5,256 research outputs found
The spatially resolved Kennicutt-Schmidt relation in the HI dominated regions of spiral and dwarf irregular galaxies
We study the Kennicutt-Schmidt relation between average star formation rate
and average cold gas surface density in the Hi dominated ISM of nearby spiral
and dwarf irregular galaxies. We divide the galaxies into grid cells varying
from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using
H-alpha emission can be biased and scatter may be introduced because of
non-uniform sampling of the IMF or because of stochastically varying star
formation. In order to alleviate these issues, we use far-ultraviolet emission
to trace SFR, and we sum up the fluxes from different bins with the same gas
surface density to calculate the average at a given value of
. We study the resulting Kennicutt-Schmidt relation in 400 pc, 1
kpc and 10 kpc scale grids in nearby massive spirals and in 400 pc scale grids
in nearby faint dwarf irregulars. We find a relation with a power law slope of
1.5 in the HI-dominated regions for both kinds of galaxies. The relation is
offset towards longer gas consumption timescales compared to the molecular
hydrogen dominated centres of spirals, but the offset is an order-of-magnitude
less than that quoted by earlier studies. Our results lead to the surprising
conclusion that conversion of gas to stars is independent of metallicity in the
HI dominated regions of star-forming galaxies. Our observed relations are
better fit by a model of star formation based on thermal and hydrostatic
equilibrium in the ISM, in which feedback driven turbulence sets the thermal
pressure.Comment: 11 pages, 7 figures, 5 tables. Accepted for publication in MNRAS Main
Journal. For the definitive version visit http://mnras.oxfordjournals.org
A WENO Algorithm of the Temperature and Ionization Profiles around a Point Source
We develop a numerical solver for radiative transfer problems based on the
weighted essentially nonoscillatory (WENO) scheme modified with anti-diffusive
flux corrections, in order to solve the temperature and ionization profiles
around a point source of photons in the reionization epoch. Algorithms for such
simulation must be able to handle the following two features: 1. the sharp
profiles of ionization and temperature at the ionizing front (I-front) and the
heating front (T-front), and 2. the fraction of neutral hydrogen within the
ionized sphere is extremely small due to the stiffness of the rate equations of
atom processes. The WENO scheme can properly handle these two features, as it
has been shown to have high order of accuracy and good convergence in capturing
discontinuities and complicated structures in fluid as well as to be
significantly superior over piecewise smooth solutions containing
discontinuities. With this algorithm, we show the time-dependence of the
preheated shell around a UV photon source. In the first stage the I-front and
T-front are coincident, and propagate with almost the speed of light. In later
stage, when the frequency spectrum of UV photons is hardened, the speeds of
propagation of the ionizing and heating fronts are both significantly less than
the speed of light, and the heating front is always beyond the ionizing front.
In the spherical shell between the I- and T-fronts, the IGM is heated, while
atoms keep almost neutral. The time scale of the preheated shell evolution is
dependent on the intensity of the photon source. We also find that the details
of the pre-heated shell and the distribution of neutral hydrogen remained in
the ionized sphere are actually sensitive to the parameters used. The WENO
algorithm can provide stable and robust solutions to study these details.Comment: 24 pages, 7 figures, accepted in New Astronom
Genetically engineered pre-microRNA-34a prodrug suppresses orthotopic osteosarcoma xenograft tumor growth via the induction of apoptosis and cell cycle arrest.
Osteosarcoma (OS) is the most common primary malignant bone tumor in children, and microRNA-34a (miR-34a) replacement therapy represents a new treatment strategy. This study was to define the effectiveness and safety profiles of a novel bioengineered miR-34a prodrug in orthotopic OS xenograft tumor mouse model. Highly purified pre-miR-34a prodrug significantly inhibited the proliferation of human 143B and MG-63 cells in a dose dependent manner and to much greater degrees than controls, which was attributed to induction of apoptosis and G2 cell cycle arrest. Inhibition of OS cell growth and invasion were associated with release of high levels of mature miR-34a from pre-miR-34a prodrug and consequently reduction of protein levels of many miR-34a target genes including SIRT1, BCL2, c-MET, and CDK6. Furthermore, intravenous administration of in vivo-jetPEI formulated miR-34a prodrug significantly reduced OS tumor growth in orthotopic xenograft mouse models. In addition, mouse blood chemistry profiles indicated that therapeutic doses of bioengineered miR-34a prodrug were well tolerated in these animals. The results demonstrated that bioengineered miR-34a prodrug was effective to control OS tumor growth which involved the induction of apoptosis and cell cycle arrest, supporting the development of bioengineered RNAs as a novel class of large molecule therapeutic agents
DETERMINATION OF SERUM ALBUMIN WITH TRIBROMOARSENAZO BY SPECTROPHOTOMETRY
The reaction of tribromoarsenazo(TB-ASA) with serum albumin in the presence of emulgent OP was studied by spectrophotometry. In a Britton-Robinson buffer solution at pH 2.9, tribromoarsenazo and bovine serum albumin can immediately form a red compound in the presence of emulgent OP with a maximum absorption wavelength at 354 nm. The presence of emulgent OP can increase the reaction sensitivity and the compound stability. The molar absorptivity of the compound is
ε354 nm = 6.13 x 105 M-1•cm-1. Beer's law is obeyed over the range of
5.0-75.0 mg•L-1 for bovine serum albumin.
The present method was applied to the determination of the total proteins in human serums with satisfactory results.
KEY WORDS: Serum albumin, Tribromoarsenazo, Emulgent OP, Human serums, Spectrophotometry
Bull. Chem. Soc. Ethiop. 2007, 21(2), 291-296
Absorption-line probes of the prevalence and properties of outflows in present-day star-forming galaxies
We analyze star forming galaxies drawn from SDSS DR7 to show how the
interstellar medium (ISM) Na I 5890, 5896 (Na D) absorption lines depend on
galaxy physical properties, and to look for evidence of galactic winds. We
combine the spectra of galaxies with similar geometry/physical parameters to
create composite spectra with signal-to-noise ~300. The stellar continuum is
modeled using stellar population synthesis models, and the continuum-normalized
spectrum is fit with two Na I absorption components. We find that: (1) ISM Na D
absorption lines with equivalent widths EW > 0.8A are only prevalent in disk
galaxies with specific properties -- large extinction (Av), high star formation
rates (SFR), high star formation rate per unit area (), or
high stellar mass (M*). (2) the ISM Na D absorption lines can be separated into
two components: a quiescent disk-like component at the galaxy systemic velocity
and an outflow component; (3) the disk-like component is much stronger in the
edge-on systems, and the outflow component covers a wide angle but is stronger
within 60deg of the disk rotation axis; (4) the EW and covering factor of the
disk component correlate strongly with dust attenuation, highlighting the
importance that dust shielding may play the survival of Na I. (5) The EW of the
outflow component depends primarily on and secondarily on
Av; (6) the outflow velocity varies from ~120 to 160km/s but shows little hint
of a correlation with galaxy physical properties over the modest dynamic range
that our sample probes (1.2 dex in log and 1 dex in log M*).Comment: 18 pages, 18 figures, accepted by A
Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates
One popular approach to prepare graphene is to grow them on transition metal
substrates via chemical vapor deposition. By using the density functional
theory with dispersion correction, we systematically investigate for the first
time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on
metal substrates. Three categories of interfacial structures are revealed. The
adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak
physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti,
Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive
band gap is opened for the two uncontacted layers of TLG. The adsorption of
B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for
the uncontacted layers. This fundamental study also helps for B(T)LG device
study due to inevitable graphene/metal contact.Comment: 1 table, 8 figure
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