13,621 research outputs found
Modeling the Dust Spectral Energy Distributions of Dwarf Galaxies
Recent efforts on the modeling of the infrared spectral energy distributions
(SEDs) of dwarf galaxies are summarised here. The characterisation of the dust
properties in these low metallicity environments is just unfolding, as a result
of recently available mid-infrared to millimetre observations. From the limited
cases we know to date, it appears that the hard radiation fields that are
present in these star-bursting dwarf galaxies, as well as the rampent
energetics of supernovae shocks and winds have modified the dust properties, in
comparison with those in the Galaxy, or other gas and dust rich galaxies. The
sophistication of the SED models is limited by the availability of detailed
data in the mid infrared and particularly in the submillimetre to millimetre
regime, which will open up in the near future with space-based missions, such
as Herschel.Comment: 8 pages presented at "The Spectral Energy Distribution of Gas-Rich
Galaxies: Confronting Models with Data" Heidelberg (Germany), October 2004.
To be published in The Spectral Energy Distribution of Gas-Rich Galaxies, ed.
C. Popescu & R. Tuffs (Melville:AIP) in pres
Interstellar Gas in Low Mass Virgo Cluster Spiral Galaxies
We have measured the strengths of the [C II] 158 micron, [N II] 122 micron,
and CO (1 - 0) lines from five low blue luminosity spiral galaxies in the Virgo
Cluster, using the Infrared Space Observatory and the NRAO 12m millimeter
telescope. Two of the five galaxies have high L([C II)]/L(CO) and L(FIR)/L(CO)
ratios compared to higher mass spirals. These two galaxies, NGC 4294 and NGC
4299, have L([C II])/L(CO) ratios of >14,300 and 15,600, respectively, which
are similar to values found in dwarf irregular galaxies. This is the first time
that such enhanced L([C II])/L(CO) ratios have been found in spiral galaxies.
This result may be due to low abundances of dust and heavy elements, which can
cause the CO (1 - 0) measurements to underestimate the molecular gas content.
Another possibility is that radiation from diffuse HI clouds may dominate the
[C II] emission from these galaxies. Less than a third of the observed [C II]
emission arises from HII regions.Comment: 24 pages, Latex, 2 Figures, 6 Tables To appear in the Astronomical
Journal, July 199
Formation of offshore tidal sandbanks triggered by a gasmined bed subsidence
Offshore gasmining is an example of a human intervention with a morphological impact. On land, it is usually attended with a dish-like bed depression. We show that, if located at sea, such a bed depression can become morphodynamically active by triggering mechanisms related to tidal sand bank formation. To that end, a simple morphological model is considered which describes an erodible bed subject to a tidal wave in a shallow sea. The continuous subsidence is modelled by a sink term in the sediment balance. Then, a linear approximation is carried out to describe the bed evolution after the onset of subsidence. The results, presented in physical space, show that the subsidence triggers the formation of a sand bank pattern that gradually spreads around the centre of subsidence, at a rate that may go up to 160 m yearÂż1, depending on the tidal transport rate and the tidal eccentricity. The dimension of the depression does not affect the spreading rate nor the orientation of the sand banks, but it does influence their spacing. The main conclusion is that the horizontal extent of the area influenced by the bed depression by far exceeds that of the direct subsidence, thus showing that bed depressions on land and at sea indeed behave in fundamentally different ways. The results suggest that nonlinear effects are worthwhile to be investigated in order to describe finite amplitude development of sand banks as well as the interaction between subsidence and bed forms
Constraints on crustal rheology and age of deformation from models of gravitational spreading in Ishtar Terra, Venus
Gravitational spreading is expected to lead to rapid relaxation of high relief due to the high surface temperature and associated weak crust on Venus. In this study, we use new Magellan radar and altimetry data to determine the extent of gravitational relaxation in Ishtar Terra, which contains the highest relief on Venus as well as areas of extremely high topographic slope. Within Ishtar Terra the only mountain belts found on Venus, Akna, Danu, Freyja, and Maxwell Montes, nearly encircle the smooth, high (3-4 km) plateau of Lakshmi Planum. Finite-element models of this process give expected timescales for relaxation of relief and failure at the surface. From these modeling results we attempt to constrain the strength of the crust and timescales of deformation in Ishtar Terra. Below we discuss observational evidence for gravitational spreading in Ishtar Terra, results from the finite-element modeling, independent age constraints, and implications for the rheology and timing of deformation
Gravitational spreading of Danu, Freyja and Maxwell Montes, Venus
The potential energy of elevated terrain tends to drive the collapse of the topography. This process of gravitational spreading is likely to be more important on Venus than on Earth because the higher surface temperature weakens the crust. The highest topography on Venus is Ishtar Terra. The high plateau of Lakshmi Planum has an average elevation of 3 km above mean planetary radius, and is surrounded by mountain belts. Freyja, Danu, and Maxwell Montes rise, on average, an additional 3, 0.5, and 5 km above the plateau, respectively. Recent high resolution Magellan radar images of this area, east of approx. 330 deg E, reveal widespread evidence for gravity spreading. Some observational evidence is described for gravity spreading and the implications are discussed in terms of simple mechanical models. Several simple models predict that gravity spreading should be an important process on Venus. One difficulty in using remote observations to infer interior properties is that the observed features may not have formed in response to stresses which are still active. Several causes of surface topography are briefly examined
A comparison of refined models for flexible subassemblies
Interactions between structure response and control of large flexible space systems have challenged current modeling techniques and have prompted development of new techniques for model improvement. Due to the geometric complexity of envisioned large flexible space structures, finite element models (FEM's) will be used to predict the dynamic characteristics of structural components. It is widely accepted that these models must be experimentally 'validated' before their acceptance as the basis for final design analysis. However, predictions of modal properties (natural frequencies, mode shapes, and damping ratios) are often in error when compared to those obtained from Experimental Modal Analysis (EMA). Recent research efforts have resulted in the development of algorithmic approaches for model improvement, also referred to as system or structure identification
Competition between Pressure and Gravity Confinement in Lyman-Alpha Forest Observations
A break in the distribution function of Ly clouds (at a typical
redshift of ) has been reported by Petitjean et al. (1993). This feature
is what would be expected from a transition between pressure confinement and
gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The
column density at which the feature occurs has been used to determine the
external confining pressure, , which could be
due to a hot, intergalactic medium. For models that provide a good fit to the
data, the contribution of the gas in clouds to is small. The specific
shape of the distribution function at the transition (predicted by models to
have a non-monotonic slope) can serve as a diagnostic of the distribution of
dark matter around Ly forest clouds, and the present data already
eliminate certain models.Comment: 10 pages plain TeX, 2 figures available upon request, submitted to
ApJ Letters, PSU-jc-
The export of national varieties of capitalism: the cases of Wal-Mart and Ikea
Using the cases of Wal-Mart and IKEA, this paper takes a productive systems approach to examine âvarieties of capitalismâ from the perspective of the ways by which production and market relations are structured and prioritised. It considers the nature of these relations and their interaction within the domestic economy and the ways that firms and national systems interact with each other
in the global economy. It examines the processes by which trading standards are transported via supply chain relationships, which ultimately become embedded in products and recognized by consumers at various stages. In this
analysis, the cases of Wal-Mart and IKEA provide insight into the ways by which national systems extend themselves globally, their contrasting effects on the business environments in host localities, and the impact of the resulting supply chain relations on organizational performance
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