940 research outputs found
The Planetary Nebulae Luminosity Function and distances to Virgo, Hydra I and Coma clusters
The luminosity function of planetary nebulae populations in galaxies within
10-15 Mpc distance has a cut-off at bright magnitudes and a functional form
that is observed to be invariant in different galaxy morphological types. Thus
it is used as a secondary distance indicator in both early and late-type
galaxies. Recent deep surveys of planetary nebulae populations in brightest
cluster galaxies (BCGs) seem to indicate that their luminosity functions
deviate from those observed in the nearby galaxies. We discuss the evidence for
such deviations in Virgo, and indicate which physical mechanisms may alter the
evolution of a planetary nebula envelope and its central star in the halo of
BCGs. We then discuss preliminary results for distances for the Virgo, Hydra I
and Coma clusters based on the observed planetary nebulae luminosity functions.Comment: 5 pages, one figure. To appear on the Proceedings of the IAU Symp.
289 "Advancing the physics of cosmic distances
The planetary nebula population in the halo of M87
We investigate the diffuse light in the outer regions of the nearby
elliptical galaxy M87 in the Virgo cluster, using planetary nebulas (PNs) as
tracers. The surveyed areas (0.43 squared degrees) cover M87 up to a radial
distance of 150 kpc, in the ransition region between galaxy halo and
intracluster light (ICL). All PNs are identified through the on-off band
technique using automatic selection criteria based on the distribution of the
detected sources in the colour-magnitude diagram and the properties of their
point-spread function. We extract a catalogue of 688 objects down to
m_5007=28.4, with an estimated residual contamination from foreground stars and
background Lyalpha galaxies, which amounts to ~35% of the sample. This is one
of the largest extragalactic PN samples in number of candidates, magnitude
depth, and radial extent, which allows us to carry out an unprecedented
photometric study of the PN population in the outer regions of M87. We find
that the logarithmic density profile of the PN distribution is shallower than
the surface brightness profile at large radii. This behaviour is consistent
with the superposition of two components associated with the halo of M87 and
with the ICL, which have different luminosity specific PN numbers, the ICL
contributing three times more PNs per unit light. Because of the depth of this
survey we are also able to study the shape of the PN luminosity function (PNLF)
in the outer regions of M87. We find a slope for the PNLF that is steeper at
fainter magnitudes than the standard analytical PNLF formula and adopt a
generalised model that treats the slope as a free parameter. Comparing the PNLF
of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87
PNLF contains fewer bright PNs and has a steeper slope towards fainter
magnitudes.Comment: 16 pages, 13 figures, 5 tables, accepted for publication in A&
Structure of self-assembled Mn atom chains on Si(001)
Mn has been found to self-assemble into atomic chains running perpendicular
to the surface dimer reconstruction on Si(001). They differ from other atomic
chains by a striking asymmetric appearance in filled state scanning tunneling
microscopy (STM) images. This has prompted complicated structural models
involving up to three Mn atoms per chain unit. Combining STM, atomic force
microscopy and density functional theory we find that a simple necklace-like
chain of single Mn atoms reproduces all their prominent features, including
their asymmetry not captured by current models. The upshot is a remarkably
simpler structure for modelling the electronic and magnetic properties of Mn
atom chains on Si(001).Comment: 5 pages, 4 figure
Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study
Atomic nanolines are one dimensional systems realized by assembling many
atoms on a substrate into long arrays. The electronic properties of the
nanolines depend on those of the substrate. Here, we demonstrate that to fully
understand the electronic properties of Bi nanolines on clean Si(001) several
different contributions must be accounted for. Scanning tunneling microscopy
reveals a variety of different patterns along the nanolines as the imaging bias
is varied. We observe an electronic phase shift of the Bi dimers, associated
with imaging atomic p-orbitals, and an electronic coupling between the Bi
nanoline and neighbouring Si dimers, which influences the appearance of both.
Understanding the interplay between the Bi nanolines and Si substrate could
open a novel route to modifying the electronic properties of the nanolines.Comment: 6 pages (main), 2 pages (SI), accepted by Phys. Rev.
Fossil group origins: VIII RXJ075243.6+455653 a transitionary fossil group
It is thought that fossil systems are relics of structure formation in the
primitive Universe. They are galaxy aggregations that have assembled their mass
at high redshift with few or no subsequent accretion. Observationally these
systems are selected by large magnitude gaps between their 1st and 2nd ranked
galaxies. Nevertheless, there is still debate over whether or not this
observational criterium selects dynamically evolved ancient systems. We have
studied the properties of the nearby fossil group RXJ075243.6+455653 in order
to understand the mass assembly of this system. Deep spectroscopic observations
allow us to construct the galaxy luminosity function (LF) of RXJ075243.6+455653
down to M*+ 6. The analysis of the faint-end of the LF in groups and clusters
provides valuable information about the mass assembly of the system. In
addition, we have analyzed the nearby large-scale structure around this group.
We identified 26 group members within r200=0.9 Mpc. The LF of the group shows a
flat faint-end slope ( -1.08 +/- 0.33). This low density of dwarf galaxies is
confirmed by the low value of the dwarf-to-giant ratio (DGR = 0.99 +/- 0.49)
for this system. Both the lack of dwarf galaxies and the low luminosity of the
BGG suggests that RXJ075243.6+455653 still has to accrete mass from its nearby
environment. This mass accretion will be achieved because it is the dominant
structure of a rich environment formed by several groups of galaxies (15)
within 7 Mpc from the group center and with +/- 1000$ km/s. RXJ075243.6+455653
is a group of galaxies that has not yet completed the process of its mass
assembly. This new mass accretion will change the fossil state of the group.
This group is an example of a galaxy aggregation selected by a large magnitude
gap but still in the process of the accretion of its mass (Abridged).Comment: 9 pages, 9 figures, accepted in A&
The influence of resilience and future orientation on academic achievement during the transition to high school: the mediating role of social support
This study applies Bronfenbrenner’s bioecological systems theory to analyse how social support impacts resilience, future outlook, and academic performance during high school transition. Self-report questionnaires were completed by 958 high school students (54.5% female) ages 13–1 (M = 13.72, SD = 0.47). After descriptive and correlational analyses, path analysis explored the mediating function of general social support and its specific manifestations (from family, friends, and significant others). The findings indicate a strong positive correlation among resilience, social support, and academic success, with resilience directly enhancing academic achievement. Both general social support and support from family, friends, and significant others are key in mediating the effects on achievement. This emphasizes the relevance of social networks in educational performance and the need for interventions to build social relationships to promote student resilience and future planning. Long-term consequences of these determinants on educational trajectories should be studied, taking cultural differences into account
School climate and academic performance of Italian students: the role of disciplinary behaviour and parental involvement
Educational researchers have increasingly recognised the importance of school climate as a malleable factor for improving academic performance. In this perspective, we exploit the data collected by the Italian Institute for the Evaluation of the Education System (INVALSI) to assess the effect of some school climate related factors on academic performance of tenth-grade Italian students. A Multilevel Bayesian Structural Equation Model (MBSEM) is adopted to highlight the effect of some relevant dimensions of school climate (students’ disciplinary behaviour and parents’ involvement) on academic performance and their role on the relationships between student socioeconomic status and achievement. The main findings show that disciplinary behaviour, on the one hand, directly influences the level of competence of the students, and, on the other hand, it partly mediates the effect of socioeconomic background whereas parents’ involvement does not appear to exert any significant effect on students’ performance
What School Factors are Associated with the Success of Socio-Economically Disadvantaged Students? An Empirical Investigation Using PISA Data
Many school-level policies, such as school funding formulae and teacher allocation mechanisms, aim at reducing the influence of students’ low socio-economic condition on academic achievement. Benchmarks and indicators based on large-scale international assessments can be used to measure academic success and identify if and when disadvantaged students are successful. We build on such work and develop a new method for identifying a cross-country comparable metric of the academic success of socio-economically disadvantaged students using data from the Programme for International Student Assessment (PISA). We estimate the prevalence of successful disadvantaged students in 56 countries, as well as changes over time between 2006 and 2015. In addition, we focus on the PISA 2015 edition and explore school factors associated with the probability that disadvantaged students will be successful academically in a subsample of 18 countries. Findings reveal that successful disadvantaged students attend schools with a better disciplinary climate and that provide additional time for instruction in key subjects
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