256 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
A statistical method for the identification of stars enriched in neutron-capture elements from medium-resolution spectra
We present an automated statistical method that uses medium-resolution
spectroscopic observations of a set of stars to select those that show evidence
of possessing significant amounts of neutron-capture elements. Our tool was
tested against a sample of F- and G-type stars distributed among
plates from the Galactic Understanding and Exploration (SEGUE) survey,
including that were directed at stellar Galaxy clusters. Focusing on five
spectral lines of europium in the visible window, our procedure ranked the
stars by their likelihood of having enhanced content of this atomic species and
identifies the objects that exhibit signs of being rich in neutron-capture
elements as those scoring in the upper . We find that several of the
cluster plates contain relatively large numbers of stars with significant
absorption around at least three of the five selected lines. The most prominent
is the globular cluster M3, where we measured a fraction of stars that are
potentially rich in heavy nuclides, representing at least .Comment: 7 pages, 2 figures, accepted for publication in Astronomy &
Astrophysics (v2
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
Scale Radii and Aggregation Histories of Dark Haloes
Relaxed dark-matter haloes are found to exhibit the same universal density
profiles regardless of whether they form in hierarchical cosmologies or via
spherical collapse. Likewise, the shape parameters of haloes formed
hierarchically do not seem to depend on the epoch in which the last major
merger took place. Both findings suggest that the density profile of haloes
does not depend on their aggregation history. Yet, this possibility is
apparently at odds with some correlations involving the scale radius r_s found
in numerical simulations. Here we prove that the scale radius of relaxed,
non-rotating, spherically symmetric haloes endowed with the universal density
profile is determined exclusively by the current values of four independent,
though correlated, quantities: mass, energy and their respective instantaneous
accretion rates. Under this premise and taking into account the inside-out
growth of haloes during the accretion phase between major mergers, we build a
simple physical model for the evolution of r_s along the main branch of halo
merger trees that reproduces all the empirical trends shown by this parameter
in N-body simulations. This confirms the conclusion that the empirical
correlations involving r_s do not actually imply the dependence of this
parameter on the halo aggregation history. The present results give strong
support to the explanation put forward in a recent paper by Manrique et al.
(2003) for the origin of the halo universal density profile.Comment: 13 pages, 8 figures, accepted for publication in MNRA
- …