937 research outputs found
The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History
Fully cosmological, high resolution N-Body + SPH simulations are used to
investigate the chemical abundance trends of stars in simulated stellar halos
as a function of their origin. These simulations employ a physically motivated
supernova feedback recipe, as well as metal enrichment, metal cooling and metal
diffusion. As presented in an earlier paper, the simulated galaxies in this
study are surrounded by stellar halos whose inner regions contain both stars
accreted from satellite galaxies and stars formed in situ in the central
regions of the main galaxies and later displaced by mergers into their inner
halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within
10 kpc of a solar-like observer are analyzed. We find that for galaxies which
have not experienced a recent major merger, in situ stars at the high [Fe/H]
end of the metallicity distribution function are more [alpha/Fe]-rich than
accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at
a given [Fe/H] results from the different potential wells within which in situ
and accreted halo stars form. These results qualitatively match recent
observations of local Milky Way halo stars. It may thus be possible for
observers to uncover the relative contribution of different physical processes
to the formation of stellar halos by observing such trends in the halo
populations of the Milky Way, and other local L* galaxies.Comment: Version accepted for publication in ApJ Part 1. This version of the
paper has been extended to include a detailed discussion of numerical issue
Stellar Population Variations in the Milky Way's Stellar Halo
If the stellar halos of disk galaxies are built up from the disruption of
dwarf galaxies, models predict highly structured variations in the stellar
populations within these halos. We test this prediction by studying the ratio
of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor
populations) to main-sequence turn-off stars (MSTO stars; a feature of all
populations) in the stellar halo of the Milky Way using data from the Sloan
Digital Sky Survey. We develop and apply an improved technique to select BHB
stars using ugr color information alone, yielding a sample of ~9000 g<18
candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across
~1/4 of the sky at the distance resolution permitted by the absolute magnitude
distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in
the stellar halo. Previously identified, stream-like halo structures have
distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo
features, e.g., the low-latitude structure, appear to be almost completely
devoid of BHB stars, whereas other structures appear to be rich in BHB stars.
The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio
along its extent, which we interpret in terms of population gradients within
the progenitor dwarf galaxy. Our detection of coherent stellar population
variations between different stellar halo substructures provides yet more
support to cosmologically motivated models for stellar halo growth.Comment: Astronomical Journal, in press. 10 pages, 5 color figures. Much
better printed in colo
The Destruction of Thin Stellar Disks Via Cosmologically Common Satellite Accretion Events
Most Galaxy-sized systems (M_host ~ 10^12 M_sun) in the LCDM cosmology are
expected to have accreted at least one satellite with a total mass M_sat ~
10^11 M_sun = 3M_disk in the past 8 Gyr. Analytic and numerical investigations
suggest that this is the most precarious type of merger for the survival of
thin galactic disks because more massive accretion events are relatively rare
and less massive ones preserve thin disk components. We use high-resolution,
dissipationless N-body simulations to study the response of an initially-thin,
fully-formed Milky-Way type stellar disk to these cosmologically common events
and show that the thin disk does not survive. Regardless of orbital
configuration, the impacts transform the disks into structures that are roughly
three times as thick and more than twice as kinematically hot as the observed
dominant thin disk component of the Milky Way. We conclude that if the Galactic
thin disk is a representative case, then the presence of a stabilizing gas
component is the only recourse for explaining the preponderance of disk
galaxies in an LCDM universe; otherwise, the disk of the Milky Way must be
uncommonly cold and thin for its luminosity, perhaps as a consequence of an
unusually quiescent accretion history.Comment: 6 pages, 4 figures; accepted by ApJL; high-resolution version and
example movie of simulation available at
http://web.me.com/purcellgalaxies/workshop/log/Entries/2008/10/15_The_Destruction_of_thin_stellar_disks_via_cosmologically_common_mergers.htm
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Testing predictions on body mass and gut contents: dissection of an African elephant Loxodonta africana Blumenbach 1797
The values reported in the literature for the total gastrointestinal tract (GIT) content mass of elephants are lower than expected from interspecific mammalian regression. This finding agrees with theoretical considerations that elephants should have less capacious GITs than other herbivorous mammals, resulting in short ingesta retention times. However, the data on elephants was so far derived from either diseased zoo specimens or free-ranging animals subjected to an unknown hunting stress. In this study, we weighed the wet contents of the GIT segments of a captive African elephant that was euthanased because of a positive serological tuberculosis test, but that was clinically healthy, did not show a reduced appetite, and ingested food up to the time of euthanasia. The animal weighed 3,140 kg and its total gut contents were 542 kg or 17% of body mass. This is in close accord with the published mammalian herbivore regression equation of Parra (Comparison of foregut and hindgut fermentation in herbivores. In: Montgomery GG (ed) The ecology of arboreal folivores. Smithsonian Institution Press, Washington DC, pp205-230, 1978) and contradicts the notion that elephants have comparatively less capacious gastrointestinal tracts. Data on the individual gut segments, however, do support earlier suspicions that elephants have a comparatively less capacious caecum and a disproportionally capacious colon
LSST Science Book, Version 2.0
A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at
http://www.lsst.org/lsst/sciboo
Clinical Trials in Head Injury
Traumatic brain injury (TBI) remains a major public health problem globally. In the United States the incidence of closed head injuries admitted to hospitals is conservatively estimated to be 200 per 100,000 population, and the incidence of penetrating head injury is estimated to be 12 per 100,000, the highest of any developed country in the world. This yields an approximate number of 500,000 new cases each year, a sizeable proportion of which demonstrate signficant long-term disabilities. Unfortunately, there is a paucity of proven therapies for this disease. For a variety of reasons, clinical trials for this condition have been difficult to design and perform. Despite promising pre-clinical data, most of the trials that have been performed in recent years have failed to demonstrate any significant improvement in outcomes. The reasons for these failures have not always been apparent and any insights gained were not always shared. It was therefore feared that we were running the risk of repeating our mistakes. Recognizing the importance of TBI, the National Institute of Neurological Disorders and Stroke (NINDS) sponsored a workshop that brought together experts from clinical, research, and pharmaceutical backgrounds. This workshop proved to be very informative and yielded many insights into previous and future TBI trials. This paper is an attempt to summarize the key points made at the workshop. It is hoped that these lessons will enhance the planning and design of future efforts in this important field of research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63185/1/089771502753754037.pd
Cosmology: small scale issues
The abundance of dark matter satellites and subhalos, the existence of
density cusps at the centers of dark matter halos, and problems producing
realistic disk galaxies in simulations are issues that have raised concerns
about the viability of the standard cold dark matter (LambdaCDM) scenario for
galaxy formation. This talk reviews these issues, and considers the
implications for cold vs. various varieties of warm dark matter (WDM). The
current evidence appears to be consistent with standard LambdaCDM, although
improving data may point toward a rather tepid version of LambdaWDM - tepid
since the dark matter cannot be very warm without violating observational
constraints.Comment: 7 pages, 1 figure, to appear in the proceedings of the 8th UCLA Dark
Matter Symposium, Marina del Rey, USA, 20-22 February 200
Risk of adverse outcomes in patients with underlying respiratory conditions admitted to hospital with COVID-19: a national, multicentre prospective cohort study using the ISARIC WHO Clinical Characterisation Protocol UK
Background: Studies of patients admitted to hospital with COVID-19 have found varying mortality outcomes associated with underlying respiratory conditions and inhaled corticosteroid use. Using data from a national, multicentre, prospective cohort, we aimed to characterise people with COVID-19 admitted to hospital with underlying respiratory disease, assess the level of care received, measure in-hospital mortality, and examine the effect of inhaled corticosteroid use. Methods: We analysed data from the International Severe Acute Respiratory and emerging Infection Consortium (ISARIC) WHO Clinical Characterisation Protocol UK (CCP-UK) study. All patients admitted to hospital with COVID-19 across England, Scotland, and Wales between Jan 17 and Aug 3, 2020, were eligible for inclusion in this analysis. Patients with asthma, chronic pulmonary disease, or both, were identified and stratified by age (<16 years, 16â49 years, and â„50 years). In-hospital mortality was measured by use of multilevel Cox proportional hazards, adjusting for demographics, comorbidities, and medications (inhaled corticosteroids, short-acting ÎČ-agonists [SABAs], and long-acting ÎČ-agonists [LABAs]). Patients with asthma who were taking an inhaled corticosteroid plus LABA plus another maintenance asthma medication were considered to have severe asthma. Findings: 75 463 patients from 258 participating health-care facilities were included in this analysis: 860 patients younger than 16 years (74 [8·6%] with asthma), 8950 patients aged 16â49 years (1867 [20·9%] with asthma), and 65 653 patients aged 50 years and older (5918 [9·0%] with asthma, 10 266 [15·6%] with chronic pulmonary disease, and 2071 [3·2%] with both asthma and chronic pulmonary disease). Patients with asthma were significantly more likely than those without asthma to receive critical care (patients aged 16â49 years: adjusted odds ratio [OR] 1·20 [95% CI 1·05â1·37]; p=0·0080; patients aged â„50 years: adjusted OR 1·17 [1·08â1·27]; p<0·0001), and patients aged 50 years and older with chronic pulmonary disease (with or without asthma) were significantly less likely than those without a respiratory condition to receive critical care (adjusted OR 0·66 [0·60â0·72] for those without asthma and 0·74 [0·62â0·87] for those with asthma; p<0·0001 for both). In patients aged 16â49 years, only those with severe asthma had a significant increase in mortality compared to those with no asthma (adjusted hazard ratio [HR] 1·17 [95% CI 0·73â1·86] for those on no asthma therapy, 0·99 [0·61â1·58] for those on SABAs only, 0·94 [0·62â1·43] for those on inhaled corticosteroids only, 1·02 [0·67â1·54] for those on inhaled corticosteroids plus LABAs, and 1·96 [1·25â3·08] for those with severe asthma). Among patients aged 50 years and older, those with chronic pulmonary disease had a significantly increased mortality risk, regardless of inhaled corticosteroid use, compared to patients without an underlying respiratory condition (adjusted HR 1·16 [95% CI 1·12â1·22] for those not on inhaled corticosteroids, and 1·10 [1·04â1·16] for those on inhaled corticosteroids; p<0·0001). Patients aged 50 years and older with severe asthma also had an increased mortality risk compared to those not on asthma therapy (adjusted HR 1·24 [95% CI 1·04â1·49]). In patients aged 50 years and older, inhaled corticosteroid use within 2 weeks of hospital admission was associated with decreased mortality in those with asthma, compared to those without an underlying respiratory condition (adjusted HR 0·86 [95% CI 0·80â0·92]). Interpretation: Underlying respiratory conditions are common in patients admitted to hospital with COVID-19. Regardless of the severity of symptoms at admission and comorbidities, patients with asthma were more likely, and those with chronic pulmonary disease less likely, to receive critical care than patients without an underlying respiratory condition. In patients aged 16 years and older, severe asthma was associated with increased mortality compared to non-severe asthma. In patients aged 50 years and older, inhaled corticosteroid use in those with asthma was associated with lower mortality than in patients without an underlying respiratory condition; patients with chronic pulmonary disease had significantly increased mortality compared to those with no underlying respiratory condition, regardless of inhaled corticosteroid use. Our results suggest that the use of inhaled corticosteroids, within 2 weeks of admission, improves survival for patients aged 50 years and older with asthma, but not for those with chronic pulmonary disease. Funding: National Institute for Health Research, Medical Research Council, NIHR Health Protection Research Units in Emerging and Zoonotic Infections at the University of Liverpool and in Respiratory Infections at Imperial College London in partnership with Public Health England
Search for direct top squark pair production in final states with two leptons in âs=13 TeV pp collisions with the ATLAS detector
The results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using 36.1 fbâ1 of integrated luminosity from protonâproton collisions at âs=13 TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark t~ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay t~âbÏ~1± into a b-quark and the lightest chargino with Ï~1±âWÏ~10 , t~âtÏ~10 into an on-shell top quark and the lightest neutralino, the three-body decay t~âbWÏ~10 and the four-body decay t~âbâÎœÏ~10. No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the t~ and Ï~01 masses. The results exclude at 95% confidence level t~ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches
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