974 research outputs found
The Nearest Group of Galaxies
The small Antlia-Sextans clustering of galaxies is located at a distance of
only 1.36 Mpc from the Sun, and 1.72 Mpc from the adopted barycenter of the
Local Group. The latter value is significantly greater than the radius of the
zero- velocity surface of the Local Group which, for an assumed age of 14 Gyr,
has Ro = 1.18 " 0.15 Mpc. This, together with the observation that the members
of the Ant-Sex group have a mean redshift of +114 " 12 km s-1 relative to the
centroid of the Local Group, suggests that the Antlia-Sextans group is not
bound to our Local Group, and that it is expanding with the Hubble flow. If
this conclusion is correct, then Antlia-Sextans may be the nearest external
clustering of galaxies. The total galaxian population of the Ant-Sex group is ~
1/5 that of the Local Group. However, the integrated luminosity of Ant-Sex is
two orders of magnitude lower than that of the Local Group.
Subject headings: Galaxies - clusters: individual (Antlia-Sextans)Comment: Has been accepted for publication in the Astrophysical Journal
Letter
The Mass of the Centaurus A Group of Galaxies
The mass M, and the radius R_h, of the Centaurus A group are estimated from
the positions and radial velocities of 30 probable cluster members. For an
assumed distance of 3.9 Mpc it is found that R_h \sim 640 kpc. The velocity
dispersion in the Cen A group is 114 \pm 21 km/s. From this value, and R_h =
640 kpc, the virial theorem yields a total mass of 1.4 \times 10^{13} M_{\sun}
for the Cen A group. The projected mass method gives a mass of 1.8 \times
10^{13} M_{\sun}. These values suggest that the Cen A group is about seven
times as massive as the Local Group. The Cen A mass-to-light ratio is found to
be M/L_B = 155-200 in solar units. The cluster has a zero-velocity radius R_0 =
2.3 Mpc.Comment: 8 pages, 1 figure, in LaTeX format; to appear in the Astronomical
Journal in January 200
Slumberland Waltzes
Lighthouse shining with woman\u27s face in beam; Moon, clouds, and ocean waves in backgroundhttps://scholarsjunction.msstate.edu/cht-sheet-music/11301/thumbnail.jp
Someone Remembers World Forgets
Two birds flying around a patterned border with flowers and stemshttps://scholarsjunction.msstate.edu/cht-sheet-music/7625/thumbnail.jp
M31 Transverse Velocity and Local Group Mass from Satellite Kinematics
We present several different statistical methods to determine the transverse
velocity vector of M31. The underlying assumptions are that the M31 satellites
on average follow the motion of M31 through space, and that the galaxies in the
outer parts of the Local Group on average follow the motion of the Local Group
barycenter through space. We apply the methods to the line-of-sight velocities
of 17 M31 satellites, to the proper motions of the 2 satellites M33 and IC 10,
and to the line-of-sight velocities of 5 galaxies near the Local Group turn
around radius, respectively. This yields 4 independent but mutually consistent
determinations of the heliocentric M31 transverse velocities in the West and
North directions, with weighted averages = -78+/-41 km/s and =
-38+/-34 km/s. The Galactocentric tangential velocity of M31 is 42 km/s, with
1-sigma confidence interval V_tan <= 56 km/s. The implied M31-Milky Way orbit
is bound if the total Local Group mass M exceeds 1.72^{+0.26}_{-0.25}x10^{12}
solar masses. If indeed bound, then the timing argument combined with the known
age of the Universe implies that M = 5.58^{+0.85}_{-0.72}x10^{12} solar masses.
This is on the high end of the allowed mass range suggested by cosmologically
motivated models for the individual structure and dynamics of M31 and the Milky
Way, respectively. It is therefore possible that the timing mass is an
overestimate of the true mass, especially if one takes into account recent
results from the Millennium Simulation that show that there is also a
theoretical uncertainty of 41 percent (Gaussian dispersion) in timing mass
estimates. The M31 transverse velocity implies that M33 is in a tightly bound
orbit around M31. This may have led to some tidal deformation of M33. It will
be worthwhile to search for observational evidence of this.Comment: ApJ in press, 14 pages, including 3 figures (has minor revisions with
respect to previously posted version to address referee comments
Orbital Eccentricity Distribution of Solar-Neighbour Halo Stars
We present theoretical calculations for the differential distribution of
stellar orbital eccentricity for a sample of solar-neighbour halo stars. Two
types of static, spherical gravitational potentials are adopted to define the
eccentricity e for given energy E and angular momentum L, such as an isochrone
potential and a Navarro-Frenk-White potential that can serve as two extreme
ends covering in-between any realistic potential of the Milky Way halo. The
solar-neighbour eccentricity distribution \Delta N(e) is then formulated, based
on a static distribution function of the form f(E,L) in which the velocity
anisotropy parameter \beta monotonically increases in the radial direction away
from the galaxy center, such that beta is below unity (near isotropic velocity
dispersion) in the central region and asymptotically approaches \sim 1
(radially anisotropic velocity dispersion) in the far distant region of the
halo. We find that \Delta N(e) sensitively depends upon the radial profile of
\beta, and this sensitivity is used to constrain such profile in comparison
with some observational properties of \Delta N_{obs}(e) recently reported by
Carollo et al. (2010). Especially, the linear e-distribution and the fraction
of higher-e stars for their sample of solar-neighbour inner-halo stars rule out
a constant profile of \beta, contrary to the opposite claim by Bond et al.
(2010). Our constraint of \beta \lesssim 0.5 at the galaxy center indicates
that the violent relaxation that has acted on the inner halo is effective
within a scale radius of \sim 10 kpc from the galaxy center. We discuss that
our result would help understand the formation and evolution of the Milky Way
halo.Comment: 13 pages, 6 figures, accepted for publication in MNRA
The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions
The highly anisotropic distribution and apparent alignment of the Galactic
satellites in polar great planes begs the question how common such
distributions are. The satellite system of M31 is the only nearby system for
which we currently have sufficiently accurate distances to study the
three-dimensional satellite distribution. We present the spatial distribution
of the 15 presently known M31 companions in a coordinate system centered on M31
and aligned with its disk. Through a detailed statistical analysis we show that
the full satellite sample describes a plane that is inclined by -56 deg with
respect to the poles of M31 and that has an r.m.s. height of 100 kpc. With 88%
the statistical significance of this plane is low and it is unlikely to have a
physical meaning. The great stellar stream found near Andromeda is inclined to
this plane by 7 deg. There is little evidence for a Holmberg effect. If we
confine our analysis to early-type dwarfs, we find a best-fit polar plane
within 5 deg to 7 deg from the pole of M31. This polar great plane has a
statistical significance of 99.3% and includes all dSphs (except for And II),
M32, NGC 147, and PegDIG. The r.m.s. distance of these galaxies from the polar
plane is 16 kpc. The nearby spiral M33 has a distance of only about 3 kpc from
this plane, which points toward the M81 group. We discuss the anisotropic
distribution of M31's early-type companions in the framework of three
scenarios, namely as remnants of the break-up of a larger progenitor, as tracer
of a prolate dark matter halo, and as tracer of collapse along large-scale
filaments. (Abridged)Comment: 14 pages, 5 figures, accepted for publication in the Astronomical
Journa
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