708 research outputs found
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
The Lyman-alpha Forest at z~4: Keck HIRES Observations of Q 0000-26
This paper describes a study of the Lyman-alpha forest absorption clouds
along the quasar sightline Q0000-26 (zem=4.1). The spectrum was obtained with
the High Resolution Spectrometer on the 10m Keck telescope. We derive accurate
H I column density and Doppler width distributions for the clouds from Voigt
profile fitting. We also analyze simulated Lyman-alpha forest spectra of
matching characteristics in order to gauge the effects of line
blending/blanketing and noise in the data. The results are compared with
similar studies at lower redshifts in order to study any possible evolution in
the clouds' properties. We also estimate the mean intensity of the UV
background at z=4 from an analysis of the proximity effect.Comment: plain TeX containing 23 PS pages, 3 PS tables, and 9 PS figures, ApJ,
Dec 1, 1996 issue replacing an earlier version which contains an corrupted
table
Submarine landslides on the upper southeast Australian passive continental margin – preliminary findings
The southeast Australian passive continental margin is narrow, steep and sediment-deficient, and characterized by relatively low rates of modern sedimentation. Upper slope (\u3c1200m) sediments comprise mixtures of calcareous and terrigenous sand and mud. Three of twelve sediment cores recovered from geologically-recent, submarine landslides located offshore New South Wales/Queensland (NSW/QLD) are interpreted to have sampled failure surfaces at depths of between 85 cm and 220 cm below the present-day seabed. Differences in sediment physical properties are recorded above and below the three slide-plane boundaries. Sediment taken directly above the inferred submarine landslide failure surfaces and presumed to be post-landslide, returned radiocarbon ages of 15.8 ka, 20.7 ka and 20.1 ka. The last two ages correspond to adjacent slide features, which are inferred to be consistent with their being triggered by a single event such as an earthquake. Slope stability models based on classical soil mechanics and measured sediment shearstrengths indicate that the upper slope sediments should be stable. However, multibeam sonar data reveal that many upper slope landslides occur across the margin and that submarine landsliding is a common process. We infer from these results that: a) an unidentified mechanism regularly acts to reduce the shear resistance of these sediments to the very low values required to enable slope failure, and/or b) the margin experiences seismic events that act to destabilise the slope sediments
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
Halo Shapes, Dynamics and Environment
In the hierarchical structure formation model cosmic halos are supposed to
form by accretion of smaller units along anisotropic direction, defined by
large-scale filamentary structures. After the epoch of primary mass aggregation
(which depend on the cosmological model), violent relaxation processes will
tend to alter the halo phase-space configuration producing quasi-spherical
halos with a relatively smooth density profiles.
Here we attempt to investigate the relation between halos shapes, their
environment and their dynamical state. To this end we have run a large ( Mpc, particles) N-body simulation of a flat low-density
cold dark matter model with a matter density , Hubble constant km s
Mpc and a normalization parameter of . The particle mass
is comparable to the mass of
one single galaxy. The halos are defined using a friends-of-friend algorithm
with a linking length given by where is the mean
density. This linking length corresponds to an overdensity at the present epoch () and the total number of halos
with more than 130 particles () is 57524.Comment: To be published in "Groups Of Galaxies In The Nearby Universe", held
in Chile, December 2005, edited by I.Saviane, V.Ivanov and J.Borissova.
Springer-Verlag series "ESO Astrophysics Symposia
Cosmological Relativity: A General-Relativistic Theory for the Accelerating Expanding Universe
Recent observations of distant supernovae imply, in defiance of expectations,
that the universe growth is accelerating, contrary to what has always been
assumed that the expansion is slowing down due to gravity. In this paper a
general-relativistic cosmological theory that gives a direct relationship
between distances and redshifts in an expanding universe is presented. The
theory is actually a generalization of Hubble's law taking gravity into account
by means of Einstein's theory of general relativity. The theory predicts that
the universe can have three phases of expansion, decelerating, constant and
accelerating, but it is shown that at present the first two cases are excluded,
although in the past it had experienced them. Our theory shows that the
universe now is definitely in the stage of accelerating expansion, confirming
the recent experimental results
An HST/WFPC Survey of Bright Young Clusters in M31. II. Photometry of Less Luminous Clusters in the Fields
We report on the properties of 89 low mass star clusters located in the
vicinity of luminous young clusters (blue globulars) in the disk of M31. 82 of
the clusters are newly detected. We have determined their integrated magnitudes
and colors, based on a series of Hubble Space Telescope Wide Field/Planetary
Camera 2 exposures in blue and red (HST filters F450W and F814W). The
integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors
indicate a wide range of ages. Stellar color-magnitude diagrams for all
clusters were obtained and those with bright enough stars were fit to
theoretical isochrones to provide age estimates. The ages range from 12 Myr to
>500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of
0.21 magnitudes, were derived from the main sequence fitting for those
clusters. Comparison of these ages and integrated colors with single population
theoretical models with solar abundances suggests a color offset of 0.085
magnitudes at the ages tested. Estimated ages for the remaining clusters are
based on their measured colors. The age-frequency diagram shows a steep decline
of number with age, with a large decrease in number per age interval between
the youngest and the oldest clusters detected.Comment: 20 pages, 9 figure
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
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