364 research outputs found
Kinematics of the southern galaxy cluster Abell 3733
We report radial velocities for 99 galaxies with projected positions within
30 arcmin of the center of the cluster A3733 obtained with the MEFOS multifiber
spectrograph at the 3.6-m ESO telescope. These measurements are combined with
39 redshifts previously published by Stein (1996) to built a collection of 112
galaxy redshifts in the field of A3733, which is used to examine the kinematics
and structure of this cluster. We assign cluster membership to 74 galaxies with
heliocentric velocities in the interval 10500-13000 km/s. From this sample of
cluster members, we infer a heliocentric systemic velocity for A3733 of
11653{+74}{-76} km/s, which implies a mean cosmological redshift of 0.0380, and
a velocity dispersion of 614{+42}{-30} km/s. The application of statistical
substructure tests to a magnitude-limited subset of the latter sample reveals
evidence of non-Gaussianity in the distribution of ordered velocities in the
form of lighter tails and possible multimodality. Spatial substructure tests do
not find, however, any significant clumpiness in the plane of the sky, although
the existence of subclustering along the line-of-sight cannot be excluded.Comment: AA-LaTeX2e style; 10 pages, 2 Postscript figures, Table 1 appended.
To be published in Astronomy and Astrophysics. Also available at
ftp://pcess1.am.ub.es/pub/AA/a3733.ps.g
Structure, mass and distance of the Virgo cluster from a Tolman-Bondi model
We have applied a relativistic Tolman-Bondi model of the Virgo cluster to a
sample of 183 galaxies with measured distances within a radius of 8 degrees
from M87. We find that the sample is significantly contaminated by background
galaxies which lead to too large a cluster mean distance if not excluded. The
Tolman-Bondi model predictions, together with the HI deficiency of spiral
galaxies, allows one to identify these background galaxies. One such galaxy is
clearly identified among the 6 calibrating galaxies with Cepheid distances. As
the Tolman-Bondi model predicts the expected distance ratio to the Virgo
distance, this galaxy can still be used to estimate the Virgo distance, and the
average value over the 6 galaxies is 15.4 +- 0.5 Mpc.
Well-known background groups of galaxies are clearly recovered, together with
filaments of galaxies which link these groups to the main cluster, and are
falling into it. No foreground galaxy is clearly detected in our sample.
Applying the B-band Tully-Fisher method to a sample of 51 true members of the
Virgo cluster according to our classification gives a cluster distance of 18.0
+- 1.2 Mpc, larger than the mean Cepheid distance.
Finally, the same model is used to estimate the Virgo cluster mass, which is
M = 1.2 10^{15} Msun within 8 degrees from the cluster center (2.2 Mpc radius),
and amounts to 1.7 virial mass.Comment: 12 pages, 7 figures. Astronomy and Astrophysics, in press (accepted
May 31, 2001
HI Distribution and Tully-Fisher Distances of Gas-Poor Spiral Galaxies in the Virgo Cluster Region
We present aperture synthesis observations in the 21 cm line of pointings
centered on the Virgo Cluster region spirals NGC 4307, NGC 4356, NGC 4411B, and
NGC 4492 using the Very Large Array (VLA) radiotelescope in its CS
configuration. These galaxies were identified in a previous study of the
three-dimensional distribution of HI emission in the Virgo region as objects
with a substantial dearth of atomic gas and Tully-Fisher (TF) distance
estimates that located them well outside the main body of the cluster. We have
detected two other galaxies located in two of our fields and observed bands,
the spiral NGC 4411A and the dwarf spiral VCC 740. We provide detailed
information of the gas morphology and kinematics for all these galaxies. Our
new data confirm the strong HI-deficiency of all the main targets but NGC
4411B, which is found to have a fairly normal neutral gas content. The VLA
observations have also been used to discuss the applicability of TF techniques
to the five largest spirals we have observed. We conclude that none of them is
actually suitable for a TF distance evaluation, whether due to the radical
trimming of their neutral hydrogen disks (NGC 4307, NGC 4356, and NGC 4492) or
to their nearly face-on orientation (NGC 4411A and B).Comment: 32 pages, 5 figures. Accepted for publication in A
Are the HI deficient galaxies on the outskirts of Virgo recent arrivals?
The presence on the Virgo cluster outskirts of spiral galaxies with gas
deficiencies as strong as those of the inner galaxies stripped by the
intracluster medium has led us to explore the possibility that some of these
peripheral objects are not newcomers. A dynamical model for the collapse and
rebound of spherical shells under the point mass and radial flow approximations
has been developed to account for the amplitude of the motions in the Virgo I
cluster (VIC) region. According to our analysis, it is not unfeasible that
galaxies far from the cluster, including those in a gas-deficient group well to
its background, went through its core a few Gyr ago. The implications would be:
(1) that the majority of the HI-deficient spirals in the VIC region might have
been deprived of their neutral hydrogen by interactions with the hot
intracluster medium; and (2) that objects spending a long time outside the
cluster cores might keep the gas deficient status without altering their
morphology.Comment: Accepted for publication in ApJ. 4 pages, 3 figures. Uses emulateapj
Forming first-ranked early-type galaxies through hierarchical dissipationless merging
We have developed a computationally competitive N-body model of a
previrialized aggregation of galaxies in a flat LambdaCDM universe to assess
the role of the multiple mergers that take place during the formation stage of
such systems in the configuration of the remnants assembled at their centres.
An analysis of a suite of 48 simulations of low-mass forming groups (of about
1E13 solar masses) demonstrates that the gravitational dynamics involved in
their hierarchical collapse is capable of creating realistic first-ranked
galaxies without the aid of dissipative processes. Our simulations indicate
that the brightest group galaxies (BGGs) constitute a distinct population from
other group members, sketching a scenario in which the assembly path of these
objects is dictated largely by the formation of their host system. We detect
significant differences in the distribution of Sersic indices and total
magnitudes, as well as a luminosity gap between BGGs and the next brightest
galaxy that is positively correlated with the total luminosity of the parent
group. Such gaps arise from both the grow of BGGs at the expense of lesser
companions and the decrease in the relevance of second-ranked objects in equal
measure. This results in a dearth of intermediate-mass galaxies which explains
the characteristic central dip detected in their luminosity functions in
dynamically young galaxy aggregations. The fact that the basic global
properties of our BGGs define a thin mass fundamental plane strikingly similar
to that followed giant early-type galaxies in the local universe reinforces
confidence in the results obtained.Comment: 25 pages, 14 figures, 3 tables. Accepted to MNRA
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