287 research outputs found
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
Classical Cosmological Tests for Galaxies of the Hubble Ultra Deep Field
Images of the Hubble Ultra Deep Field are analyzed to obtain a catalog of
galaxies for which the angular sizes, surface brightness, photometric
redshifts, and absolute magnitudes are found. The catalog contains a total of
about 4000 galaxies identified at a high signal-to-noise ratio, which allows
the cosmological relations angular size{redshift and surface
brightness-redshift to be analyzed. The parameters of the evolution of linear
sizes and surface brightness of distant galaxies in the redshift interval
0.5-6.5 are estimated in terms of a grid of cosmological models with different
density parameters. The distribution of photometric redshifts of galaxies is
analyzed and possible superlarge inhomogeneities in the radial distribution of
galaxies are found with scale lengths as large as 2000 Mpc.Comment: 23 pages, 9 figures, 1 tabl
A large-scale survey for variable stars in M33
We have started a survey of M 33 in order to find variable stars and Cepheids
in particular. We have obtained more than 30 epochs of g'r'i' data with the
CFHT and the one-square-degree camera MegaCam. We present first results from
this survey, including the search for variable objects and a basic
characterization of the various groups of variable stars.Comment: To appear in the proceedings of the "Nonlinear stellar
hydrodynamics", conference in honor of Robert Buchler's 65th birthday, July
2007, Pari
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
Structure of the Galaxies in the NGC 80 Group
BV-bands photometric data obtained at the 6-m telescope of the Special
Astrophysical Observatory are used to analyze the structure of 13 large disk
galaxies in the NGC 80 group. Nine of the 13 galaxies under consideration are
classified by us as lenticular galaxies. The stellar populations in the
galaxies are very different, from old ones with ages of T>10 Gyrs (IC 1541) to
relatively young, with the ages of T<2-3 Gyr (IC 1548, NGC 85). In one case,
current star formation is known (UCM 0018+2216). In most of the galaxies, more
precisely in all of them more luminous than M(B) -18, two-tiered
(`antitruncated') stellar disks are detected, whose radial surface brightness
profiles can be fitted by two exponential segments with different scalelengths
-- shorter near the center and longer at the periphery. All dwarf S0 galaxies
with single-scalelength exponential disks are close companions to giant
galaxies. Except for this fact, no dependence of the properties of S0 galaxies
on distance from the center of the group is found. Morphological traces of
minor merger are found in the lenticular galaxy NGC 85. Basing on the last two
points, we conclude that the most probable mechanisms for the transformation of
spirals into lenticular galaxies in groups are gravitational ones, namely,
minor mergers and tidal interactions.Comment: 24 pages, 9 figures, slightly improved version of the paper published
in the December, 2009, issue of the Astronomy Report
Superclusters of galaxies in the 2dF redshift survey. III. The properties of galaxies in superclusters
We use catalogues of superclusters of galaxies from the 2dF Galaxy Redshift
Survey to study the properties of galaxies in superclusters. We compare the
properties of galaxies in high and low density regions of rich superclusters,
in poor superclusters and in the field, as well as in groups, and of isolated
galaxies in superclusters of various richness. We show that in rich
superclusters the values of the luminosity density smoothed on a scale of 8
\Mpc are higher than in poor superclusters: the median density in rich
superclusters is , in poor superclusters . Rich superclusters contain high density cores with densities while in poor superclusters such high density cores are absent. The
properties of galaxies in rich and poor superclusters and in the field are
different: the fraction of early type, passive galaxies in rich superclusters
is slightly larger than in poor superclusters, and is the smallest among the
field galaxies. Most importantly, in high density cores of rich superclusters
() there is an excess of early type, passive galaxies in groups
and clusters, as well as among those which do not belong to groups or clusters.
The main galaxies of superclusters have a rather limited range of absolute
magnitudes. The main galaxies of rich superclusters have larger luminosities
than those of poor superclusters and of groups in the field. Our results show
that both the local (group/cluster) environments and global (supercluster)
environments influence galaxy morphologies and their star formation activity.Comment: 13 pages, 10 figures, submitted to Astronomy and Astrophysic
The Apparent and Intrinsic Shape of the APM Galaxy Clusters
We estimate the distribution of intrinsic shapes of APM galaxy clusters from
the distribution of their apparent shapes. We measure the projected cluster
ellipticities using two alternative methods. The first method is based on
moments of the discrete galaxy distribution while the second is based on
moments of the smoothed galaxy distribution. We study the performance of both
methods using Monte Carlo cluster simulations covering the range of APM cluster
distances and including a random distribution of background galaxies. We find
that the first method suffers from severe systematic biases, whereas the second
is more reliable. After excluding clusters dominated by substructure and
quantifying the systematic biases in our estimated shape parameters, we recover
a corrected distribution of projected ellipticities. We use the non-parametric
kernel method to estimate the smooth apparent ellipticity distribution, and
numerically invert a set of integral equations to recover the corresponding
distribution of intrinsic ellipticities under the assumption that the clusters
are either oblate or prolate spheroids. The prolate spheroidal model fits the
APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA
Cosmographic Hubble fits to the supernova data
The Hubble relation between distance and redshift is a purely cosmographic
relation that depends only on the symmetries of a FLRW spacetime, but does not
intrinsically make any dynamical assumptions. This suggests that it should be
possible to estimate the parameters defining the Hubble relation without making
any dynamical assumptions. To test this idea, we perform a number of
inter-related cosmographic fits to the legacy05 and gold06 supernova datasets.
Based on this supernova data, the "preponderance of evidence" certainly
suggests an accelerating universe. However we would argue that (unless one uses
additional dynamical and observational information) this conclusion is not
currently supported "beyond reasonable doubt". As part of the analysis we
develop two particularly transparent graphical representations of the
redshift-distance relation -- representations in which acceleration versus
deceleration reduces to the question of whether the relevant graph slopes up or
down. Turning to the details of the cosmographic fits, three issues in
particular concern us: First, the fitted value for the deceleration parameter
changes significantly depending on whether one performs a chi^2 fit to the
luminosity distance, proper motion distance or other suitable distance
surrogate. Second, the fitted value for the deceleration parameter changes
significantly depending on whether one uses the traditional redshift variable
z, or what we shall argue is on theoretical grounds an improved
parameterization y=z/(1+z). Third, the published estimates for systematic
uncertainties are sufficiently large that they certainly impact on, and to a
large extent undermine, the usual purely statistical tests of significance. We
conclude that the supernova data should be treated with some caution.Comment: 28 pages, 4 figure
M31N 2005-09c: a fast FeII nova in the disk of M31
Classical novae are quite frequent in M~31. However, very few spectra of M31
novae have been studied to date, especially during the early decline phase. Our
aim is to study the photometric and spectral evolution of a M31 nova event
close to outburst. We present photometric and spectroscopic observations of
M31N 2005-09c, a classical nova in the disk of M31, using the 1.3m telescope of
the Skinakas Observatory in Crete (Greece), starting on the 28th September,
i.e. about 5 days after outburst, and ending on the 5th October 2005, i.e.
about 12 days after outburst. We also have supplementary photometric
observations from the La Sagra Observatory in Northern Andalucia, Spain, on
September 29 and 30, October 3, 6 and 9 and November 1, 2005. The wavelength
range covered by the spectra is from 3565 A to 8365 A. The spectra are of high
S/N allowing the study of the evolution of the equivalent widths of the Balmer
lines, as well as the identification of non-Balmer lines. The nova displays a
typical early decline spectrum that is characterized by many weak FeII
multiplet emissions. It is classified as a P nova. From the nova light
curve, we have also derived its speed class, t=14+-2.5 days. As the nova
evolved the Balmer lines became stronger and narrower. The early decline of the
expansion velocity of the nova follows a power law in time with an exponent of
\~-0.2
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