2,377 research outputs found
Kinematic detection of the double nucleus in M31
Using a spectrum obtained under moderate (of order 1 arcsecond) seeing, we
show that the double nucleus in M31 produces a strong kinematic signature even
though the individual components are not spatially resolved. The signature
consists of a significant asymmetric wing in the stellar velocity distribution
close to the center of the system. The properties of the second nucleus derived
from this analysis agree closely with those measured from high-spatial
resolution Hubble Space Telescope images. Even Space Telescope only has
sufficient resolution to study the structure of very nearby galactic nuclei
photometrically; this spectroscopic approach offers a tool for detecting
structure such as multiple nuclei in a wider sample of galaxy cores.Comment: 4 pages of uuencoded compressed postscript, figures included.
Accepted for publication in MNRA
Disc heating in NGC 2985
Various processes have been proposed to explain how galaxy discs acquire
their thickness. A simple diagnostic for ascertaining this ``heating''
mechanism is provided by the ratio of the vertical to radial velocity
dispersion components. In a previous paper we have developed a technique for
measuring this ratio, and demonstrated its viability on the Sb system NGC 488.
Here we present follow-up observations of the morphologically similar Sab
galaxy NGC 2985, still only the second galaxy for which this ratio has been
determined outside of the solar neighbourhood. The result is consistent with
simple disc heating models which predict ratios of less
than oneComment: 5 pages, 4 figures. Accepted for publication in MNRA
The shape of the velocity ellipsoid in NGC 488
Theories of stellar orbit diffusion in disk galaxies predict different rates
of increase of the velocity dispersions parallel and perpendicular to the disk
plane, and it is therefore of interest to measure the different velocity
dispersion components in galactic disks of different types. We show that it is
possible to extract the three components of the velocity ellipsoid in an
intermediate-inclination disk galaxy from measured line-of-sight velocity
dispersions on the major and minor axes. On applying the method to observations
of the Sb galaxy NGC 488, we find evidence for a higher ratio of vertical to
radial dispersion in NGC 488 than in the solar neighbourhood of the Milky Way
(the only other place where this quantity has ever been measured). The
difference is qualitatively consistent with the notion that spiral structure
has been relatively less important in the dynamical evolution of the disk of
NGC 488 than molecular clouds.Comment: 5 pages LaTex, including 2 figures, mn.sty, submitted to MNRA
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
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
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