30 research outputs found
Mass-Luminosity Relation for White-Dwarf Stars
Recebido em 19 de Fevereiro de 1971 Improving the model of Milne in which the white-dwarf star is considered as consisting of an envelope of perfect gas and an interna1 part of degenerate electron gas, we investigated the relation between its mass and luminosity. In conclusion, we found thai, though the masses are the same, the luminosity of the white dwarfs which contain light elements in their interior is greater than that of those which contain heavier elements. Aperfeiçoando o modèlo de Milne em que a estrela anã branca é considerada como sendo constituida de um envelope de gás perfeito e de uma parte interna de gás de elétrons degenerado, investigamos a relação entre a sua massa e luminosidade. Como conclusão, achamos que, mesmo quando as massas sejam iguais, a luminosidade das anãs brancas que contenham elementos leves no seu interior é maior que a daquelas que contenham elementos pesados
Evidence of Substructure in the Cluster of Galaxies A3558
We investigate the dynamical properties of the cluster of galaxies A3558
(Shapley 8). Studying a region of one square degree ( 3 Mpc) centered
on the cluster cD galaxy, we have obtained a statistically complete photometric
catalog with positions and magnitudes of 1421 galaxies (down to a limiting
magnitude of ). This catalog has been matched to the recent velocity
data obtained by Mazure et al. (1997) and from the literature, yielding a
radial velocity catalog containing 322 galaxies. Our analysis shows that the
position/velocity space distribution of galaxies shows significant
substructure. A central bimodal core detected previously in preliminary studies
is confirmed by using the Adaptive Kernel Technique and Wavelet Analysis. We
show that this central bimodal subtructure is nevertheless composed of a
projected feature, kinematically unrelated to the cluster, plus a group of
galaxies probably in its initial merging phase into a relaxed core. The cD
velocity offset with respect to the average cluster redshift, reported earlier
by several authors, is completely eliminated as a result of our dynamical
analysis. The untangling of the relaxed core component also allows a better,
more reliable determination of the central velocity dispersion, which in turn
eliminates the ``-problem'' for A3558. The cluster also shows a
``preferential'' distribution of subclumps coinciding with the direction of the
major axis position angle of the cD galaxy and of the central X-ray emission
ellipsoidal distribution, in agreement with an anisotropic merger scenario.Comment: 35 pages in latex, 17 figures in Postscript, accepted for publication
in the Astrophysical Journa
Decision Tree Classifiers for Star/Galaxy Separation
We study the star/galaxy classification efficiency of 13 different decision
tree algorithms applied to photometric objects in the Sloan Digital Sky Survey
Data Release Seven (SDSS DR7). Each algorithm is defined by a set of parameters
which, when varied, produce different final classification trees. We
extensively explore the parameter space of each algorithm, using the set of
SDSS objects with spectroscopic data as the training set. The
efficiency of star-galaxy separation is measured using the completeness
function. We find that the Functional Tree algorithm (FT) yields the best
results as measured by the mean completeness in two magnitude intervals: () and (). We compare the performance of the
tree generated with the optimal FT configuration to the classifications
provided by the SDSS parametric classifier, 2DPHOT and Ball et al. (2006). We
find that our FT classifier is comparable or better in completeness over the
full magnitude range , with much lower contamination than all but
the Ball et al. classifier. At the faintest magnitudes (), our classifier
is the only one able to maintain high completeness (80%) while still
achieving low contamination (). Finally, we apply our FT classifier
to separate stars from galaxies in the full set of SDSS
photometric objects in the magnitude range .Comment: Submitted to A
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray Properties of Clusters Detected as Extended X-ray Sources
In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination and galactic latitude and comprises sources with
a count rate counts s and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...Comment: 61 pages; ApJS in press; fixed bug in table file; also available at
(better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS
X-ray Galaxy Clusters in NoSOCS: Substructure and the Correlation of Optical and X-ray Properties
We present a comparison of optical and X-ray properties of galaxy clusters in
the northern sky. We determine the recovery rate of X-ray detected clusters in
the optical as a function of richness, redshift and X-ray luminosity, showing
that the missed clusters are typically low contrast systems when observed
optically. We employ four different statistical tests to test for the presence
of substructure using optical two-dimensional data, finding that approximately
35% of the clusters show strong signs of substructure. However, the results are
test-dependent, with variations also due to the magnitude range and radius
utilized.We have also performed a comparison of X-ray luminosity and
temperature with optical galaxy counts (richness). We find that the slope and
scatter of the relations between richness and the X-ray properties are heavily
dependent on the density contrast of the clusters. The selection of
substructure-free systems does not improve the correlation between X-ray
luminosity and richness, but this comparison also shows much larger scatter
than one obtained using the X-ray temperature. In the latter case, the sample
is significantly reduced because temperature measurements are available only
for the most massive (and thus high contrast) systems. However, the comparison
between temperature and richness is very sensitive to the exclusion of clusters
showing signs of substructure. The correlation of X-ray luminosity and richness
is based on the largest sample to date ( 750 clusters), while tests
involving temperature use a similar number of objects as previous works
(\lsim100). The results presented here are in good agreement with existing
literature.Comment: 32 pages, 18 figures, ApJ in press, including minor changes following
the ApJ's editio
On the Origin of S0 Galaxies
I will review the basic properties of S0 galaxies in the local Universe in
relation to both elliptical and spiral galaxies, their neighbours on the Hubble
sequence, and also in relation to dwarf spheroidal (dSph) galaxies. This will
include colours, luminosities, spectral features, information about the age and
metallicity composition of their stellar populations and globular clusters,
about their ISM content, as well as kinematic signatures and their implications
for central black hole masses and past interaction events, and the number
ratios of S0s to other galaxy types in relation to environmental galaxy
density. I will point out some caveats as to their morphological discrimination
against other classes of galaxies, discuss the role of dust and the wavelength
dependence of bulge/disk light ratios. These effects are of importance for
investigations into the redshift evolution of S0 galaxies -- both as individual
objects and as a population. The various formation and transformation scenarios
for S0 and dSph galaxies will be presented and confronted with the available
observations.Comment: Invited Review, 18 pages, ``BARS 2004'' Conference, South Africa,
June 2004, eds.: K. C. Freeman, D. L. Block, I. Puerari, R. Groess, Kluwer,
in pres
Palinologia holocênica do testemunho Bom Jesus, margem leste da ilha do Marajó, Pará, Amazônia
Searching for filaments and large-scale structure around DAFT/FADA clusters
International audienceContext. Clusters of galaxies are located at the intersection of cosmic filaments and are still accreting galaxies and groups along these preferential directions. However, because of their relatively low contrast on the sky, filaments are difficult to detect (unless a large amount of spectroscopic data are available), and unambiguous detections have been limited until now to relatively low redshifts (z \textless similar to 0.3). Aims. This project is aimed at searching for extensions and filaments around clusters, traced by galaxies selected to be at the cluster redshift based on the red sequence. In the 0.4 \textless z \textless 0.9 redshift range of our sample, clusters are believed to be already well formed, but still to be accreting material along filaments. Methods. We have searched for extensions and filaments around the thirty clusters of the DAFT/FADA survey for which we had deep wide field photometric data. For each cluster, based on a colour-magnitude diagram, we selected galaxies that were likely to belong to the red sequence, and hence to be at the cluster redshift, and built density maps. By computing the background for each of these maps and drawing 3 sigma contours, we estimated the elongations of the structures detected in this way. Whenever possible, we identified the other structures detected on the density maps with clusters listed in NED. Results. We find clear elongations in twelve clusters out of thirty, with sizes that can reach up to 7.6 Mpc. Eleven other clusters have neighbouring structures, but the zones linking them are not detected in the density maps at a 3 sigma level. Three clusters show no extended structure and no neighbours, and four clusters are of too low contrast to be clearly visible on our density maps. Conclusions. The simple method we have applied appears to work well to show the existence of filaments and/or extensions around a number of clusters in the redshift range 0.4 \textless z \textless 0.9. We plan to apply it to other large cluster samples such as the clusters detected in the CFHTLS and SDSS-Stripe 82 surveys in the near future