8,371 research outputs found
Supersymmetry and Phenomenology of Heterotic and Type I Superstring Models
This thesis is the discussion of heterotic and type I string phenomenology.
The heterotic string model is based on the free--fermionic formalism. This is
the first case where non--Abelian VEV's, as opposed to singlet VEV's are
required for the cancellation of the Fayet--Iliopoulos term. It is noted that
non--Abelian fields are the only fields that can give rise to the satisfaction
of the D--flat constraints in this model.
The type I models are based on T^6/Z_2x(Z_2)^s and T^6/Z_2xZ_2x(Z_2)^s
compactifications. The first example has N=2 supersymmetry and includes a rank
reduction of the D5 gauge groups as a result of using a freely acting Kaluza
Klein shift Z_2^s. The second case is an N=1 model. One has a choice of sign
epsilon=+/-1 from terms not related to the principle orbits by S and T
transformations. This allows the breaking of supersymmetry with the
introduction of antibranes. For epsilon=-1 there is a problem with respect to
particle interpretation.
I magnetize the T^6/Z_2xZ_2 model for the epsilon=-1 case. This leads to
tadpole complications for the g and f twisted sectors, but allows the h twisted
sector to behave normally
Starburst and AGN activity in ultraluminous infrared galaxies
(Abridged) We examine the power source of 41 local Ultraluminous Infrared
Galaxies using archival infrared and optical photometry. We fit the observed
Spectral Energy Distributions (SEDs) with starburst and AGN components; each
component being drawn from a family of templates. We find all of the sample
require a starburst, whereas only half require an AGN. In 90% of the sample the
starburst provides over half the IR emission, with a mean fractional luminosity
of 82%. When combined with other galaxy samples we find that starburst and AGN
luminosities correlate over 6 decades in IR luminosity suggesting that a common
factor governs both luminosities, plausibly the gas masses in the nuclear
regions. We find that the mid-IR 7.7 micron line-continuum ratio is no
indication of the starburst luminosity, or the fractional AGN luminosity, and
therefore that this ratio is not a reliable diagnostic of the power source in
ULIRGs. We propose that the scatter in the radio-IR correlation in ULIRGs is
due to a skewed starburst IMF and/or relic relativistic electrons from a
previous starburst, rather than contamination from an obscured AGN. We show
that most ULIRGs undergo multiple starbursts during their lifetime, and by
inference that mergers between more than two galaxies may be common amongst
ULIRGs. Our results support the evolutionary model for ULIRGs proposed by
Farrah et al 2001, where they can follow many different evolutionary paths of
starburst and AGN activity in transforming merging spiral galaxies into
elliptical galaxies, but that most do not go through an optical QSO phase. The
lower level of AGN activity in our local sample than in z~1 HLIRGs implies that
the two samples are distinct populations. We postulate that different galaxy
formation processes at high-z are responsible for this difference.Comment: 24 pages, 8 figures. Accepted for publication in MNRA
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
How to Complete an Interactive Configuration Process?
When configuring customizable software, it is useful to provide interactive
tool-support that ensures that the configuration does not breach given
constraints.
But, when is a configuration complete and how can the tool help the user to
complete it?
We formalize this problem and relate it to concepts from non-monotonic
reasoning well researched in Artificial Intelligence. The results are
interesting for both practitioners and theoreticians. Practitioners will find a
technique facilitating an interactive configuration process and experiments
supporting feasibility of the approach. Theoreticians will find links between
well-known formal concepts and a concrete practical application.Comment: to appear in SOFSEM 201
Ground-based detection of a cloud of methanol from Enceladus: When is a biomarker not a biomarker?
Saturn's moon Enceladus has vents emerging from a sub-surface ocean, offering
unique probes into the liquid environment. These vents drain into the larger
neutral torus in orbit around Saturn. We present a methanol (CH3OH) detection
observed with IRAM 30-m from 2008 along the line-of-sight through Saturn's
E-ring. Additionally, we also present supporting observations from the Herschel
public archive of water (ortho-H2O; 1669.9 GHz) from 2012 at a similar
elongation and line-of-sight. The CH3OH 5(1,1)-4(1,1) transition was detected
at 5.9 sigma confidence. The line has 0.43 km/s width and is offset by +8.1
km/s in the moon's reference frame. Radiative transfer models allow for gas
cloud dimensions from 1750 km up to the telescope beam diameter ~73000 km.
Taking into account the CH3OH lifetime against solar photodissociation and the
redshifted line velocity, there are two possible explanations for the CH3OH
emission: methanol is primarily a secondary product of chemical interactions
within the neutral torus that (1) spreads outward throughout the E-ring or (2)
originates from a compact, confined gas cloud lagging Enceladus by several
km/s. We find either scenario to be consistent with significant redshifted H2O
emission (4 sigma) measured from the Herschel public archive. The measured
CH3OH:H2O abundance (> 0.5 per cent) significantly exceeds the observed
abundance in the direct vicinity of the vents (~0.01 per cent), suggesting
CH3OH is likely chemically processed within the gas cloud with methane (CH4) as
its parent species.Comment: 16 pages, 4 figures, accepted for publication in the International
Journal of Astrobiology (IJA
Optical Imaging of Very Luminous Infrared Galaxy Systems: Photometric Properties and Late Evolution
A sample of 19 low redshift (0.03z0.07) very luminous infrared galaxy
(VLIRG: L[8-1000 m] ) systems (30
galaxies) has been imaged in , , and . These objects cover a
luminosity range that is key to linking the most luminous infrared galaxies
with the population of galaxies at large. We have obtained photometry for all
of these VLIRG systems, the individual galaxies (when detached), and their
nuclei, and the relative behavior of these classes has been studied in optical
color-magnitude diagrams. The photometric properties of the sample are also
compared with previously studied samples of ULIRGs. The mean observed
photometric properties of VLIRG and ULIRG samples, considered as a whole, are
indistinguishable at optical wavelengths. This suggests that not only ULIRG,
but also the more numerous population of VLIRGs, have similar rest-frame
optical photometric properties as the submillimeter galaxies (SMG), reinforcing
the connection between low-{\it z} LIRGs -- high-{\it z} SMGs. When the nuclei
of the {\it young} and {\it old} interacting systems are considered separately,
some differences between the VLIRG and the ULIRG samples are found. In
particular, the old VLIRGs are less luminous and redder than old ULIRG systems.
If confirmed with larger samples, this behavior suggests that the late-stage
evolution is different for VLIRGs and ULIRGs. Specifically, as suggested from
spectroscopic data, the present photometric observations support the idea that
the activity during the late phases of VLIRG evolution is dominated by
starbursts, while a higher proportion of ULIRGs could evolve into a QSO type of
object.Comment: 27 pages, 5 figures (degraded to reduce space). Figures 1 and 2 are
multiple page figures (i.e. Fig 1a,b and Fig2a-g
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
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