832 research outputs found
Shapes of clusters and groups of galaxies: Comparison of model predictions with observations
We study the properties of the 3-dimensional and projected shapes of haloes
using high resolution numerical simulations and observational data where the
latter comes from the 2PIGG (Eke et al. 2004) and SDSS-DR3GC group catalogues
(Merchan & Zandivarez 2005). We investigate the dependence of halo shape on
characteristics such as mass and number of members. In the 3-dimensional case,
we find a significant correlation between the mass and halo shape; massive
systems are more prolate than small haloes. We detect a source of strong
systematics in estimates of the triaxiality of a halo, which is found to be a
strong function of the number of members; LCDM haloes usually characterised by
triaxial shapes, slightly bent toward prolate forms, appear more oblate when
taking only a small subset of the halo particles. The ellipticities of observed
2PIGG and SDSS-DR3GC groups are found to be strongly dependent on the number of
group members, so that poor groups appear more elongated than rich ones.
However, this is again an artifact caused by poor statistics and not an
intrinsic property of the galaxy groups, nor an effect from observational
biases. We interpret these results with the aid of a GALFORM mock 2PIGG
catalogue. When comparing the group ellipticities in mock and real catalogues,
we find an excellent agreement between the trends of shapes with number of
group members. When carefully taking into account the effects of low number
statistics, we find that more massive groups are consistent with more elongated
shapes. Finally, our studies find no significant correlations between the shape
of observed 2PIGG or SDSS-DR3GC groups with the properties of galaxy members
such as colour or spectral type index.Comment: 9 pages, 10 figures, submitted to MNRA
Meandering periods and asymmetries in light curves of Miras: Observational evidence for low mass-loss rates
Some Miras -- long-period variables in late evolutionary stages -- have
meandering pulsation periods and light curve asymmetries, the causes of which
are still unclear. We aim to understand better the origin of these phenomena by
investigating a sample of solar-neighbourhood Miras. We characterised this
group of stars and related their variability characteristics to other stellar
parameters. We analysed observations from several databases to obtain light
curves with maximum time span and temporal coverage for a sample of 548 Miras.
We determined their pulsation period evolution over a time span of many
decades, searched for changes in the periods, and determined the amplitude of
the period change. We also analysed the Fourier spectra with respect to
possible secondary frequency maxima. The sample was divided into two groups
with respect to the presence of light curve bumps. IR colours and indicators of
the third dredge-up were collected to study the sample stars' mass-loss and
deep mixing properties. Our analysis revealed one new star, T~Lyn, with a
continuously changing period. The group of Miras with meandering period changes
is exclusively made up of M-type stars. The Fourier spectra of the meandering
period Miras have no prominent additional peaks, suggesting that additional
pulsation modes are not the cause of the meandering periods. We confirm that
bumps are more common among S and C Miras and show, for the first time, that
Miras with bumps have lower mass-loss rates than those with regular, symmetric
light curves. Also Miras with meandering period changes have relatively little
mass loss. We conclude that Miras with strongly changing periods or asymmetries
in their light curves have relatively low dust mass-loss rates. Meandering
period changes and light curve asymmetries could be connected to He-shell
flashes and third dredge-up episodes.Comment: 13 pages (plus 13 pages Appendix), 14 Figures, accepted for
publication in A&
Correlation Function of Galaxy Groups
We use the Updated Zwicky Catalog of galaxies (Falco et al. 1999) to generate
a catalog of groups, by means of a friend-of-friend algorithm. The correlation
length of the total sample is well fitted with a power law with parameters and for values of . Three subsamples defined by
the range of group virial masses were used to have their clustering
properties examined throughout the autocorrelation function. We find an
increase of the amplitude of the correlation function according to the group
masses which extends the results of the relation for galaxy systems
at small . For completeness we have also analyzed a sample of groups
obtained from the Southern Sky Redshift Survey (da Costa et al.1998) in the
range of virial masses to compare the results with those obtained from
GUZC.Comment: 9 figures, accepted for publication in Ap
La colección herpetológica de Bolivia depositada en la Estación Biológica de Doñana
The collection consists of 822 specimens, of which 529 are amphibians, all of them anurans (5 families, 17 genera and 51 species) and 293 specimens are reptiles (10 families, 28 genera and 49 species). The collection has around 25% of the amphibians species known to occur in Bolivia and about 19% of the reptile species. They come from 55 localities of the Bolivian Departments of Beni, Chuquisaca, Cochabamba, La Paz, Potosà and Santa Cruz and represent the following bioregions: Puna, Chaco, Chiquitanian Forest, Wet Savannas, Ceja de Montaña, Yungas, Interandean Dry Valleys and Humid Lowland Forest. The specimens of Scinax chiquitanus and Phrynopus kempffi are paratypes. The record of Pleurodema borelli is the first for the Santa Cruz Department and second for Bolivia. Liolaemus dorbignyi also constitutes the second report for the country and Tropidurus melanopleurus is cited for the first time for the Beni Department.La colección se compone de 822 ejemplares, 529 anfibios y 293 reptiles. Los anfibios son todos anuros, pertenecientes a 51 especies de 17 géneros y cinco familias. Los reptiles estan representados por 49 especies, incluidas en 28 géneros de 10 familias. Los ejemplares provienen de 55 localidades repartidas en los Departamentos bolivianos de Beni, Chuquisaca, Cochabamba, La Paz, Potosà y Santa Cruz, y representan las siguientes bioregiones: Puna, Chaco, Bosque Chiquitano, Sabanas Húmedas, Ceja de Montaña, Yungas, Valles Secos Interandinos y Bosque Húmedo de Llanura. Los ejemplares de Scinax chiquitanus y Phrynopus kempffi son paratipos. Pleurodema borelli es citada por primera vez para el Departamento de Santa Cruz y por segunda vez para Bolivia; también el registro de Liolaemus dorbignyi constituye el segundo para el paÃs y el de Tropidurus melanopleurus el primero para el Departamento Beni
Assessment of the potential energy hypersurfaces in thymine within multiconfigurational theory: CASSCF vs. CASPT2
The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess the accuracy of these different computational strategies under study based on a variety of numerical thresholds and optimization methods. Several basis sets and active spaces have also been calibrated to understand to what extent they can influence the resulting geometries and subsequent interpretation of the photochemical decay channels. The study shows small discrepancies between CASSCF and CASPT2 PEHs, displaying a shallow planar or twisted 1(ππ*) minimum, respectively, and thus featuring a qualitatively similar scenario for supporting the ultrafast bi-exponential deactivation registered in thymine upon UV-light exposure. A deeper knowledge of the PEHs at different levels of theory provides useful insight into its correct characterization and subsequent interpretation of the experimental observations. The discrepancies displayed by the different methods studied here are then discussed and framed within their potential consequences in on-the-fly non-adiabatic molecular dynamics simulations, where qualitatively diverse outcomes are expected
Quasar-galaxy and AGN-galaxy cross-correlations
We compute quasar-galaxy and AGN-galaxy cross-correlation functions for
samples taken from the \cite{VCV98} catalog of quasars and active galaxies,
using tracer galaxies taken from the Edinburgh/Durham Southern Catalog. The
sample of active galaxy targets shows positive correlation at projected
separations consistent with the usual power-law. On the
other hand, we do not find a statistically significant positive quasar-galaxy
correlation signal except in the range
where we find similar AGN-galaxy and quasar-galaxy correlation amplitudes. At
separations a strong decline of quasar-galaxy correlations
is observed, suggesting a significant local influence of quasars in galaxy
formation. In an attempt to reproduce the observed cross-correlation between
quasars and galaxies, we have performed CDM cosmological hydrodynamical
simulations and tested the viability of a scenario based on the model developed
by \cite{silkrees98}. In this scheme a fraction of the energy released by
quasars is considered to be transferred into the baryonic component of the
intergalactic medium in the form of winds. The results of the simulations
suggest that the shape of the observed quasar-galaxy cross-correlation function
could be understood in a scenario where a substantial amount of energy is
transferred to the medium at the redshift of maximum quasar activity.Comment: 11 pages, 9 figures. Accepted for publication in Ap
Galaxy groups in the 2dF redshift survey: Galaxy Spectral Type Segregation in Groups
The behaviour of the relative fraction of galaxies with different spectral
types in groups is analysed as a function of projected local galaxy density and
the group-centric distance. The group sample was taken from the 2dF Group
Galaxy Calatogue constructed by Merch\'an & Zandivarez. Our group sample was
constrained to have a homogeneous virial mass distribution with redshift.
Galaxies belonging to this group sample were selected in order to minimize
possible biases such as preferential selection of high luminosity objects. We
find a clear distinction between high virial mass groups (M_V\gsim 10^{13.5}
M_{\odot}) and the less massive ones. While the massive groups show a
significant dependence of the relative fraction of low star formation galaxies
on local galaxy density and group-centric radius,groups with lower masses show
no significant trends. We also cross-correlate our group subsample with the
previously identified clusters finding that this sample shows a very similar
behaviour as observed in the high virial mass group subsample.Comment: 3 figures replaced, accepted for publication in MNRA
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