6,333 research outputs found
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
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
Speedup of Optical Scanner Characterization Subsystem
Small image deformations such as those introduced by optical scanners significantly reduce the accuracy rate of optical character recognition (OCR) software. Characterization of the scanner used in the OCR process may diminish the impact on recognition rates. Theoretical methods have been developed to characterize a scanner based on the bi-level image resulting from scanning a high contrast document. Two bottlenecks in the naïve implementation of these algorithms have been identified, and techniques were developed to improve the execution time of the software. The algorithms are described and analyzed. Since approximations are used in one of the techniques, the error of the approximations is examined
Block shear capacity of bolted connections in cold-reduced steel sheets
This paper examines the mechanisms for block shear failures of bolted connections in steel plates postulated in the design equations specified in the North American, European and Australian steel structures codes. It explains that there is only one feasible mechanism for the limit state of conventional block shear failure, that which involves tensile rupture and shear yielding, irrespective of the steel material ductility. It describes the fundamental shortcomings of various code equations for determining the block shear capacity of a bolted connection. Based on the tensile rupture and shear yielding mechanism, an in-plane shear lag factor, and the active shear resistance planes identified in the present work, this paper proposes a rational equation that is demonstrated to provide more accurate results compared to all the code equations in predicting the block shear capacities of bolted connections in G450 steel sheets subjected to concentric loading. The resistance factor of 0.8 for the proposed equation is computed with respect to the LRFD approach given in the North American specification for the design of cold-formed steel structures
Active Shear Planes in Block Shear Failure of Bolted Connections
In the AISI Specification for the Design of Cold-formed Steel Structural Members 2007, there are two types of shear planes used to determine the resistance of a bolted connection to block shear failure. When the block shear failure occurs by shear yielding and tensile rupture, the shear failure plane is taken to be the gross shear plane. Conversely, when the block shear failure is deemed to occur by simultaneous shear and tensile ruptures, the shear failure plane is assumed to be the net shear plane. Such an approach is not logical since the shear failure planes should be unique irrespective of the block shear failure mechanism. Through finite element analysis presented in this paper, the shear failure plane is shown to be neither the gross nor the net shear plane, and to be midway between the two shear planes assumed in design specifications. This shear failure plane is termed the active shear plane. The veracity of the active shear area is demonstrated in terms of the ability of the resulting block shear equation to predict the governing failure modes of test specimens consistently, in comparison against the equations assuming the gross and the net shear areas
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
Evolution of the far-infrared luminosity functions in the Spitzer Wide-area Infrared Extragalactic Legacy Survey
We present new observational determination of the evolution of the rest-frame
70 and 160 micron and total infrared (TIR) galaxy luminosity functions (LFs)
using 70 micron data from the Spitzer Wide-area Infrared Extragalactic Legacy
Survey (SWIRE). The LFs were constructed for sources with spectroscopic
redshifts only in the XMM-LSS and Lockman Hole fields from the SWIRE
photometric redshift catalogue. The 70 micron and TIR LFs were constructed in
the redshift range 0<z<1.2 and the 160 micron LF was constructed in the
redshift range 0<z<0.5 using a parametric Bayesian and the vmax methods. We
assume in our models, that the faint-end power-law index of the LF does not
evolve with redshifts. We find the the double power-law model is a better
representation of the IR LF than the more commonly used power-law and Gaussian
model. We model the evolution of the FIR LFs as a function of redshift where
where the characteristic luminosity, evolve as
\propto(1+z)^{\alpha_\textsc{l}}. The rest-frame 70 micron LF shows a strong
luminosity evolution out to z=1.2 with alpha_l=3.41^{+0.18}_{-0.25}. The
rest-frame 160 micron LF also showed rapid luminosity evolution with
alpha_l=5.53^{+0.28}_{-0.23} out to z=0.5. The rate of evolution in luminosity
is consistent with values estimated from previous studies using data from IRAS,
ISO and Spitzer. The TIR LF evolves in luminosity with
alpha_l=3.82^{+0.28}_{-0.16} which is in agreement with previous results from
Spitzer 24 micron which find strong luminosity evolution. By integrating the LF
we calculated the co-moving IR luminosity density out to z=1.2, which confirm
the rapid evolution in number density of LIRGs and ULIRGs which contribute
~68^{+10}_{-07} % to the co-moving star formation rate density at z=1.2. Our
results based on 70 micron data confirms that the bulk of the star formation at
z=1 takes place in dust obscured objects.Comment: 17 pages, 14 figure
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
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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