2,472 research outputs found
The Infrared Continuum of Active Galaxies
We discuss the different physical processes contributing to the infrared
continuum of AGN, assuming that both photoionization from the active center and
shocks ionize and heat the gas and dust contained in an ensemble of clouds
surrounding the nucleus. Radiation transfer of primary and secondary radiation
throughout a cloud is calculated consistently with collisional processes due to
the shock. We consider that the observed continuum corresponds to reprocessed
radiation from both dust and gas in the clouds. The model is applied to the
continuum of Seyfert galaxies from which best estimate of the nuclear, stellar
subtracted, emission is available. The results show that radiation-dominated
high velocity clouds are more numerous in Seyfert 1-1.5 whereas shock-dominated
low velocity clouds are dominant in Seyfert type 2 in full agreement with the
unified model for AGN. In type 2 objects, radiation is partly suppressed by a
central dusty medium with a high dust-to-gas ratio. A grid of models is used to
provide a phenomenological analysis of the observed infrared spectral energy
distribution.Comment: 14 pages, 10 figures. in press in MNRA
The Narrow Line Region of Ark 564
The continuum and emission-line spectrum of the narrow-line Seyfert 1 galaxy
Ark 564 is used to investigate, for the first time, the physical conditions and
structure of its narrow line region (NLR). For this purpose, composite models,
accounting for the coupled effect of photoionization and shocks, are employed.
The emission-line spectrum of Ark 564, which ranges from the ultraviolet to the
near-infrared, shows a rich forbidden line spectrum. Strong emphasis is given
to the study of the coronal line region. The diversity of physical conditions
deduced from the observations requires multi-cloud models to reproduce the
observed lines and continuum. We find that a combination of high velocity (Vs =
1500 km/s) shock-dominated clouds as well as low velocity (Vs = 150 km/s)
radiation-dominated clouds explains the coronal lines, while the optical
low-ionization lines are mainly explained by shock-dominated clouds. The
results for Ark 564 are compared with those obtained for other Seyfert galaxies
previously analyzed such as NGC 5252, Circinus, NGC 4051 and NGC 4151. The
model results for the ultraviolet and optical permitted lines suggest that the
broad line region may contribute up to 80%, depending on the emission-line,
being of about 30% for Hbeta. The consistency of the multi-cloud model is
checked by comparing the predicted and observed continuum, from radio to X-ray,
and indicate that the dust-to-gas ratio in the clouds varies from 10^{-15} to
10^{-12}.Comment: 18 pages, 1 figure. Accepted in A&
Nelson Goodman and Project Zero: art, cognition, and education
Annamaria Contini\u2019s article \u201cNelson Goodman and Project Zero: art, cognition, and education\u201d was published in a special issue of the journal "Studi di Aesthetic" (eds Annamaria Contini & Lorenzo Manera) on \u201cAesthetic Education\u201d. The article reconstructs in an interdisciplinary perspective a little-known but very interesting chapter both for aesthetics and for the history and theories of education. The protagonist of this chapter is one of the most influential figures in contemporary aesthetics and analytic philosophy in general: Nelson Goodman.
In 1967, Nelson Goodman founded Project Zero, a program of basic research into art education, which, even today, is an international reference point. This article reviews the relationships between the theses set out in Languages of Art and the most original results obtained by Project Zero between 1967 and 1971, when Goodman was the director. Thus emerges the role of general symbol theory in developing an educational approach that attempts to overcome the dichotomy between art and science, between the emotive and the cognitive. The article also analyzes the effects of Goodman\u2019s participation in Project Zero on his aesthetic-philosophical reflections, using it to interpret both the exemplar function taken on by art in Ways of Worldmaking, and the emergence of new concepts such as that of \u201cimplementation\u201d
A Multi-Cloud Warm-Absorber Model for NGC 4051
A multi-cloud model is presented which explains the soft X-ray excess in NGC
4051 and, consistently, the optical line spectrum and the SED of the continuum.
The clouds are heated and ionized by the photoionizing flux from the active
center and by shocks. Diffuse radiation, partly absorbed throughout the clouds,
nicely fits the bump in the soft X-ray domain, while bremsstrahlung radiation
from the gaseous clouds contribute to the fit of the continuum SED. Debris of
high density fragmented clouds are necessary to explain the absorption oxygen
throats observed at 0.87 keV and 0.74 keV. The debris are heated by shocks of
about 200-300 km/s. Low velocity (100 km/s)-density (100 cm-3) clouds
contribute to the line and continuum spectra, as well as high velocity (1000
km/s)-density (8000 cm-3) clouds which are revealed by the FWHM of the line
profiles. The SED in the IR is explained by reradiation of dust, however, the
dust-to-gas ratio is not particularly high. Radio emission is well fitted by
synchrotron radiation created at the shock front by Fermi mechanism.Comment: 19 pages + 3 figures PostScrip
Metaphor
The encyclopedia entry aims to trace the fundamental coordinates of today's debate on metaphor, with particular reference to the metaphor-knowledge nexus and the relationship between thought and language
On the formation and physical properties of the Intra-Cluster Light in hierarchical galaxy formation models
We study the formation of the Intra-Cluster Light (ICL) using a semi-analytic
model of galaxy formation, coupled to merger trees extracted from N-body
simulations of groups and clusters. We assume that the ICL forms by (1) stellar
stripping of satellite galaxies and (2) relaxation processes that take place
during galaxy mergers. The fraction of ICL in groups and clusters predicted by
our models ranges between 10 and 40 per cent, with a large halo-to-halo scatter
and no halo mass dependence. We note, however, that our predicted ICL fractions
depend on the resolution: for a set of simulations with particle mass one order
of magnitude larger than that adopted in the high resolution runs used in our
study, we find that the predicted ICL fractions are ~30-40 per cent larger than
those found in the high resolution runs. On cluster scale, large part of the
scatter is due to a range of dynamical histories, while on smaller scale it is
driven by individual accretion events and stripping of very massive satellites,
, that we find to be the major contributors
to the ICL. The ICL in our models forms very late (below ), and a
fraction varying between 5 and 25 per cent of it has been accreted during the
hierarchical growth of haloes. In agreement with recent observational
measurements, we find the ICL to be made of stars covering a relatively large
range of metallicity, with the bulk of them being sub-solar.Comment: Accepted for Publication in MNRAS, 19 pages, 13 figures, 1 tabl
Rescoring Virtual Screening Results with the MM-PBSA Methods: Beware of Internal Dielectric Constants
With the potential of improving virtual screening outcome, MM-PB/GBSA has become a disputed method that requires extensive testing and tuning to provide the optimal results. One of the tuning factors is the internal or solute dielectric constant. We have applied three test sets with receptors of different categories and libraries from different sources to investigate the underlying issue related to this constant. We discovered that increasing internal dielectric value does not improve the virtual screening enrichment qualitatively. More interestingly, nonpolar and polar calculated energies act differently in libraries with different molecular weight distributions. From this work, the performance of MM-PBSA rescoring in virtual screening is more library- than receptor-dependent
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