120 research outputs found
Two-fluid model for VLBI jets. I. Homogeneous and stationary synchrotron emission simulations
In this series of papers, we develop a two-fluid model for VLBI jets. The
idea is that the jet itself is non- or mildly-relativistic (electrons and
protons), while the radiating blobs are relativistic electron-positron `clouds'
moving on helical paths wrapped around the jet. In this work, the emphasis is
on the physical description of the clouds, and not on the structure or origin
of the trajectory. In the simple case where the magnetic field is uniform and
homogeneous accross the cloud, and the properties of the cloud are constant,
the present paper shows synthetic maps of VLBI jets in different
configurations, as well as the variation of different observational parameters
along the trajectory.Comment: to appear in A&A, 8 pages and 10 figure
The Fundamental Plane of Early-Type Galaxies as a Confounding Correlation
Early-type galaxies are characterized by many scaling relations. One of them,
the so-called fundamental plane is a relatively tight correlation between three
variables, and has resisted a clear physical understanding despite many years
of intensive research. Here, we show that the correlation between the three
variables of the fundamental plane can be the artifact of the effect of another
parameter influencing all, so that the fundamental plane may be understood as a
confounding correlation. Indeed, the complexity of the physics of galaxies and
of their evolution suggests that the main confounding parameter must be related
to the level of diversification reached by the galaxies. Consequently, many
scaling relations for galaxies are probably evolutionary correlations
An optical counterjet in 3C66B?
Long exposure observations at the Canada-France-Hawaii Telescope of 3C66B in
the I filter are presented. After subtraction of the galactic background,
optical emission on the counterjet side is detected in 10 knots coincident with
the radio counterjet. Their radio-to-optical spectral indices (0.5--0.6) are
typical of synchrotron emission in extragalactic jets, so that these knots
possibly are the optical counterparts of the radio counterjet. If this is
confirmed, 3C66B would be the first double-sided extragalactic optical jet. The
optical counterjet would also be brighter than what is predicted from the
relativistic beaming interpretation of brightness asymmetry between the two
jets. This would thus prove that the radiation properties are intrinsically
different in the two jets. Alternatively, these knots could have nothing to do
with the counterjet. But it seems that in this case, the optical counterjet
would be fainter than expected from the relativistic beaming interpretation,
favouring intrinsic asymmetry as well. In addition, two new optical components
are found in the jet.Comment: to appear in MNRAS; 6 pages, 5 figure
A six-parameter space to describe galaxy diversification
Galaxy diversification proceeds by transforming events like accretion,
interaction or mergers. These explain the formation and evolution of galaxies
that can now be described with many observables. Multivariate analyses are the
obvious tools to tackle the datasets and understand the differences between
different kinds of objects. However, depending on the method used,
redundancies, incompatibilities or subjective choices of the parameters can
void the usefulness of such analyses. The behaviour of the available parameters
should be analysed before an objective reduction of dimensionality and
subsequent clustering analyses can be undertaken, especially in an evolutionary
context. We study a sample of 424 early-type galaxies described by 25
parameters, ten of which are Lick indices, to identify the most structuring
parameters and determine an evolutionary classification of these objects. Four
independent statistical methods are used to investigate the discriminant
properties of the observables and the partitioning of the 424 galaxies:
Principal Component Analysis, K-means cluster analysis, Minimum Contradiction
Analysis and Cladistics. (abridged)Comment: Accepted for publicationin A\&
The environment of formation as a second parameter for globular cluster classification
We perform an evolutionary multivariate analysis of a sample of 54 Galactic globular clusters with high-quality colour-magnitude diagrams and well-determined ages. The four parameters adopted for the analysis are: metallicity, age, maximum temperature on the horizontal branch and absolute V magnitude. Our cladistic analysis breaks the sample into three novel groups. An a posteriori kinematical analysis puts groups 1 and 2 in the halo, and group 3 in the thick disc. The halo and disc clusters separately follow a luminosity-metallicity relation of much weaker slope than galaxies. This property is used to propose a new criterion for distinguishing halo and disc clusters. A comparison of the distinct properties of the two halo groups with those of Galactic halo field stars indicates that the clusters of group 1 originated in the inner halo, while those of group 2 formed in the outer halo of the Galaxy. The inner halo clusters were presumably initially the most massive one, which allowed the formation of more strongly helium-enriched second generation stars, thus explaining the presence of Cepheids and of very hot horizontal-branch stars exclusively in this group. We thus conclude that the âsecond parameter' is linked to the environment in which globular clusters form, the inner halo favouring the formation of the most massive clusters which subsequently become more strongly self-enriched than their counterparts of the galactic outer halo and dis
An Optical Study of 3C 31, 3C 66B, 3C 120, and Their Jets
Paper freely available at http://cdsads.u-strasbg.fr/cgi-bin/nph-iarticle_query?1991AJ....102..562F&data_type=PDF_HIGH&type=PRINTERWe present the results of BVRI CCD photometry of the radiogalaxies 3C 31, 3C 66B, and 3C 120, and V polarimetry of 3C 120. The photometry of the jet of 3C 66B definitively establishes the synchrotron nature of the optical emission. No optical counterpart of the radio counterjet in 3C 66B and of the radio jets in 3C 31 and 3C 120 is found. A rotating ring and an ionized region are present respectively in 3C 31 (NGC 383) and its companion galaxy NGC 382, but we find no isophotal distortions which could have revealed a gravitational interaction between the two galaxies as it is the case in 3C 66B. The elliptical isophotes of 3C 120 shows a slight off-centering, roughly in the direction of the radio jet, very much like 3C 66B. We find an upper limit of 20% for the polarization level of the condensations in 3C 120
First phylogenetic analyses of galaxy evolution
The Hubble tuning fork diagram, based on morphology, has always been the
preferred scheme for classification of galaxies and is still the only one
originally built from historical/evolutionary relationships. At the opposite,
biologists have long taken into account the parenthood links of living entities
for classification purposes. Assuming branching evolution of galaxies as a
"descent with modification", we show that the concepts and tools of
phylogenetic systematics widely used in biology can be heuristically transposed
to the case of galaxies. This approach that we call "astrocladistics" has been
first applied to Dwarf Galaxies of the Local Group and provides the first
evolutionary galaxy tree. The cladogram is sufficiently solid to support the
existence of a hierarchical organization in the diversity of galaxies, making
it possible to track ancestral types of galaxies. We also find that morphology
is a summary of more fundamental properties. Astrocladistics applied to
cosmology simulated galaxies can, unsurprisingly, reconstruct the correct
"genealogy". It reveals evolutionary lineages, divergences from common
ancestors, character evolution behaviours and shows how mergers organize galaxy
diversity. Application to real normal galaxies is in progress. Astrocladistics
opens a new way to analyse galaxy evolution and a path towards a new
systematics of galaxies
Towards a Phylogenetic Analysis of Galaxy Evolution : a Case Study with the Dwarf Galaxies of the Local Group
Context: The Hubble tuning fork diagram has always been the preferred scheme
for classification of galaxies. It is based on morphology only. At the
opposite, biologists have long taken into account the genealogical relatedness
of living entities for classification purposes. Aims: Assuming branching
evolution of galaxies as a 'descent with modification', we show here that the
concepts and tools of phylogenetic systematics widely used in biology can be
heuristically transposed to the case of galaxies. Methods: This approach that
we call "astrocladistics" is applied to Dwarf Galaxies of the Local Group and
provides the first evolutionary tree for real galaxies. Results: The trees that
we present here are sufficiently solid to support the existence of a
hierarchical organization in the diversity of dwarf galaxies of the Local
Group. This also shows that these galaxies all derive from a common ancestral
kind of objects. We find that some kinds of dIrrs are progenitors of both dSphs
and other kinds of dIrrs.We also identify three evolutionary groups, each one
having its own characteristics and own evolution. Conclusions: The present work
opens a new way to analyze galaxy evolution and a path towards a new
systematics of galaxies. Work on other galaxies in the Universe is in progress.Comment: 13 pages 5 figures with 3 online onl
Linear polarization and composition of VLBI jets
It is shown that linear polarization data can be used to constrain the
composition (normal or pair plasma) of pc-scale extragalactic jets.
A simple criterion, based on synchrotron and Faraday depolarization
properties, is established. It does not depend on the particle density and the
length of the emitting region along the line of sight, thus eliminating two
physical unknowns.Comment: 4 pages, 1 figur
Galaxies and Cladistics
The Hubble tuning fork diagram, based on morphology and established in the
1930s, has always been the preferred scheme for classification of galaxies.
However, the current large amount of multiwavelength data, most often spectra,
for objects up to very high distances, asks for more sophisticated statistical
approaches. Interpreting formation and evolution of galaxies as a ?transmission
with modification' process, we have shown that the concepts and tools of
phylogenetic systematics can be heuristically transposed to the case of
galaxies. This approach, which we call ?astrocladistics', has successfully been
applied on several samples. Many difficulties still remain, some of them being
specific to the nature of both galaxies and their diversification processes,
some others being classical in cladistics, like the pertinence of the
descriptors in conveying any useful evolutionary information.Comment: Talk given at the "12th Evolutionary Biology Meeting" held in
Marseille, France, Sept. 24-26, 200
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