19,423 research outputs found
The essential signature of a massive starburst in a distant galaxy
Observations of carbon monoxide (CO) emission in high redshift (z>2) galaxies
indicate the presence of large amounts of molecular gas. Many of these galaxies
contain an active galactic nucleus (AGN) powered by accretion of gas onto a
supermassive black hole, and a key question is whether their extremely high
infrared luminosities result from the AGN, or from bursts of massive star
formation (associated with the molecular gas), or both. In the Milky Way,
high-mass stars form in the dense cores of interstellar molecular clouds; gas
densities are n(H2)>105 cm-3 in the cores. Recent surveys show that virtually
all galactic sites of high-mass star formation have similarly high densities.
The bulk of the cloud material traced by CO observations is at a much lower
density. In galaxies in the local Universe, the HCN(J=1-0) line is an effective
tracer of the high-density molecular gas. Here we report observations of HCN
emission in the early Universe from the infrared luminous 'Cloverleaf' quasar
(at a redshift z=2.5579). The HCN line luminosity indicates the presence of 10
billion solar masses of very dense gas, an essential feature of an immense
starburst that contributes, together with the AGN it harbors, to its high
infrared luminosity.Comment: PDF pape
Homogeneous and heterogeneous chemistry along air parcel trajectories
The study of coupled heterogeneous and homogeneous chemistry due to polar stratospheric clouds (PSC's) using Lagrangian parcel trajectories for interpretation of the Airborne Arctic Stratosphere Experiment (AASE) is discussed. This approach represents an attempt to quantitatively model the physical and chemical perturbation to stratospheric composition due to formation of PSC's using the fullest possible representation of the relevant processes. Further, the meteorological fields from the United Kingdom Meteorological office global model were used to deduce potential vorticity and inferred regions of PSC's as an input to flight planning during AASE
MEVTV Workshop on Nature and Composition of Surface Units on Mars
Topics addressed include: SNC meteorites and their potential for providing information about the geochemical evolution of Mars; remote sensing; photogeological inferences of Martian surface compositions; and interactions of the surface with volatiles in either the surface or the atmosphere
Using handheld pocket computers in a wireless telemedicine system
Objectives: To see if senior emergency nurse practitioners can provide support to
inexperienced ones in a Minor Injuries Unit by using a wireless LAN system of
telemedicine transmitting images to a PDA when they were on duty. In addition,
whether such a system could be sufficiently accurate to make clinical diagnoses with
a high level of diagnostic confidence. This would permit an overall lower grade of
nurse to be employed to manage most of the cases as they arrive with a proportionate
lowering of costs.
Methods: The wireless LAN equipment could roam in the Minor Injuries Unit and
the experienced emergency Nurse practitioners could be at home, shopping or even
at a considerable distance from the centre.
Thirty pictorial images of patients who had been sent to the Review Clinic were
transmitted to a PDA various distances of one to sixteen miles from the centre. Two
senior emergency nurse practitioners viewed the images and opined on the diagnosis,
their degree of confidence in the diagnosis and their opinion of the quality of the
image.
Results: the images of patients were sharp, clear, and of diagnostic quality. The
image quality was only uncertain, as was the level of confidence of the diagnosis if
the patient was very dark skinned.
Conclusions: The wireless LAN system works with a remote PDA in this clinical
situation. However there are question marks over the availability of enough
experienced emergency nurse practitioners to staff a service that provides senior
cover for longer parts of the day and at weekends
Bimetric gravity is cosmologically viable
Bimetric theory describes gravitational interactions in the presence of an
extra spin-2 field. Previous work has suggested that its cosmological solutions
are generically plagued by instabilities. We show that by taking the Planck
mass for the second metric, , to be small, these instabilities can be
pushed back to unobservably early times. In this limit, the theory approaches
general relativity with an effective cosmological constant which is,
remarkably, determined by the spin-2 interaction scale. This provides a
late-time expansion history which is extremely close to CDM, but with
a technically-natural value for the cosmological constant. We find should
be no larger than the electroweak scale in order for cosmological perturbations
to be stable by big-bang nucleosynthesis. We further show that in this limit
the helicity-0 mode is no longer strongly-coupled at low energy scales.Comment: 8+2 pages, 2 tables. Version published in PLB. Minor typo corrections
from v
Instabilities in tensorial nonlocal gravity
We discuss the cosmological implications of nonlocal modifications of general
relativity containing tensorial structures. Assuming the presence of standard
radiation- and matter-dominated eras, we show that, except in very particular
cases, the nonlocal terms contribute a rapidly growing energy density. These
models therefore generically do not have a stable cosmological evolution.Comment: 10 pages, 2 figures. v2: version published in PR
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