411 research outputs found
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
The Distance to the Cygnus Loop from Hubble Space Telescope Imaging of the Primary Shock Front
We present a Hubble Space Telescope/WFPC2 narrow-band H-alpha image of a
region on the northeastern limb of the Cygnus Loop supernova remnant. This
location provides a detailed example of where the primary blast wave first
encounters the surrounding interstellar medium. The filament structure is seen
in exquisite detail in this image, which was obtained primarily as an EARLY
ACQuisition image for a follow-up spectroscopic program. We compare the HST
image to a digitized version of the POSS-I red plate to measure the proper
motion of this filament. By combining this value for the proper motion with
previous measurements of the shock velocity at this position we find that the
distance to the Cygnus Loop is 440 (+130, -100) pc, considerably smaller than
the canonical value of 770 pc. We briefly discuss the ramifications of this new
distance estimate for our understanding of this prototypical supernova remnant.Comment: 18 pages, 3 Figures (2 JPEG and one Postscript
C stars in the outer spheroid of NGC 6822
From a 2 x 2 degree survey of NGC 6822 we have previously established that
this Local Group dwarf irregular galaxy possesses a huge spheroid having more
than one degree in length. This spheroid is in rotation but its rotation curve
is known only within ~15' from the center. It is therefore critical to identify
bright stars belonging to the spheroid to characterize, as far as possible, its
outer kinematics. We use the new wide field near infrared imager CPAPIR,
operated by the SMARTS consortium, to acquire J, Ks images of two 34.8' x 34.8'
areas in the outer spheroid to search for C stars. The colour diagram of the
fields allows the identification of 192 C stars candidates but a study of the
FWHM of the images permits the rejection of numerous non-stellar objects with
colours similar to C stars. We are left with 75 new C stars, their mean Ks
magnitude and mean colour are similar to the bulk of known NGC 6822 C stars.
This outer spheroid survey confirms that the intermediate-age AGB stars are a
major contributor to the stellar populations of the spheroid. The discovery of
some 50 C stars well beyond the limit of the previously known rotation curve
calls for a promising spectroscopic follow-up to a major axis distance of 40'.Comment: 13 page
The natural science of cosmology
The network of cosmological tests is tight enough now to show that the
relativistic Big Bang cosmology is a good approximation to what happened as the
universe expanded and cooled through light element production and evolved to
the present. I explain why I reach this conclusion, comment on the varieties of
philosophies informing searches for a still better cosmology, and offer an
example for further study, the curious tendency of some classes of galaxies to
behave as island universes.Comment: Keynote lecture at the seventh International Conference on
Gravitation and Cosmology, Goa India, December 201
Dependence of Spiral Galaxy Distribution on Viewing Angle in RC3
The normalized inclination distributions are presented for the spiral
galaxies in RC3. The results show that, except for the bin of
-, in which the apparent minor isophotal diameters that
are used to obtain the inclinations, are affected by the central bulges, the
distributions for Sa, Sab, Scd and Sd are well consistent with the Monte-Carlo
simulation of random inclinations within 3-, and Sb and Sbc almost, but
Sc is different. One reason for the difference between the real distribution
and the Monte-Carlo simulation of Sc may be that some quite inclined spirals,
the arms of which are inherently loosely wound on the galactic plane and should
be classified to Sc galaxies, have been incorrectly classified to the earlier
ones, because the tightness of spiral arms which is one of the criteria of the
Hubble classification in RC3 is different between on the galactic plane and on
the tangent plane of the celestial sphere. Our result also implies that there
might exist biases in the luminosity functions of individual Hubble types if
spiral galaxies are only classified visually.Comment: 5 pages + 8 figures, LaTe
Panoramic Views of the Cygnus Loop
We present a complete atlas of the Cygnus Loop supernova remnant in the light
of [O III] (5007), H alpha, and [S II] (6717, 6731). Despite its shell-like
appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast
wave. Rather, the optical emission traces interactions of the supernova blast
wave with clumps of gas. The surrounding interstellar medium forms the walls of
a cavity through which the blast wave now propagates, including a nearly
complete shell in which non-radiative filaments are detected. The Cygnus Loop
blast wave is not breaking out of a dense cloud, but is instead running into
confining walls. The interstellar medium dominates not only the appearance of
the Cygnus Loop but also the continued evolution of the blast wave. If this is
a typical example of a supernova remnant, then global models of the
interstellar medium must account for such significant blast wave deceleration.Comment: 28 pages AAS Latex, 28 black+white figures, 6 color figures. To be
published in The Astrophysical Journal Supplement Serie
Speakable and unspeakable in cosmology: dark matter vs. gravitational self energies. Hubble's constant, the cosmological term and all that
The inadequacy of the present cosmological picture is underlined. The central
issue of energy and particles-photons number conservation is addressed. It is
shown that consideration of gravitational self energy is paramount both for
matter and for radiation to bring present data estimates of matter and
radiation density and the radius of the universe towards agreement with the
Planck scale quantities from which it should have consistently evolved.
Particle creation is proven to play a fundamental role in the evolution of the
Universe. It is argued that we might be living inside an expanding black hole
A Century of Cosmology
In the century since Einstein's anno mirabilis of 1905, our concept of the
Universe has expanded from Kapteyn's flattened disk of stars only 10 kpc across
to an observed horizon about 30 Gpc across that is only a tiny fraction of an
immensely large inflated bubble. The expansion of our knowledge about the
Universe, both in the types of data and the sheer quantity of data, has been
just as dramatic. This talk will summarize this century of progress and our
current understanding of the cosmos.Comment: Talk presented at the "Relativistic Astrophysics and Cosmology -
Einstein's Legacy" meeting in Munich, Nov 2005. Proceedings will be published
in the Springer-Verlag "ESO Astrophysics Symposia" series. 10 pages Latex
with 2 figure
Sedimentology, structure and age estimate of five continental slope submarine landslides, eastern Australia
Sedimentological and accelerator mass spectrometry (AMS) <sup>14</sup>C data provide estimates of the structure and age of five submarine landslides (âŒ0.4â3 km<sup>3</sup>) present on eastern Australia's continental slope between Noosa Heads and Yamba. Dating of the post-slide conformably deposited sediment indicates sediment accumulation rates between 0.017 m ka<sup>â1</sup> and 0.2 m ka<sup>â1</sup>, which is consistent with previous estimates reported for this area. Boundary surfaces were identified in five continental slope cores at depths of 0.8 to 2.2 m below the present-day seafloor. Boundary surfaces present as a sharp colour-change across the surface, discernible but small increases in sediment stiffness, a slight increase in sediment bulk density of 0.1 g cm<sup>â3</sup>, and distinct gaps in AMS <sup>14</sup>C ages of at least 25 ka. Boundary surfaces are interpreted to represent a slide plane detachment surface but are not necessarily the only ones or even the major ones. Sub-bottom profiler records indicate that: (1) the youngest identifiable sediment reflectors upslope from three submarine landslides terminate on and are truncated by slide rupture surfaces; (2) there is no obvious evidence for a post-slide sediment layer draped over, or burying, slide ruptures or exposed slide detachment surfaces; and (3) the boundary surfaces identified within the cores are unlikely to be near-surface slide surfaces within an overall larger en masse dislocation. These findings suggest that these submarine landslides are geologically recent (<25 ka), and that the boundary surfaces are either: (a) an erosional features that developed after the landslide, in which case the boundary surface age provides a minimum age for the landslide; or (b) detachment surfaces from which slabs of near-surface sediment were removed during landsliding, in which case the age of the sediment above the boundary surface indicates the approximate age of landsliding. While an earthquake-triggering mechanism is favoured for the initiation of submarine landslides on the eastern Australian margin, further evidence is required to confirm this interpretation
Testing The Friedmann Equation: The Expansion of the Universe During Big-Bang Nucleosynthesis
In conventional general relativity, the expansion rate H of a
Robertson-Walker universe is related to the energy density by the Friedmann
equation. Aside from the present day, the only epoch at which we can constrain
the expansion history in a model-independent way is during Big-Bang
Nucleosynthesis (BBN). We consider a simple two-parameter characterization of
the behavior of H during BBN and derive constraints on this parameter space,
finding that the allowed region of parameter space is essentially
one-dimensional. We also study the effects of a large neutrino asymmetry within
this framework. Our results provide a simple way to compare an alternative
cosmology to the observational requirement of matching the primordial
abundances of the light elements.Comment: 18 pages, Final version to be published in Phys. Rev.
- âŠ