3,030 research outputs found
The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions
We have used SCUBA to observe a complete sample of 104 galaxies selected at
60 microns from the IRAS BGS and we present here the 850 micron measurements.
Fitting the 60,100 and 850 micron fluxes with a single temperature dust model
gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the
possibility of dust which is colder than this, if a 20 K component was present
then our dust masses would increase by factor 1.5-3. We present the first
measurements of the luminosity and dust mass functions, which were well fitted
by Schechter functions (unlike those 60 microns). We have correlated many
global galaxy properties with the submillimetre and find that there is a
tendancy for less optically luminous galaxies to contain warmer dust and have
greater star formation efficiencies (cf. Young 1999). The average gas-to-dust
ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen),
significantly higher than the Galactic value of 160. We believe this
discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly
tight correlation between dust mass and the mass of molecular hydrogen as
estimated from CO measurements, with an intrinsic scatter of ~50%.Comment: 24 pages, 15 figures, 8 tables, accepted for publication in MNRA
A census of metals at high and low redshifts and the connection between submillimetre sources and spheroid formation
Deep surveys in many wavebands have shown that the rate at which stars were
forming was at least a factor of 10 higher at z > 1 than today. Heavy elements
(metals) are produced by stars, and the star formation history deduced by these
surveys implies that a significant fraction of metals in the universe today
should already exist at z~2-3. However, only 10% of the total metals expected
to exist at this epoch have so far been accounted for (in DLAs and the Lyman
forest). In this paper, we use the results of submillimetre surveys of the
local and high redshift universe to show that there was much more dust in
galaxies in the past. We find that a large proportion of the missing metals are
traced by this dust, bringing the metals implied from the star formation
history and observations into agreement. We also show that the observed
distribution of dust masses at high redshift can be reproduced remarkably well
by a simple model for the evolution of dust in spheroids, suggesting that the
descendants of the dusty galaxies found in the deep submm surveys are the
relatively dust-free spiral bulges and ellipticals in the universe today.Comment: 11 pages, 6 figures, MNRAS in pres
Dust-to-Gas Ratio and Metallicity in Dwarf Galaxies
We examine the dust-to-gas ratio as a function of metallicity for dwarf
galaxies [dwarf irregular galaxies (dIrrs) and blue compact dwarf galaxies
(BCDGs)]. Using a one-zone model and adopting the instantaneous recycling
approximation, we prepare a set of basic equations which describes processes of
dust formation and destruction in a galaxy. Four terms are included for the
processes: dust formation from heavy elements ejected by stellar mass loss,
dust destruction in supernova remnants, dust destruction in star-forming
regions, and accretion of heavy elements onto preexisting dust grains. Solving
the equations, we compare the result with observational data of nearby dIrrs
and BCDGs. The solution is consistent with the data within the reasonable
ranges of model parameters constrained by the previous examinations. This means
that the model is successful in understanding the dust amount of nearby
galaxies. We also show that the accretion rate of heavy element onto
preexisting dust grains is less effective than the condensation of heavy
elements in dwarf galaxies.Comment: 14 pages LaTeX, 4 figures, to appear in Ap
Cold Dust in Kepler's Supernova Remnant
The timescales to replenish dust from the cool, dense winds of Asymptotic
Giant Branch stars are believed to be greater than the timescales for dust
destruction. In high redshift galaxies, this problem is further compounded as
the stars take longer than the age of the Universe to evolve into the dust
production stages. To explain these discrepancies, dust formation in supernovae
(SNe) is required to be an important process but until very recently dust in
supernova remnants has only been detected in very small quantities. We present
the first submillimeter observations of cold dust in Kepler's supernova remnant
(SNR) using SCUBA. A two component dust temperature model is required to fit
the Spectral Energy Distribution (SED) with K and K. The total mass of dust implied for Kepler is -
1000 times greater than previous estimates. Thus SNe, or their progenitors may
be important dust formation sites.Comment: 12 pages, 2 figures, accepted to ApJL, corrected proof
Submillimetre Emission from Eta Carinae
We present critical, long-wavelength observations of Eta Carinae in the
submillimetre using SCUBA on the JCMT at 850 and 450 um to confirm the presence
of a large mass of warm dust around the central star. We fit a two-component
blackbody to the IR-submm spectral energy distribution and estimate between
0.3-0.7 solar masses of dust exists in the nebula depending on the dust
absorption properties and the extent of contamination from free-free emission
at the SCUBA wavelengths. These results provide further evidence that Eta
Carinae's circumstellar nebula contains > 10 solar masses of gas, although this
may have been ejected on a longer timescale than previously thought.Comment: 7 pages, 5 figures, accepted MNRAS, corrected typo
Modeling the radial abundance distribution of the transition galaxy ngc 1313
NGC 1313 is the most massive disk galaxy showing a flat radial abundance
distribution in its interstellar gas, a behavior generally observed in
magellanic and irregular galaxies. We have attempted to reproduce this flat
abundance distribution using a multiphase chemical evolution model, which has
been previously used sucessfully to depict other spiral galaxies along the
Hubble morphological sequence. We found that it is not possible to reproduce
the flat radial abundance distribution in NGC 1313, and at the same time, be
consistent with observed radial distributions of other key parameters such the
surface gas density and star formation profiles. We conclude that a more
complicated galactic evolution model including radial flows, and possibly mass
loss due to supernova explosions and winds, is necessary to explain the
apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri
Antibody responses to nasopharyngeal carriage of Streptococcus pneumoniae in adults: A longitudinal household study
Background. Natural immunity to Streptococcus pneumoniae is thought to be induced by exposure to S. pneumoniae or cross-reactive antigens. No longitudinal studies of carriage of and immune responses to S. pneumoniae have been conducted using sophisticated immunological laboratory techniques.Methods. We enrolled 121 families with young children into this study. Nasopharyngeal (NP) swabs were collected monthly for 10 months from all family members and were cultured in a standard fashion. Cultured S. pneumoniae isolates were serotyped. At the beginning (month 0) and end (month 10) of the study, venous blood was collected from family members 118 years old. Serotype-specific antipolysaccharide immunoglobulin G (IgG) and functional antibody and antibodies to pneumolysin, pneumococcal surface protein A (PspA), and pneumococcal surface antigen A (PsaA) were measured in paired serum samples.Results. Levels of anticapsular IgG increased significantly after carriage of serotypes 9V, 14, 18C, 19F, and 23F by an individual or family member. For serotype 14, a higher level of anticapsular IgG at the beginning of the study was associated with reduced odds of carriage (P = .0006). There was a small (similar to 20%) but significant increase in titers of antibodies to PsaA and pneumolysin but no change in titers of antibody to PspA.Conclusions. Adults respond to NP carriage by mounting anticapsular and weak antiprotein antibody responses, and naturally induced anticapsular IgG can prevent carriage
Modelling the hepatitis B vaccination programme in prisons
A vaccination programme offering hepatitis B (HBV) vaccine at reception into prison has been introduced into selected prisons in England and Wales. Over the coming years it is anticipated this vaccination programme will be extended. A model has been developed to assess the potential impact of the programme on the vaccination coverage of prisoners, ex-prisoners, and injecting drug users (IDUs). Under a range of coverage scenarios, the model predicts the change over time in the vaccination status of new entrants to prison, current prisoners and IDUs in the community. The model predicts that at baseline in 2012 57% of the IDU population will be vaccinated with up to 72% being vaccinated depending on the vaccination scenario implemented. These results are sensitive to the size of the IDU population in England and Wales and the average time served by an IDU during each prison visit. IDUs that do not receive HBV vaccine in the community are at increased risk from HBV infection. The HBV vaccination programme in prisons is an effective way of vaccinating this hard-to-reach population although vaccination coverage on prison reception must be increased to achieve this
Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field
We measure nebular oxygen abundances for 204 emission-line galaxies with
redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North
(GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We
also provide an updated analytic prescription for estimating oxygen abundances
using the traditional strong emission line ratio, R_{23}, based on the
photoionization models of Kewley & Dopita (2003). We include an analytic
formula for very crude metallicity estimates using the [NII]6584/Halpha ratio.
Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1
corresponding to metallicities between 0.3 and 2.5 times the solar value. This
sample of galaxies exhibits a correlation between rest-frame blue luminosity
and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z
correlations of previously-studied intermediate-redshift samples. The zero
point of the L-Z relation evolves with redshift in the sense that galaxies of a
given luminosity become more metal poor at higher redshift. Galaxies in
luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance
by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that
28\pm0.07% of metals in local galaxies have been synthesized since z=1, in
reasonable agreement with the predictions based on published star formation
rate densities which show that ~38% of stars in the universe have formed during
the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The
Astrophysical Journa
A scattering rate approach to the understanding of absorption line broadening in near-infrared AlGaN/GaN quantum wells
There has been much interest in the advancement of III-Nitride growth technology to fabricate AlGaN/GaN heterostructures for intersubband transitions (ISBTs). The large conduction band offset in these structures (up to 2 eV) allows transition energies in the near- to the far-infrared region, which have applications from telecommunications, such as in all-optical switches, to infra-red detectors for sensing and imaging. To date, ISBT electroluminescence has been elusive and absorption measurements remain an important method to verify band structure calculations. The growth quality can be inferred from the absorption spectrum, which will have line broadening with contributions that are both inhomogeneous (large-scale interface roughness, and non-parabolicity) and homogeneous (electron scattering related lifetime broadening). In the present work we calculated the contributions of various homogeneous broadening mechanisms (electron interaction with longitudinal-optical (LO) phonons, acoustic phonons, impurities and alloy disorder) to the full linewidth, and also the contribution of band non-parabolicity, which contributes to the inhomogeneous broadening. Calculations are then compared to the measured absorption spectra of several samples
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