1,036 research outputs found
RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution
We explore the mass-to-light ratio in galaxy clusters and its relation to the
cluster mass. We study the relations among the optical luminosity (),
the cluster mass () and the number of cluster galaxies within
() in a sample of 217 galaxy clusters with confirmed 3D
overdensity. We correct for projection effects, by determining the galaxy
surface number density profile in our cluster sample. This is best fitted by a
cored King profile in low and intermediate mass systems. The core radius
decreases with cluster mass, and, for the highest mass clusters, the profile is
better represented by a generalized King profile or a cuspy Navarro, Frenk &
White profile. We find a very tight proportionality between and
, which, in turn, links the cluster mass-to-light ratio to the Halo
Occupation Distribution vs. . After correcting for
projection effects, the slope of the and
relations is found to be , close, but still significantly less
than unity. We show that the non-linearity of these relations cannot be
explained by variations of the galaxy luminosity distributions and of the
galaxy M/L with the cluster mass. We suggest that the nonlinear relation
between number of galaxies and cluster mass reflects an underlying nonlinear
relation between number of subhaloes and halo mass.Comment: 15 pages, 15 figures, accepted for publication in A&
Detection of X-ray Clusters of Galaxies by Matching RASS Photons and SDSS Galaxies within GAVO
A new method for a simultaneous search for clusters of galaxies in X-ray
photon maps and optical galaxy maps is described. The merging of X-ray and
optical data improves the source identification so that a large amount of
telescope time for spectroscopic follow-up can be saved. The method appears
thus ideally suited for the analysis of the recently proposed wide-angle X-ray
missions like DUO and ROSITA. As a first application, clusters are extracted
from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of
the Sloan Digital Sky Survey (SDSS). The time-consuming computations are
performed within the German Astrophysical Virtual Observatory (GAVO). On a test
area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray
flux limit of in the
ROSAT energy band 0.1-2.4 keV. The clusters have redshifts . The
survey thus fills the gap between traditional large-area X-ray surveys and
serendipitous X-ray cluster searches based on pointed observations, and has the
potential to yield about 4,000 X-ray clusters after completion of SDSS.Comment: 19 pages, low-resolution figures, accepted for publication in
Astronomy and Astrophysic
RASS-SDSS Galaxy Cluster Survey. VI. The dependence of the cluster SFR on the cluster global properties
Using a subsample of 79 nearby clusters from the RASS-SDSS galaxy cluster
catalogue of Popesso et al. (2005a), we perform a regression analysis between
the cluster integrated star formation rate (Sigma_SFR) the cluster total
stellar mass (M_star), the fractions of star forming (f_SF) and blue (f_b)
galaxies and other cluster global properties, namely its richness (N_gal, i.e.
the total number of cluster members within the cluster virial radius), velocity
dispersion (sigma_v), virial mass (M_200), and X-ray luminosity (L_X). All
cluster global quantities are corrected for projection effects before the
analysis. Galaxy SFRs and stellar masses are taken from the catalog of
Brinchmann et al. (2004), which is based on SDSS spectra. We only consider
galaxies with M_r <= -20.25 in our analysis, and exclude AGNs. We find that
both Sigma_SFR and M_star are correlated with all the cluster global
quantities. A partial correlation analysis show that all the correlations are
induced by the fundamental one between Sigma_SFR and N_gal, hence there is no
evidence that the cluster properties affect the mean SFR or M_star per galaxy.
The relations between Sigma_SFR and M_star, on one side, and both N_gal and
M_200, on the other side, are linear, i.e. we see no evidence that different
clusters have different SFR or different M_star per galaxy and per unit mass.
The fraction f_SF does not depend on any cluster property considered, while f_b
does depend on L_X. We note that a significant fraction of star-forming cluster
galaxies are red (~25% of the whole cluster galaxy population). We conclude
that the global cluster properties are unable to affect the SF properties of
cluster galaxies, but the presence of the X-ray luminous intra-cluster medium
can affect their colors, perhaps through the ram-pressure stripping mechanism.Comment: 14 pages, 12 figures, accepted for publication on A&A; corrected
coefficient in Tab.
A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields
Results are presented of a new VLA-ROSAT study that probes the magnetic field
strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1)
galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity,
and were selected to be free of (unusual) strong radio cluster halos, and
widespread cooling flows. Consistent with these criteria, most clusters show a
relaxed X-ray morphology and little or no evidence for recent merger activity.
Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM
excess out to ~0.5 Mpc from cluster centers. The results, combined with RM
imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the
hot intergalactic gas within these ``normal'' clusters is permeated with a high
filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2}
microGauss, where l is the field correlation length. These results lead to a
global estimate of the total magnetic energy in clusters, and give new insight
into the ultimate energy origin, which is likely gravitational. These results
also shed some light on the cluster evolutionary conditions that existed at the
onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ
Metal transport by gas sloshing in M87
We present the results of an XMM-Newton mosaic covering the central ~200 kpc
of the nearby Virgo cluster. We focus on a strong surface brightness
discontinuity in the outskirts of the brightest cluster galaxy, M87. Using both
XMM-Newton and Suzaku, we derive accurate temperature and metallicity profiles
across this feature and show that it is a cold front probably due to sloshing
of the Virgo ICM. It is also associated with a discontinuity in the chemical
composition. The gas in the inner, bright region of the front is ~40% more
abundant in Fe than the gas outside the front, suggesting the important role of
sloshing in transporting metals through the ICM. For the first time, we provide
a quantitative estimate of the mass of Fe transported by a cold front. This
amounts to ~6% of the total Fe mass within the radial range affected by
sloshing, significantly more than the amount of metals transported by the AGN
in the same cluster core. The very low Fe abundance of only ~0.2 solar
immediately outside the cold front at a radius of 90 kpc suggests we are
witnessing first-hand the transport of higher metallicity gas into a pristine
region, whose abundance is typical of the cluster outskirts. The Mg/Fe and O/Fe
abundance ratios remain approximately constant over the entire radial range
between the centre of M87 and the faint side of the cold front, which requires
the presence of a centrally peaked distribution not only for Fe but also for
core-collapse type supernova products. This peak may stem from the star
formation triggered as the BCG assembled during the protocluster phase.Comment: accepted for publication in MNRA
The REFLEX Galaxy Cluster Survey VII: Omega_m and sigma_8 from cluster abundance and large-scale clustering
For the first time the large-scale clustering and the mean abundance of
galaxy clusters are analysed simultaneously to get precise constraints on the
normalized cosmic matter density and the linear theory RMS
fluctuations in mass . A self-consistent likelihood analysis is
described which combines, in a natural and optimal manner, a battery of
sensitive cosmological tests where observational data are represented by the
(Karhunen-Lo\'{e}ve) eigenvectors of the sample correlation matrix. This method
breaks the degeneracy between and . The cosmological tests
are performed with the ROSAT ESO Flux-Limited X-ray (REFLEX) cluster sample.
The computations assume cosmologically flat geometries and a non-evolving
cluster population mainly over the redshift range . The REFLEX sample
gives the cosmological constraints and their random errors of
and . Possible systematic errors are evaluated by estimating the
effects of uncertainties in the value of the Hubble constant, the baryon
density, the spectral slope of the initial scalar fluctuations, the mass/X-ray
luminosity relation and its intrinsic scatter, the biasing scheme, and the
cluster mass density profile. All these contributions sum up to total
systematic errors of and
.Comment: 10 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Temperature gradients in XMM-Newton observed REFLEX-DXL galaxy clusters at z~0.3
We present XMM-Newton results on the temperature profiles of a volume-limited
sample of galaxy clusters at redshifts z~0.3, selected from the REFLEX survey
(REFLEX-DXL sample). In the spectral analysis, where only the energies above 1
keV were considered, we obtained consistent results on the temperature derived
from the EPN, EMOS1 and EMOS2 data. Useful temperature measurements could be
performed out to radii with overdensity 500 (r500) for all nine clusters. We
discovered a diversity in the temperature gradients at the outer cluster radii
with examples of both flat and strongly decreasing profiles. Using the total
mass and the gas mass profiles for the cluster RXCJ0307.0-2840 we demonstrate
that the errors on the mass estimates for the REFLEX-DXL clusters are within
25% up to r500.Comment: Revised version accepted for publication in A&A, 18 page
Ways of knowing of farmers and scientists: tree and soil management in the Ethiopian Highlands
The Ethiopian Highlands have been studied extensively, hosting a large amount of research for development projects in agriculture and forestry over several decades. The encounters in these projects were also encounters of different ways of knowing that were negotiated by the actors meeting in the space provided by the projects. This research explores these encounters and the social worlds they are embedded in, drawing on actor-oriented approaches as well as theories of narratives and framing. Ways of knowing and citizen epistemologies are taken as a lens to understand the role of identities in knowledge production and use.
The two case studies were agroforestry research projects in the Ethiopian Highlands. The research followed a range of qualitative and ethnographic research methods. Different types of farmers and scientists meet in the case studies. I recognise that they all have individual agency, nevertheless I use the terms âscientistâ and âfarmerâ in this thesis. I use the terms to describe certain groups of actors who all draw on different ways of knowing, and different value systems, when interacting with each other and their environment.
The results indicate that the importance of social worlds at different scales and the contexts of research projects tend to be underestimated. In spite of good intentions scientific methodologies, terminologies and narratives tend to dominate. Scientists in the case studies acknowledged the existence of farmersâ âindigenousâ knowledge, but they determined the value of knowledge by its scientific applicability and the replicability of experiments. Research systems force the scientists into a certain modus operandi with limited possibilities to experiment and to respond to the complexities and diversities of people's social worlds.
Farmers in the case studies preferred observation from their parents, observing from others or the environment as a way of learning and gaining knowledge. Depending on their personalities and their life histories they also relied on alternative ways of knowing rooted in spirituality, emotions and memories. Powerful influences on ways of knowing resulted from the way languages and authority had been used. These often led to the exclusion of marginalised community members from access to knowledge and technologies.
Unfortunately, common narratives prevailed in the case studies, and alternative ways of knowing were often marginalised. By acknowledging different ways of knowing and the importance of different social worlds and different ways of doing research, both scientists and farmers could benefit and develop more sustainable pathways for agricultural and forestry land use
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