277 research outputs found
Diffuse Hard X-ray Emission in Starburst Galaxies as Synchrotron from Very High Energy Electrons
[Abdriged] The origin of the diffuse hard X-ray (2 - 10 keV) emission from
starburst galaxies is a long-standing problem. We suggest that synchrotron
emission of 10 - 100 TeV electrons and positrons (e+/-) can contribute to this
emission, because starbursts have strong magnetic fields. We consider three
sources of e+/- at these energies: (1) primary electrons directly accelerated
by supernova remnants; (2) pionic secondary e+/- created by inelastic
collisions between CR protons and gas nuclei in the dense ISMs of starbursts;
(3) pair e+/- produced between the interactions between 10 - 100 TeV gamma-rays
and the intense far-infrared (FIR) radiation fields of starbursts. We create
one-zone steady-state models of the CR population in the Galactic Center (R <=
112 pc), NGC 253, M82, and Arp 220's nuclei, assuming a power law injection
spectrum for electrons and protons. We compare these models to extant radio and
GeV and TeV gamma-ray data for these starbursts, and calculate the diffuse
synchrotron X-ray and Inverse Compton (IC) luminosities of these starbursts. If
the primary electron spectrum extends to ~PeV energies and has a
proton/electron injection ratio similar to the Galactic value, we find that
synchrotron contributes 2 - 20% of their unresolved, diffuse hard X-ray
emission. Inverse Compton emission is likewise a minority of the unresolved
X-ray emission in these starbursts, from 0.1% in the Galactic Center to 10% in
Arp 220's nuclei. We also model generic starbursts, including submillimeter
galaxies, in the context of the FIR--X-ray relation, finding that up to 2% in
the densest starbursts with our fiducial assumptions. Neutrino and TeV
gamma-ray data can further constrain the synchrotron X-ray emission of
starbursts. Our models do not constrain hard synchrotron X-ray emission from
any additional hard components of primary e+/- from sources like pulsars in
starbursts.Comment: Accepted by ApJ; 31 pages, emulateapj forma
Polymer-based paclitaxel-eluting stents reduce in-stent neointimal tissue proliferation A serial volumetric intravascular ultrasound analysis from the TAXUS-IV trial
ObjectivesThe aim of this study was to use serial volumetric intravascular ultrasound (IVUS) to evaluate the effects of polymer-based, paclitaxel-eluting stents on in-stent neointima formation and late incomplete stent apposition.BackgroundThe TAXUS-IV trial demonstrated that the slow-release, polymer-based, paclitaxel-eluting stent reduces angiographic restenosis and the need for repeat revascularization procedures. Serial IVUS studies reveal details of the pattern of vascular responses provoked by stent implantation that provide insight into device safety and efficacy.MethodsIn the TAXUS-IV trial, patients were randomized to the slow-release, polymer-based, paclitaxel-eluting TAXUS stent or a bare-metal EXPRESS stent (Boston Scientific Corp., Natick, Massachusetts). As part of a formal substudy, complete volumetric IVUS data were available in 170 patients, including 88 TAXUS patients and 82 controls, at implantation and at nine-month follow-up.ResultsNo baseline differences were present in the clinical characteristics or IVUS parameters between the control and TAXUS groups. At nine-month follow-up, IVUS lumen volumes were larger in the TAXUS group (123 ± 43 mm3vs. 104 ± 44 mm3, p = 0.005), due to a reduction in neointimal volume (18 ± 18 mm3vs. 41 ± 23 mm3, p < 0.001). Millimeter-by-millimeter analysis within the stent demonstrated uniform suppression of neointimal growth along the entire stent length. Late lumen loss was similar at the proximal edge of the stent between the two groups, and reduced with the TAXUS stent at the distal edge (p = 0.004). Incomplete stent apposition at nine months was observed in only 3.0% of control and 4.0% of TAXUS stents (p = 0.12).ConclusionsPolymer-based, paclitaxel-eluting TAXUS stents are effective in inhibiting neointimal tissue proliferation, and do not result in late incomplete stent apposition
On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253
The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and
VERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts.
We argue that the gamma rays are predominantly hadronic in origin, as expected
by previous studies. The measured fluxes imply that pionic losses are efficient
for CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of
the energy injected in high energy primary CR protons is lost to inelastic
proton-proton collisions (pion production) before escape, producing gamma rays,
neutrinos, and secondary electrons and positrons. We discuss the factor ~2
uncertainties in this estimate, including supernova rate and leptonic
contributions to the GeV-TeV emission. We argue that gamma-ray data on ULIRGs
like Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we
present upper limits on Arp 220 from the Fermi data. We show that the observed
ratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron
cooling processes are important for cooling the CR electron/positron
population. We briefly reconsider previous predictions in light of the
gamma-ray detections, including the starburst contribution to the gamma-ray
background and CR energy densities. Finally, as a guide for future studies, we
list the brightest star-forming galaxies on the sky and present updated
predictions for their gamma-ray and neutrino fluxes.Comment: 15 pages, emulateapj format, accepted to ApJ, Table 1 fixe
Preheating by Previrialization and its Impact on Galaxy Formation
We use recent observations of the HI-mass function to constrain galaxy
formation. The data conflicts with the standard model where most of the gas in
a low-mass dark matter halo is assumed to settle into a disk of cold gas that
is depleted by star formation and supernova-driven outflows until the disk
becomes gravitationally stable. A consistent model can be found if low-mass
haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev
cm^2. Such a model simultaneously matches the faint-end slope of the galaxy
luminosity function. We propose a preheating model where the medium around
low-mass haloes is preheated by gravitational pancaking. Since gravitational
tidal fields suppress the formation of low-mass haloes while promoting that of
pancakes, the formation of massive pancakes precedes that of the low-mass
haloes within them. We demonstrate that the progenitors of present-day dark
matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses
~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to
a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas
has a cooling time that exceeds the age of the Universe at z~2, and has a
specific entropy of ~15Kev cm^2, almost exactly the amount required to explain
the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA
GECO: Galaxy Evolution COde - A new semi-analytical model of galaxy formation
We present a new semi-analytical model of galaxy formation, GECO (Galaxy
Evolution COde), aimed at a better understanding of when and how the two
processes of star formation and galaxy assembly have taken place. Our model is
structured into a Monte Carlo algorithm based on the Extended Press-Schechter
theory, for the representation of the merging hierarchy of dark matter halos,
and a set of analytic algorithms for the treatment of the baryonic physics,
including classical recipes for the gas cooling, the star formation
time-scales, galaxy mergers and SN feedback. Together with the galaxies, the
parallel growth of BHs is followed in time and their feedback on the hosting
galaxies is modelled. We set the model free parameters by matching with data on
local stellar mass functions and the BH-bulge relation at z=0. Based on such
local boundary conditions, we investigate how data on the high-redshift
universe constrain our understanding of the physical processes driving the
evolution, focusing in particular on the assembly of stellar mass and on the
star formation history. Since both processes are currently strongly constrained
by cosmological near- and far-IR surveys, the basic physics of the Lambda CDM
hierarchical clustering concept of galaxy formation can be effectively tested
by us by comparison with the most reliable set of observables. Our
investigation shows that when the time-scales of the stellar formation and mass
assembly are studied as a function of dark matter halo mass and the single
galaxy stellar mass, the 'downsizing' fashion of star formation appears to be a
natural outcome of the model, reproduced even in the absence of the AGN
feedback. On the contrary, the stellar mass assembly history turns out to
follow a more standard hierarchical pattern progressive in cosmic time, with
the more massive systems assembled at late times mainly through dissipationless
mergers.Comment: Accepted for publication in A&A, 24 pages, 15 figure
The intriguing HI gas in NGC 5253: an infall of a diffuse, low-metallicity HI cloud?
(Abridged) We present new, deep HI line and 20-cm radio continuum data of the
very puzzling blue compact dwarf galaxy NGC 5253, obtained with the ATCA as
part of the `Local Volume HI Survey' (LVHIS). Our low-resolution HI maps show
the disturbed HI morphology that NGC 5253 possesses, including tails, plumes
and detached HI clouds. The high-resolution map reveals an HI plume at the SE
and an HI structure at the NW that surrounds an Ha shell. We confirm that the
kinematics of the neutral gas are highly perturbed and do not follow a rotation
pattern. We discuss the outflow and infall scenarios to explain such disturbed
kinematics, analyze the environment in which it resides, and compare it
properties with those observed in similar star-forming dwarf galaxies. The
radio-continuum emission of NGC 5253 is resolved and associated with the
intense star-forming region at the center of the galaxy. We complete the
analysis using multiwavelength data extracted from the literature. We estimate
the SFR using this multiwavelength approach. NGC 5253 does not satisfy the
Schmidt-Kennicutt law of star-formation, has a very low HI mass-to-light ratio
when comparing with its stellar mass, and seems to be slightly metal-deficient
in comparison with starbursts of similar baryonic mass. Taking into account all
available multiwavelength data, we conclude that NGC 5253 is probably
experiencing the infall of a diffuse, low-metallicity HI cloud along the minor
axis of the galaxy, which is comprising the ISM and triggering the powerful
starburst. The tidally disturbed material observed at the east and north of the
galaxy is a consequence of this interaction, which probably started more than
100 Myr ago. The origin of this HI cloud may be related with a strong
interaction between NGC 5253 and the late-type spiral galaxy M 83 in the past.Comment: 19 pages, 12 figures, accepted for publication in MNRA
The Star-Forming Galaxy Contribution to the Cosmic MeV and GeV Gamma-Ray Background
While star-forming galaxies could be major contributors to the cosmic GeV
-ray background, they are expected to be MeV-dim because of the "pion
bump" falling off below ~100 MeV. However, there are very few observations of
galaxies in the MeV range, and other emission processes could be present. We
investigate the MeV background from star-forming galaxies by running one-zone
models of cosmic ray populations, including Inverse Compton and bremsstrahlung,
as well as nuclear lines (including Al), emission from core-collapse
supernovae, and positron annihilation emission, in addition to the pionic
emission. We use the Milky Way and M82 as templates of normal and starburst
galaxies, and compare our models to radio and GeV--TeV -ray data. We
find that (1) higher gas densities in high-z normal galaxies lead to a strong
pion bump, (2) starbursts may have significant MeV emission if their magnetic
field strengths are low, and (3) cascades can contribute to the MeV emission of
starbursts if they emit mainly hadronic -rays. Our fiducial model
predicts that most of the unresolved GeV background is from star-forming
galaxies, but this prediction is uncertain by an order of magnitude. About ~2%
of the claimed 1 MeV background is diffuse emission from star-forming galaxies;
we place a firm upper limit of <~10% based on the spectral shape of the
background. The star-formation contribution is constrained to be small, because
its spectrum is peaked, while the observed background is steeply falling with
energy through the MeV-GeV range.Comment: Published in ApJ, 27 pages, emulateapj format. Readers may be
interested in the concurrent paper by Chakraborty and Fields
(arXiv:1206.0770), a calculation of the Inverse Compton background from
star-forming galaxie
Controlling the shape of a quantum wavefunction
The ability to control the shape and motion of quantum states(1,2) may lead to methods for bond-selective chemistry and novel quantum technologies, such as quantum computing. The classical coherence of laser light has been used to guide quantum systems into desired target states through interfering pathways(3-5). These experiments used the control of target properties-such as fluorescence from a dye solution(6), the current in a semiconductor(7,8) 8 Or the dissociation fraction of an excited molecule(9)-to infer control over the quantum state. Here we report a direct approach to coherent quantum control that allows us to actively manipulate the shape of an atomic electron's radial wavefunction, We use a computer-controlled laser to excite a coherent state in atomic caesium. The shape of the wavefunction is then measured(10) and the information fed back into the laser control system, which reprograms the optical field. The process is iterated until the measured shape of the wavefunction matches that of a target wavepacket, established at the start of the experiment. We find that, using a variation of quantum holography(11) to reconstruct the measured wavefunction, the quantum state can be reshaped to match the target within two iterations of the feedback loop.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62625/1/397233a0.pd
Coordinated Monitoring of the Eccentric O-star Binary Iota Orionis: The X-ray Analysis
We analyse two ASCA observations of the highly eccentric O9III + B1III binary
Iota Orionis obtained at periastron and apastron. Based on the assumption of a
strong colliding winds shock between the stellar components, we expected to see
significant variation in the X-ray emission between these phases. The
observations proved otherwise: the X-ray luminosities and spectral
distributions were remarkably similar. The only noteworthy feature was the hint
of a proximity effect during periastron passage, supported also in the optical.
We discuss the accuracy of our results, and also analyse archival ROSAT
observations. We investigate why we do not see a clear colliding winds
signature. A simple model shows that the wind attenuation to the expected
position of the shock apex is negligible throughout the orbit, which poses the
puzzling question of why the expected 1/D variation (ie. a factor of 7.5) in
the intrinsic luminosity is not seen in the data. Two scenarios are proposed:
either the colliding winds emission is unexpectedly weak such that intrinsic
shocks in the winds dominate the emission, or, alternatively, that the emission
observed is colliding winds emission but in a more complex form than we would
naively expect. Complex hydrodynamical models are then analyzed. Despite
strongly phase-variable emission from the models, both were consistent with the
observations. We find that if the mass-loss rates of the stars are low then
intrinsic wind shocks could dominate the emission. However, when we assume
higher mass-loss rates of the stars, we find that the observed emission could
also be consistent with a purely colliding winds origin. To distinguish between
the different models X-ray observations with improved phase coverage will be
necessary.Comment: 18 pages, 14 figures, uses mn.st
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