1,634 research outputs found
The Lantern Vol. 25, No. 2, Spring 1957
⢠Samarra Train ⢠Caleb: A Story of the Civil War ⢠Pursued ⢠Coup d\u27Etat ⢠Medieval Portraits, Somewhat Diagrammatic ⢠London: 1951, 1952 ⢠Spot: A Tale of the Sea ⢠The Big Mistake ⢠Poetry and College Poetryhttps://digitalcommons.ursinus.edu/lantern/1071/thumbnail.jp
Nuclear gas dynamics in Arp 220 - sub-kiloparsec scale atomic hydrogen disks
We present new, high angular resolution (~0.22") MERLIN observations of
neutral hydrogen (HI) absorption and 21-cm radio continuum emission across the
central ~900 parsecs of the ultraluminous infrared galaxy, Arp220. Spatially
resolved HI absorption is detected against the morphologically complex and
extended 21-cm radio continuum emission, consistent with two counterrotating
disks of neutral hydrogen, with a small bridge of gas connecting the two.
We propose a merger model in which the two nuclei represent the galaxy cores
which have survived the initial encounter and are now in the final stages of
merging, similar to conclusions drawn from previous CO studies (Sakamoto,
Scoville & Yun 1999). However, we suggest that instead of being coplanar with
the main CO disk (in which the eastern nucleus is embedded), the western
nucleus lies above it and, as suggested by bridge of HI connecting the two
nuclei, will soon complete its final merger with the main disk. We suggest that
the collection of radio supernovae (RSN) detected in VLBA studies in the more
compact western nucleus represent the second burst of star formation associated
with this final merger stage and that free-free absorption due to ionised gas
in the bulge-like component can account for the observed RSN distribution.
(Abridged)Comment: 26 pages including 8 figures and 1 table; accepted for publication in
Ap
The Lantern Vol. 26, No. 3, June 1958
⢠Reflections ⢠Recession: A Matter of Opinion ⢠His Name Was... ⢠The Outward Bound ⢠A Champion There Was ⢠Notes from the Sukura ⢠Undauntedhttps://digitalcommons.ursinus.edu/lantern/1075/thumbnail.jp
3D Printed PLA Scaffolds to Promote Healing of Large Bone Defects
One challenge modern medicine faces is the ability to repair large bone defects and stimulate healing. Small defects typically heal naturally, but large bone defects do not and current solutions are to replace the missing tissue with biologically inert materials such as titanium. This limits the amount of bone healing as the defect is not repaired but rather replaced. The focus of our research is to develop a method of using 3D printing to create biodegradable scaffolds which promote bone in-growth and replacement. To accomplish this we used poly lactic acid (PLA) filament and a desktop 3D printer. To promote bone healing and provide mechanical support our team investigated different design methodologies to provide a scaffold of customizable stiffness while allowing cell attachment and in-growth. Our team used CAD modeling to create unique architecture design systems which we analyzed for stiffness using Finite Element Analysis (FEA). We developed a unit cell method of scaffold construction that allowed for customized stiffness of irregular shapes. We 3D printed our designs using a desktop 3D printer and verified our stiffness through mechanical tension and compression testing. We investigated cell viability of the scaffolds by immersing test specimens in culturing media and fibroblast cells. Fibroblast cells are from the same lineage as osteoblast cells but are much faster growing, allowing for more efficient testing. Specimens were left in the media for one week then a total cell count was performed. Scaffold designs were then evaluated based on stiffness and cell viability. We have produced several different viable models with appropriate stiffness for human trabecular bone and good cellular adhesion
Momentum of an electromagnetic wave in dielectric media
Almost a hundred years ago, two different expressions were proposed for the
energy--momentum tensor of an electromagnetic wave in a dielectric. Minkowski's
tensor predicted an increase in the linear momentum of the wave on entering a
dielectric medium, whereas Abraham's tensor predicted its decrease. Theoretical
arguments were advanced in favour of both sides, and experiments proved
incapable of distinguishing between the two. Yet more forms were proposed, each
with their advocates who considered the form that they were proposing to be the
one true tensor. This paper reviews the debate and its eventual conclusion:
that no electromagnetic wave energy--momentum tensor is complete on its own.
When the appropriate accompanying energy--momentum tensor for the material
medium is also considered, experimental predictions of all the various proposed
tensors will always be the same, and the preferred form is therefore
effectively a matter of personal choice.Comment: 23 pages, 3 figures, RevTeX 4. Removed erroneous factor of mu/mu_0
from Eq.(44
The Universe Was Reionized Twice
We show the universe was reionized twice, first at z~15-16 and second at z~6.
Such an outcome appears inevitable, when normalizing to two well determined
observational measurements, namely, the epoch of the final cosmological
reionization at z~6 and the density fluctuations at z~6, which in turn are
tight ly constrained by Lyman alpha forest observations at z~3. These two
observations most importantly fix the product of star formation efficiency and
ionizing photon escape fraction from galaxies at high redshift. To the extent
that the relative star formation efficiencies in gaseous minihalos with H2
cooling and large halos with atomic cooling at high redshift are still unknown,
the primary source for the first reionization could be Pop III stars either in
minihalos or in large halos. We show that gas in minihalos can be cooled
efficiently by H2 molecules and star formation can continue to take place
largely unimpeded throughout the first reionization period, thanks to two new
mechanisms for generating a high X-ray background during the Pop III era, put
forth here. Moreover, an important process for producing a large number of H2
molecules in relic HII regions of Pop III galaxies, first pointed out by
Ricotti, Gnedin, & Shull, is quantified here. It is shown that the Lyman-Werner
background may never build up during the Pop III era. The long cosmological
reionization and reheating history is complex. We discuss a wide range of
implications and possible tests for this new reionization picture. In
particular, Thomson scattering optical depth is increased to 0.10 +- 0.03,
compared to 0.027 for the case of only one rapid reionization at z=6. Upcoming
Microwave Anisotropy Probe observation of the polarization of the cosmic
microwave background should be able to distinguish between these two scenarios.Comment: submitted to ApJ, 69 pages, substantial revision made and conclusions
strengthene
An Action-Based Approach to Presence: Foundations and Methods
This chapter presents an action-based approach to presence. It starts by briefly describing the theoretical and empirical foundations of this approach, formalized into three key notions of place/space, action and mediation. In the light of these notions, some common assumptions about presence are then questioned: assuming a neat distinction between virtual and real environments, taking for granted the contours of the mediated environment and considering presence as a purely personal state. Some possible research topics opened up by adopting action as a unit of analysis are illustrated. Finally, a case study on driving as a form of mediated presence is discussed, to provocatively illustrate the flexibility of this approach as a unified framework for presence in digital and physical environment
High redshift quasars and the supermassive black hole mass budget: constraints on quasar formation models
We investigate the constraints on models of supermassive black hole (SMBH)
and quasar formation obtainable from two recent observational developments: the
discovery of luminous quasars at z~6, and estimates of the local mass density
of SMBHs. If ~90 per cent of this mass was accreted at redshifts z<3, as
suggested by the observed quasar luminosity functions, these joint constraints
pose a challenge for models, which must account for the observed luminous
quasar population at z~6 within a very limited `mass budget'. We investigate a
class of models based within the hierarchical structure formation scenario, in
which major mergers lead to black hole formation and fuelling, and the
resulting quasars shine at their Eddington-limited rate until their fuel is
exhausted. We show that the simplest such model, in which a constant fraction
of the gas within the halo is accreted in each major merger, cannot satisfy
both constraints simultaneously. When this model is normalized to reproduce the
number density of luminous quasars at z~6, the mass budget is grossly exceeded
due to an overabundance of lower mass SMBHs. We explore a range of
modifications to the simple model designed to overcome this problem. We show
that both constraints can be satisfied if the gas accretion fraction scales as
a function of the halo virial velocity. Similar scalings have been proposed in
order to reproduce the local M-sigma relation. Successful models can also be
constructed by restricting the formation of seed black holes to redshifts above
z~11.5 or to haloes above a velocity threshold ~55 km/s, or assuming that only
a fraction of major mergers result in formation of a seed SMBH. (abridged)Comment: 19 pages, 6 figures, 1 table. v2: Corrected references. v3: Extended
Section 5.1, corrected Section 3.2, various other corrections and additions
suggested by referee. Accepted by MNRAS in this for
A relational, indirect, meso-level approach to CSCL design in the next decade
This paper reviews some foundational issues that we believe will affect the progress of CSCL over the next ten years. In particular, we examine the terms technology, affordance, and infrastructure and we propose a relational approach to their use in CSCL. Following a consideration of networks, space, and trust as conditions of productive learning, we propose an indirect approach to design in CSCL. The work supporting this theoretical paper is based on the outcomes of two European networks: E-QUEL, a network investigating e-quality in e-learning; and Kaleidoscope, a European Union Framework 6 Network of Excellence. In arguing for a relational understanding of affordance, infrastructure, and technology we also argue for a focus on what we describe as meso-level activity. Overall this paper does not aim to be comprehensive or summative in its review of the state of the art in CSCL, but rather to provide a view of the issues currently facing CSCL from a European perspective
The effect of bars on the M*- e relation: offset, scatter and residuals correlations
We analyse a set of collisionless disc galaxy simulations to study the consequences of bar formation and evolution on the Mâ˘-Ďe relation of supermassive black holes (SMBHs). The redistribution of angular momentum driven by bars leads to a mass increase within the central region, raising the velocity dispersion of the bulge, Ďe, on average by Ë12 per cent and as much as Ë20 per cent. If a disc galaxy with an SMBH satisfying the Mâ˘-Ďe relation forms a bar, and the SMBH does not grow in the process, then the increase in Ďe moves the galaxy off the Mâ˘-Ďe relation. We explore various effects that can affect this result including contamination from the disc and anisotropy. The displacement from the Mâ˘-Ďe relation for individual model barred galaxies correlates with both the bulge-to-total stellar mass ratio, M(B)/M(B + D), and the 2D anisotropy, βĎ(B + D), both measured within the effective radius of the bulge. Overall, this process leads to an Mâ˘-Ďe for barred galaxies offset from that of unbarred galaxies, as well as an increase in its scatter. We assemble samples of observed unbarred and barred galaxies with classical bulges and find tentative hints of an offset between the two consistent with the predicted. Including all barred galaxies, rather than just those with a classical bulge, leads to a significantly larger offset, which is mostly driven by the significantly larger offset of pseudo bulge
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