4,691 research outputs found
Shocks, cooling and the origin of star formation rates in spiral galaxies
Understanding star formation is problematic as it originates in the large
scale dynamics of a galaxy but occurs on the small scale of an individual star
forming event. This paper presents the first numerical simulations to resolve
the star formation process on sub-parsec scales, whilst also following the
dynamics of the interstellar medium (ISM) on galactic scales. In these models,
the warm low density ISM gas flows into the spiral arms where orbit crowding
produces the shock formation of dense clouds, held together temporarily by
their external pressure. Cooling allows the gas to be compressed to
sufficiently high densities that local regions collapse under their own gravity
and form stars. The star formation rates follow a Schmidt-Kennicutt
\Sigma_{SFR} ~ \Sigma_{gas}^{1.4} type relation with the local surface density
of gas while following a linear relation with the cold and dense gas. Cooling
is the primary driver of star formation and the star formation rates as it
determines the amount of cold gas available for gravitational collapse. The
star formation rates found in the simulations are offset to higher values
relative to the extragalactic values, implying a constant reduction, such as
from feedback or magnetic fields, is likely to be required. Intriguingly, it
appears that a spiral or other convergent shock and the accompanying thermal
instability can explain how star formation is triggered, generate the physical
conditions of molecular clouds and explain why star formation rates are tightly
correlated to the gas properties of galaxies.Comment: 13 pages, 12 figures. MNRAS in pres
The role of course development and design in an itinerant schooling program: the perceptions of staff members of the School of Distance Education in Brisbane, Queensland
This paper examines the perceptions of teachers associated with the Brisbane School of Distance Education (Queensland, Australia), concerning their role in the establishment and implementation of a primary education program for children of the Showmen's Guild of Australasia. Interviews with five itinerant teachers revealed that their responsibilities include assessing correspondence papers from students and maintaining telephone contact with students, home tutors, and parents, as well as working in selected towns on a short-term basis to teach 'face-to-face' lessons to itinerant students. Each teacher worked with between 15 and 20 children, usually in family groups across grade levels. Teachers expressed concerns about the show children's lifestyle and how this has affected their educational and social development. However, all teachers felt that the distance education program had improved the children's educational opportunities and adequately addressed their educational needs. Disadvantages of the children's itinerant lifestyle that the program was unable to address were lack of routine, lack of continuity, dependence on the support of the home tutor, role conflicts of local teachers, and insufficient program funding. Implications for other itinerant education projects include recognizing the importance of teacher attitudes when implementing an educational program for a marginalized group. Contains 20 references. (LP
On the Determination of Star Formation Rates in Evolving Galaxy Populations
The redshift dependence of the luminosity density in certain wavebands (e.g.
UV and H-alpha) can be used to infer the history of star formation in the
populations of galaxies producing this luminosity. This history is a useful
datum in studies of galaxy evolution. It is therefore important to understand
the errors that attend the inference of star formation rate densities from
luminosity densities. This paper explores the self-consistency of star
formation rate diagnostics by reproducing commonly used observational
procedures in a model with known galaxy populations, evolutionary histories and
spectral emission properties. The study reveals a number of potential sources
of error in the diagnostic processes arising from the differential evolution of
different galaxy types. We argue that multi-wavelength observations can help to
reduce these errors.Comment: 13 pages (including 5 encapsulated postscript figures), aastex,
accepted for publication in Ap
Adult T-cell leukemia/lymphoma—pathobiology and implications for modern clinical management
Adult T-cell leukemia/lymphoma (ATL) is a highly aggressive malignancy that arises in 2-5% of carriers of human T-cell lymphotropic virus type 1 (HTLV-1). The median overall survival of acute and lymphoma subtypes remains approximately 9–13 months and depressingly, with chemotherapy based approaches survival is largely unchanged in the ~40 years since it was first described. There is a clear and urgent need to conduct clinical trials of novel therapies in this disease. A high proviral load (PVL) (>4%, percentage of HTLV-1 infected mononuclear cells), male gender and smoking were previously the only major known risk factors for developing ATL, and so it has been difficult to advise patients about their individual risk of future ATL. Here, we describe the recent evidence that malignant disease does not occur randomly amongst all asymptomatic carriers but is more likely to arise in a subset of high PVL individuals with abnormally abundant clonal expansions of circulating HTLV-1 infected T-cells which typically express CD3dim+ CD4+ CD5-CD7- CD25+ CCR4+ with monoclonal TCRVβ. These clones also typically harbour known ATL driver mutations such as PLCG1, PRKCB, CARD11, STAT3, VAV1, NOTCH1, IRF4, CCR4, CCR7, TP53 and CDKN2, and may be detectable 10 years prior to disease presentation providing an opportunity to identify at risk individuals prior to clinical ATL. We describe the current classification and clinical features of ATL, and the exciting work of the last few years that underpins our new understanding of the genetic and epigenetic landscape with implications for future therapy. Whilst current therapy for aggressive ATL remain largely ineffective, recent advances may allow for early identification of at-risk individuals, and for pre-emptive therapies, and hope for a new era of effective targeted biological agents
The star-formation history of the universe - an infrared perspective
A simple and versatile parameterized approach to the star formation history
allows a quantitative investigation of the constraints from far infrared and
submillimetre counts and background intensity measurements.
The models include four spectral components: infrared cirrus (emission from
interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN
dust torus. The 60 m luminosity function is determined for each chosen
rate of evolution using the PSCz redshift data for 15000 galaxies. The
proportions of each spectral type as a function of 60 m luminosity are
chosen for consistency with IRAS and SCUBA colour-luminosity relations, and
with the fraction of AGN as a function of luminosity found in 12 m
samples. The luminosity function for each component at any wavelength can then
be calculated from the assumed spectral energy distributions. With assumptions
about the optical seds corresponding to each component and, for the AGN
component, the optical and near infrared counts can be accurately modelled.
A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850
m can be found with pure luminosity evolution in all 3 cosmological models
investigated: = 1, = 0.3 ( = 0), and
= 0.3, = 0.7.
All 3 models also give an acceptable fit to the integrated background
spectrum. Selected predictions of the models, for example redshift
distributions for each component at selected wavelengths and fluxes, are shown.
The total mass-density of stars generated is consistent with that observed,
in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details
of models can be found at http://astro.ic.ac.uk/~mrr/countmodel
Mapping of hydrothermal alternation zones and regional rock types using computer enhanced ERTS MSS images
A combination of digital computer processing and color compositing of ERTS MSS images has been used to map hydrothermal alternation zones and regional rock types in south-central Nevada. The technique is based on enhancement of subtle visible and near infrared reflectivity differences between mineralogically dissimilar rocks, especially unaltered and altered rocks. MSS spectral bands are ratioed, pixel by pixel, in the computer and subsequently stretched. These ratio values are used to produce a new black and white image which shows the subtle spectral reflectivity differences. Additional enhancement is achieved by preparing color composites of two or more stretched ratio images. The choice of MSS bands for rationing depends on the spectral reflectance properties of the rocks to be discriminated. Although this technique is in the initial stage of development and is untested in other areas, it already appears to have considerable potential for targeting mineral prospects and for regional geologic mapping
Detection and mapping of hydrothermally altered rocks in the vicinity of the comstock lode, Virginia Range, Nevada, using enhanced LANDSAT images
The author has identified the following significant results. LANDSAT images enhanced by the band-ratioing method can be used for reconnaissance alteration mapping in moderately heavily vegetated semiarid terrain as well as in sparsely vegetated to semiarid terrain where the technique was originally developed. Significant vegetation cover in a scene, however, requires the use of MSS ratios 4/5, 4/6, and 6/7 rather than 4/5, 5/6, and 6/7, and requires careful interpretation of the results. Supplemental information suitable to vegetation identification and cover estimates, such as standard LANDSAT false-color composites and low altitude aerial photographs of selected areas is desirable
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
Optical/Near-Infrared Imaging of Infrared-Excess Palomar-Green QSOs
Ground-based high spatial-resolution (FWHM < 0.3-0.8") optical and
near-infrared imaging (0.4-2.2um) is presented for a complete sample of
optically selected Palomar-Green QSOs with far-infrared excesses at least as
great as those of "warm" AGN-like ultraluminous infrared galaxies
(L_ir/L_big-blue-bump > 0.46). In all cases, the host galaxies of the QSOs were
detected and most have discernable two-dimensional structure. The QSO host
galaxies and the QSO nuclei are similar in magnitude at H-band. H-band
luminosities of the hosts range from 0.5-7.5 L* with a mean of 2.3 L*, and are
consistent with those found in ULIGs. Both the QSO nuclei and the host galaxies
have near-infrared excesses, which may be the result of dust associated with
the nucleus and of recent dusty star formation in the host. These results
suggest that some, but not all, optically-selected QSOs may have evolved from
an infrared-active state triggered by the merger of two similarly-sized L*
galaxies, in a manner similar to that of the ultraluminous infrared galaxies.Comment: Aastex format, 38 pages, 4 tables, 10 figures. Higher quality figures
are available in JPG forma
- …