44 research outputs found
The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency
We study the rich globular cluster (GC) system in the center of the massive
cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational
lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude
I_814=29 with >90% completeness and sample the brightest ~5% of the GC system.
Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we
estimate a total population of N(GC_total) = 162,850 GCs within a projected
radius of 400kpc. As many as half may comprise an intracluster component. Even
with the sizable uncertainties, which mainly result from the uncertain GCLF
parameters, this is by far the largest GC system studied to date. The specific
frequency S_N is high, but not uncommon for central galaxies in massive
clusters, rising from S_N~5 near the center to ~12 at large radii. Passive
galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass
profiles of the GCs, stars, intracluster gas, and lensing-derived total mass,
and we compare the mass fractions as a function of radius. The estimated mass
in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar
mass of the Milky Way. The shape of the GC mass profile appears intermediate
between those of the stellar light and total cluster mass. Despite the extreme
nature of this system, the ratios of the GC mass to the baryonic and total
masses, and thus the GC formation efficiency, are typical of those in other
rich clusters when comparing at the same physical radii. The GC formation
efficiency is not constant, but varies with radius, in a manner that appears
similar for different clusters; we speculate on the reasons for this similarity
in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap
The dynamical state of brightest cluster galaxies and the formation of clusters
A large sample of Abell clusters of galaxies, selected for the likely
presence of a dominant galaxy, is used to study the dynamical properties of
brightest cluster members (BCMs). From visual inspection of Digitized Sky
Survey images combined with redshift data we identify 1426 candidate BCMs in
1221 redshift components in 1169 different Abell clusters, the largest such
sample published so far. By our own morphological classification we find ~92%
of these BCMs to be early-type galaxies, and 48% of cD type. We confirm
previous findings based on much smaller samples, namely that a large fraction
of BCMs have significant peculiar velocities. For a subsample of 452 clusters
with at least 10 measured radial velocities, we find a median BCM peculiar
velocity of 32% of their host clusters' radial velocity dispersion. This
suggests that most BCMs are not at rest in the potential well of their
clusters, and that the phenomenon is thus not a special trait of clusters
hosting cD galaxies. We show that the peculiar velocity of the BCM is
independent of cluster richness and only slightly dependent on the Bautz-Morgan
type. We also find a weak trend for the peculiar velocity to rise with the
cluster velocity dispersion. The strongest dependence is with the morphological
type of the BCM: cD galaxies tend to have lower relative peculiar velocities
than elliptical galaxies. This result points to a connection between the
formation of the BCMs and that of their clusters. Our data are qualitatively
consistent with the merging-groups scenario, where BCMs in clusters formed
first in smaller subsystems comparable to compact groups of galaxies. In this
scenario, clusters would have formed recently from the mergers of many such
groups and would still be in a dynamically unrelaxed state.Comment: 114 pages, 9 figures, full version of tables 1 and 2; accepted to
appear in A
Globular cluster systems in fossil groups: NGC6482, NGC1132 and ESO306-017
We study the globular cluster (GC) systems in three representative fossil
group galaxies: the nearest (NGC6482), the prototype (NGC1132) and the most
massive known to date (ESO306-017). This is the first systematic study of GC
systems in fossil groups. Using data obtained with the Hubble Space Telescope
Advanced Camera for Surveys in the F475W and F850LP filters, we determine the
GC color and magnitude distributions, surface number density profiles, and
specific frequencies. In all three systems, the GC color distribution is
bimodal, the GCs are spatially more extended than the starlight, and the red
population is more concentrated than the blue. The specific frequencies seem to
scale with the optical luminosities of the central galaxy and span a range
similar to that of the normal bright elliptical galaxies in rich environments.
We also analyze the galaxy surface brightness distributions to look for
deviations from the best-fit S\'ersic profiles; we find evidence of recent
dynamical interaction in all three fossil group galaxies. Using X-ray data from
the literature, we find that luminosity and metallicity appear to correlate
with the number of GCs and their mean color, respectively. Interestingly,
although NGC6482 has the lowest mass and luminosity in our sample, its GC
system has the reddest mean color, and the surrounding X-ray gas has the
highest metallicity.Comment: 16 pages, 13 figures. Accepted for publication in A&
Method to estimate the medullar rays angle in pieces of wood based on tree-ring structure: application to planks of Quercus petraea
ProducciĂłn CientĂficaEstimating wood parameters employing non-destructive methods has been widely studied in recent years. The choice of wood used to build wine ageing barrels (cooperage) is strongly influenced by wood anatomy and specifically by the orientation of medullar rays among other aspects. In this article, a method based on the regularities of the tree-ring structure to estimate the medullar ray angle of the cross section of a piece of wood is proposed. This angle shows the direction of the best linear path to evaluate several tree-ring features and could be employed to automate tasks, such as introducing an analysis path or rotating the image prior to the analysis, which some dendro analysis methods require. A dataset of 26,992 synthetic images and 110 real oak wood images was used to validate the approach. The medullar ray angle of each image considered was measured manually and estimated using the method proposed here, which employs the fast Fourier transform (FFT) to take advantage of the tree-ring structure regularities and find the direction angle of the best linear path to evaluate several tree-ring features. The results obtained demonstrate a mean squared error of 0.29° and 8.19° and a mean absolute error of 0.19° and a 5.91° for the synthetic and oak wood images, respectively. These data suggest the suitability of the proposed method as part of an automated system to inspect and analyse the growth rings in oak wood planks