7,463 research outputs found
Candidate High Redshift and Primeval Galaxies in Hubble Deep Field South
We present the results of colour selection of candidate high redshift
galaxies in Hubble Deep Field South (HDF-S) using the Lyman dropout scheme. The
HDF-S data we discuss were taken in a number of different filters extending
from the near--UV (F300W) to the infrared (F222M) in two different fields. This
allows us to select candidates with redshifts from z~3 to z~12. We find 15
candidate z~3 objects (F300W dropouts), 1 candidate z~4 object (F450W dropout)
and 16 candidate z5 objects (F606W dropouts) in the ~ 4.7 arcmin^2 WFPC-2
field, 4 candidate z~6 (optical dropouts) and 1 candidate z~8 (F110W dropout)
in the 0.84 arcmin^2 NICMOS-3 field. No F160W dropouts are found (z~12). We
compare our selection technique with existing data for HDF-North and discuss
alternative interpretations of the objects. We conclude that there are a number
of lower redshift interlopers in the selections, including one previously
identified object (Treu et al. 1998), and reject those objects most likely to
be foreground contaminants. Even after this we conclude that the F606W dropout
list is likely to still contain substantial foreground contamination. The lack
of candidate very high redshift UV-luminous galaxies supports earlier
conclusions by Lanzetta et al. (1998). We discuss the morphologies and
luminosity functions of the high redshift objects, and their cosmological
implications.Comment: Accepted for publication in MNRA
Chandra Observations of Arp 220: The Nuclear Source
We present the first results from 60ks of observations of Arp 220 using the
ACIS-S instrument on Chandra. We report the detection of several sources near
the galaxy's nucleus, including a point source with a hard spectrum that is
coincident with the western radio nucleus B. This point source is mildly
absorbed (N_H ~ 3 x 10^22 cm^-2) and has an estimated luminosity of 4 x 10^40
erg/s. In addition, a fainter source may coincide with the eastern nucleus A.
Extended hard X-ray emission in the vicinity raises the total estimated nuclear
2-10 keV X-ray luminosity to 1.2 x 10^41 erg/s, but we cannot rule out a hidden
AGN behind columns exceeding 5 x 10^24 cm^-2. We also detect a peak of soft
X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from
the nucleus with a luminosity of 6 x 10^39 erg/s.Comment: Accepted for publication in Ap
Chandra Observations of Extended X-ray Emission in Arp 220
We resolve the extended X-ray emission from the prototypical ultraluminous
infrared galaxy Arp 220. Extended, faint edge-brightened, soft X-ray lobes
outside the optical galaxy are observed to a distance of 10 to 15 kpc on each
side of the nuclear region. Bright plumes inside the optical isophotes coincide
with the optical line emission and extend 11 kpc from end to end across the
nucleus. The data for the plumes cannot be fit by a single temperature plasma,
and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from
bright, diffuse circumnuclear emission in the inner 3 kpc centered on the
Halpha peak, which is displaced from the radio nuclei. There is a close
morphological correspondence between the Halpha and soft X-ray emission on all
spatial scales. We interpret the plumes as a starburst-driven superwind, and
discuss two interpretations of the emission from the lobes in the context of
simulations of the merger dynamics of Arp 220.Comment: Accepted for publication in ApJ; see also astro-ph/0208477 (Paper 1
Chandra Observations of Arp 220: The Nuclear Source
We present the first results from 60ks of observations of Arp 220 using the
ACIS-S instrument on Chandra. We report the detection of several sources near
the galaxy's nucleus, including a point source with a hard spectrum that is
coincident with the western radio nucleus B. This point source is mildly
absorbed (N_H ~ 3 x 10^22 cm^-2) and has an estimated luminosity of 4 x 10^40
erg/s. In addition, a fainter source may coincide with the eastern nucleus A.
Extended hard X-ray emission in the vicinity raises the total estimated nuclear
2-10 keV X-ray luminosity to 1.2 x 10^41 erg/s, but we cannot rule out a hidden
AGN behind columns exceeding 5 x 10^24 cm^-2. We also detect a peak of soft
X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from
the nucleus with a luminosity of 6 x 10^39 erg/s.Comment: Accepted for publication in Ap
Excitations in confined helium
We design models for helium in matrices like aerogel, Vycor or Geltech from a
manifestly microscopic point of view. For that purpose, we calculate the
dynamic structure function of 4He on Si substrates and between two Si walls as
a function of energy, momentum transfer, and the scattering angle. The
angle--averaged results are in good agreement with the neutron scattering data;
the remaining differences can be attributed to the simplified model used here
for the complex pore structure of the materials. A focus of the present work is
the detailed identification of coexisting layer modes and bulk--like
excitations, and, in the case of thick films, ripplon excitations. Involving
essentially two--dimensional motion of atoms, the layer modes are sensitive to
the scattering angle.Comment: Phys. Rev. B (2003, in press
Simulation study of magnetic holes at the Earth's collisionless bow shock
Recent observations by the Cluster and Double Star spacecraft at the Earth's bow shock have revealed localized magnetic field and density holes in the solar wind plasma. These structures are characterized by a local depletion of the magnetic field and the plasma density, and by a strong increase of the plasma temperature inside the magnetic and density cavities. Our objective here is to report results of a hybrid-Vlasov simulations of ion-Larmor-radius sized plasma density cavities with parameters that are representative of the high-beta solar wind plasma at the Earth's bow shock. We observe the asymmetric self-steepening and shock-formation of the cavity, and a strong localized temperature increase (by a factor of 5–7) of the plasma due to reflections and shock surfing of the ions against the collisionless shock. Temperature maxima are correlated with density minima, in agreement with Cluster observations. For oblique incidence of the solar wind, we observe efficient acceleration of ions along the magnetic field lines by the shock drift acceleration process
Submillimeter Observations of the Ultraluminous BAL Quasar APM 08279+5255
With an inferred bolometric luminosity of 5\times10^{15}{\rm \lsun}, the
recently identified z=3.87, broad absorption line quasar APM 08279+5255 is
apparently the most luminous object currently known. As half of its prodigious
emission occurs in the infrared, APM 08279+5255 also represents the most
extreme example of an Ultraluminous Infrared Galaxy. Here, we present new
submillimeter observations of this phenomenal object; while indicating that a
vast quantity of dust is present, these data prove to be incompatible with
current models of emission mechanisms and reprocessing in ultraluminous
systems. The influence of gravitational lensing upon these models is considered
and we find that while the emission from the central continuum emitting region
may be significantly enhanced, lensing induced magnification cannot easily
reconcile the models with observations. We conclude that further modeling,
including the effects of any differential magnification is required to explain
the observed emission from APM 08279+5255.Comment: 12 Pages with Two figures. Accepted for publication in the
Astrophysical Journal Letter
Modeling the growth of multicellular cancer spheroids in a\ud bioengineered 3D microenvironment and their treatment with an\ud anti-cancer drug
A critical step in the dissemination of ovarian cancer cells is the formation of multicellular spheroids from cells shed from the primary tumor. The objectives of this study were to establish and validate bioengineered three-dimensional (3D) microenvironments for culturing ovarian cancer cells in vitro and simultaneously to develop computational models describing the growth of multicellular spheroids in these bioengineered matrices. Cancer cells derived from human epithelial ovarian carcinoma were embedded within biomimetic hydrogels of varying stiffness and cultured for up to 4 weeks. Immunohistochemistry was used to quantify the dependence of cell proliferation and apoptosis on matrix stiffness, long-term culture and treatment with the anti-cancer drug paclitaxel.\ud
\ud
Two computational models were developed. In the first model, each spheroid was treated as an incompressible porous medium, whereas in the second model the concept of morphoelasticity was used to incorporate details about internal stresses and strains. Each model was formulated as a free boundary problem. Functional forms for cell proliferation and apoptosis motivated by the experimental work were applied and the predictions of both models compared with the output from the experiments. Both models simulated how the growth of cancer spheroids was influenced by mechanical and biochemical stimuli including matrix stiffness, culture time and treatment with paclitaxel. Our mathematical models provide new perspectives on previous experimental results and have informed the design of new 3D studies of multicellular cancer spheroids
Mitigation of Electrostatic Hazards on Spacecraft
Spacecraft are complex systems operating in challenging environments that require customized testing procedures designed to mitigate the unique hazards of a space launch environment. As an example of those testing procedures, we describe the test methodology and recommendations developed to mitigate electrostatic discharges that could have triggered an explosion of the Space Shuttle during launch, return to launch site, abort after one orbit, or during normal landing
Growth of confined cancer spheroids: a combined experimental and mathematical modelling approach
We have integrated a bioengineered three-dimensional platform by generating multicellular cancer spheroids in a controlled microenvironment with a mathematical model to investigate\ud
confined tumour growth and to model its impact on cellular processes
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