus confirming a correlation
between the global dark-to-luminous mass virial ratio (f_DM=M_DM/M_star) and
the galaxy luminosity and mass.Comment: 5 pages, 2 figures. To appear in the proceedings of the IAU Symposium
244 "Dark Galaxies and Lost Baryons", Cardiff 25-29 June 2007, eds. J.I.
Davies & M.J. Disne
On the bar pattern speed determination of NGC 3367
An important dynamic parameter of barred galaxies is the bar pattern speed.
Among several methods that are used for the determination of the pattern speed
the Tremaine-Weinberg method has the advantage of model independency and
accuracy. In this work we apply the method to a simulated bar including gas
dynamics and study the effect of 2D spectroscopy data quality on robustness of
the method. We added a white noise and a Gaussian random field to the data and
measured the corresponding errors in the pattern speed. We found that a signal
to noise ratio in surface density ~5 introduces errors of ~20% for the Gaussian
noise, while for the white noise the corresponding errors reach ~50%. At the
same time the velocity field is less sensitive to contamination. On the basis
of the performed study we applied the method to the NGC 3367 spiral galaxy
using H{\alpha} Fabry-Perot interferometry data. We found for the pattern speed
43 \pm 6 km/s/kpc for this galaxy.Comment: Accepted for publication in ApJ. 16 pages, 16 figure
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