1,518 research outputs found
Hyperbolic metamaterials by directed self-assembly of block copolymers
Hyperbolic materials are high uniaxial anisotropic materials that display hyperbolic dispersion with distinctive properties, including negative refraction index, control over light propagation and enhanced Purcell factor. Naturally-occurring hyperbolic materials exhibit these properties only in reduced wavelength ranges, thus limiting their implementation into integrated optical devices. In order to tune the hyperbolic dispersion over broader bandwidths, artificial structures capable to guarantee a greater flexibility, i.e. hyperbolic metamaterials (HMMs), are required. So far, the realization of HMMs that work in the visible and near-infrared wavelength regions has been limited to the out-of-plane configuration due to technological costraints in the fabrication of periodic structures at sub-wavelength dimensions. Here we propose a novel concept of HMMs working in the in-plane configuration, based on the use of block copolymers (BCPs) capable to self-assemble into highly ordered polimeric masks with nanometric feature sizes and periodicity, serving as templates for the subsequent fabrication of hybrid metal-dielectric HMMs. This new class of HMMs can be exploited for metrological applications such as the enhancement of single photon source's (SPS) emission properties
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
Oxygenation response to NO in newborns with severe pulmonary hypertension
Oxygenation Response to NO in Newborns With Severe Pulmonary Hypertensio
Tagaeri Taromenane: popoli incontattati dell\u2019Amazzonia Ecuadoriana ed espansione della frontiera petrolifera, quali territori per l\u2019autodeterminazione e i diritti umani?
Nel 1999 l\u2019Ecuador, primo paese al mondo, istituiva una zona Intangibile riservata al diritto all\u2019autodeterminazione dei popoli incontattati Tagaeri-
Taromenane. Nel 2007 la Zona Intangibile Tagaeri-Taromenane (ZITT) veniva finalmente delimitata occupando un\u2019area di 7500 km2 della regione amazzonica
ai confini con il Peru, perpetuamente vietata ad ogni attivit\ue0 industriale.
Contemporaneamente veniva istituita anche una buffer zone di 10 km per garantire una ulteriore area di rispetto. Tuttavia trattandosi di popolazioni
nomadi che per secoli si sono mosse su un\u2019area di circa 20.000 km2 tra i fiumi Napo e Curaray (in direzione Nord sud) e tra i primi rilievi andini e la
confluenza del Nashino con il Curaray (in direzione ovest est) la Zona Intangibile non risulta adeguata alla territorialit\ue0 Tagaeri-Taromenane.
Accanto alla mobilit\ue0 dei popoli in isolamento si assiste alla mobilit\ue0 della frontiera petrolifera. Il lavoro utilizza il ruolo della scala (cartografica e
geografica) e le sue implicazioni cognitive per osservare ci\uf2 che sta accadendo attorno, vicino e all'interno della zona intangibile, da una visione continentale
al dettaglio della sua storia e della sua istituzione.
Gli strumenti cartografici e geografici ci aiutano a visualizzare l\u2019oggi e immaginare il domani, sapendo che il destino di questo angolo dell'Amazzonia
non \ue8 necessariamente definito: si tratta di una regione ad alta complessit\ue0 territoriale con la possibilit\ue0 di articolare una rete tra aree protette, territori
indigeni e corridoi ecologici culturali, alla ricerca di percorsi alternativi di sviluppo locale
AGN heating, thermal conduction and Sunyaev-Zeldovich effect in galaxy groups and clusters
(abridged) We investigate in detail the role of active galactic nuclei on the
physical state of the gas in galaxy groups and clusters, and the implications
for anisotropy in the CMB from Sunyaev-Zeldovich effect. We include the effect
of thermal conduction, and find that the resulting profiles of temperature and
entropy are consistent with observations. Unlike previously proposed models,
our model predicts that isentropic cores are not an inevitable consequence of
preheating. The model also reproduces the observational trend for the density
profiles to flatten in lower mass systems. We deduce the energy E_agn required
to explain the entropy observations as a function of mass of groups and
clusters M_cl and show that E_agn is proportional to M_cl^alpha with alpha~1.5.
We demonstrate that the entropy measurements, in conjunction with our model,
can be translated into constraints on the cluster--black hole mass relation.
The inferred relation is nonlinear and has the form M_bh\propto M_cl^alpha.
This scaling is an analog and extension of a similar relation between the black
hole mass and the galactic halo mass that holds on smaller scales. We show that
the central decrement of the CMB temperature is reduced due to the enhanced
entropy of the ICM, and that the decrement predicted from the plausible range
of energy input from the AGN is consistent with available data of SZ decrement.
We show that AGN heating, combined with the observational constraints on
entropy, leads to suppression of higher multipole moments in the angular power
spectrum and we find that this effect is stronger than previously thought.Comment: accepted for publication in The Astrophysical Journa
Discovery of an active supermassive black hole in the bulge-less galaxy NGC 4561
We present XMM-Newton observations of the Chandra-detected nuclear X-ray
source in NGC 4561. The hard X-ray spectrum can be described by a model
composed of an absorbed power-law with Gamma= 2.5^{+0.4}_{-0.3}, and column
density N_H=1.9^{+0.1}_{-0.2} times 10^{22} atoms cm^{-2}. The absorption
corrected luminosity of the source is L(0.2 - 10.0 keV) = 2.5 times 10^{41}
ergs s^{-1}, with bolometric luminosity over 3 \times 10^{42} ergs s^{-1}.
Based on the spectrum and the luminosity, we identify the nuclear X-ray source
in NGC 4561 to be an AGN, with a black hole of mass M_BH > 20,000 solar masses.
The presence of a supermassive black hole at the center of this bulge-less
galaxy shows that black hole masses are not necessarily related to bulge
properties, contrary to the general belief. Observations such as these call
into question several theoretical models of BH--galaxy co-evolution that are
based on merger-driven BH growth; secular processes clearly play an important
role. Several emission lines are detected in the soft X-ray spectrum of the
source which can be well parametrized by an absorbed diffuse thermal plasma
with non-solar abundances of some heavy elements. Similar soft X-ray emission
is observed in spectra of Seyfert 2 galaxies and low luminosity AGNs,
suggesting an origin in the circumnuclear plasma.Comment: To appear in Ap
Supermassive Black Holes and Galaxy Formation
The formation of supermassive black holes (SMBH) is intimately related to
galaxy formation, although precisely how remains a mystery. I speculate that
formation of, and feedback from, SMBH may alleviate problems that have arisen
in our understanding of the cores of dark halos of galaxies.Comment: Talk at conference on Matter in the Universe, March 2001, ISSI Ber
Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope
The detection of high energy (HE) {\gamma}-ray emission up to about 3 GeV
from the giant lobes of the radio galaxy Centaurus A has been recently reported
by the Fermi-LAT Collaboration based on ten months of all-sky survey
observations. A data set more than three times larger is used here to study the
morphology and photon spectrum of the lobes with higher statistics. The larger
data set results in the detection of HE {\gamma}-ray emission (up to about 6
GeV) from the lobes with a significance of more than 10 and 20 {\sigma} for the
North and the South lobe, respectively. Based on a detailed spatial analysis
and comparison with the associated radio lobes, we report evidence for a
substantial extension of the HE {\gamma}-ray emission beyond the WMAP radio
image in the case of the Northern lobe of Cen A. We reconstruct the spectral
energy distribution (SED) of the lobes using radio (WMAP) and Fermi-LAT data
from the same integration region. The implications are discussed in the context
of hadronic and leptonic scenarios
Ionized gas and stellar kinematics of seventeen nearby spiral galaxies
Ionized gas and stellar kinematics have been measured along the major axes of
seventeen nearby spiral galaxies of intermediate to late morphological type. We
discuss the properties of each sample galaxy distinguishing between those
characterized by regular or peculiar kinematics. In most of the observed
galaxies ionized gas rotates more rapidly than stars and have a lower velocity
dispersion, as is to be expected if the gas is confined in the disc and
supported by rotation while the stars are mostly supported by dynamical
pressure. In a few objects, gas and stars show almost the same rotational
velocity and low velocity dispersion, suggesting that their motion is dominated
by rotation.
Incorporating the spiral galaxies studied by Bertola et al. (1996), Corsini
et al. (1999, 2003) and Vega Beltran et al. (2001) we have compiled a sample of
50 S0/a-Scd galaxies, for which the major-axis kinematics of the ionized gas
and stars have been obtained with the same spatial (~1'') and spectral
(~50km/s) resolution, and measured with the same analysis techniques. This
allowed us to address the frequency of counterrotation in spiral galaxies. It
turns out that less than 12% and less than 8% (at the 95% confidence level) of
the sample galaxies host a counterrotating gaseous and stellar disc,
respectively. The comparison with S0 galaxies suggests that the retrograde
acquisition of small amounts of external gas gives rise to counterrotating
gaseous discs only in gas-poor S0s, while in gas-rich spirals the newly
acquired gas is swept away by the pre-existing gas. Counterrotating gaseous and
stellar discs in spirals are formed only from the retrograde acquisition of
large amounts of gas exceeding that of pre-existing gas, and subsequent star
formation, respectively.Comment: 14 pages, 33 figures, A&A accepte
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