333 research outputs found
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
BLAST05: Power Spectra of Bright Galactic Cirrus at Submillimeter Wavelengths
We report multi-wavelength power spectra of diffuse Galactic dust emission
from BLAST observations at 250, 350, and 500 microns in Galactic Plane fields
in Cygnus X and Aquila. These submillimeter power spectra statistically
quantify the self-similar structure observable over a broad range of scales and
can be used to assess the cirrus noise which limits the detection of faint
point sources. The advent of submillimeter surveys with the Herschel Space
Observatory makes the wavelength dependence a matter of interest. We show that
the observed relative amplitudes of the power spectra can be related through a
spectral energy distribution (SED). Fitting a simple modified black body to
this SED, we find the dust temperature in Cygnus X to be 19.9 +/- 1.3 K and in
the Aquila region 16.9 +/- 0.7 K. Our empirical estimates provide important new
insight into the substantial cirrus noise that will be encountered in
forthcoming observations.Comment: Submitted to the Astrophysical Journal. Maps and other data are
available at http://blastexperiment.info
Deconvolution of Images from BLAST 2005: Insight into the K3-50 and IC 5146 Star-Forming Regions
We present an implementation of the iterative flux-conserving Lucy-Richardson
(L-R) deconvolution method of image restoration for maps produced by the
Balloon-borne Large Aperture Submillimeter Telescope (BLAST). We have analyzed
its performance and convergence extensively through simulations and
cross-correlations of the deconvolved images with available highresolution
maps. We present new science results from two BLAST surveys, in the Galactic
regions K3-50 and IC 5146, further demonstrating the benefits of performing
this deconvolution.
We have resolved three clumps within a radius of 4.'5 inside the star-forming
molecular cloud containing K3-50. Combining the well-resolved dust emission map
with available multi-wavelength data, we have constrained the Spectral Energy
Distributions (SEDs) of five clumps to obtain masses (M), bolometric
luminosities (L), and dust temperatures (T). The L-M diagram has been used as a
diagnostic tool to estimate the evolutionary stages of the clumps. There are
close relationships between dust continuum emission and both 21-cm radio
continuum and 12CO molecular line emission.
The restored extended large scale structures in the Northern Streamer of IC
5146 have a strong spatial correlation with both SCUBA and high resolution
extinction images. A dust temperature of 12 K has been obtained for the central
filament. We report physical properties of ten compact sources, including six
associated protostars, by fitting SEDs to multi-wavelength data. All of these
compact sources are still quite cold (typical temperature below ~ 16 K) and are
above the critical Bonner-Ebert mass. They have associated low-power Young
Stellar Objects (YSOs). Further evidence for starless clumps has also been
found in the IC 5146 region.Comment: 13 pages, 12 Figures, 3 Table
The Galactic dynamics revealed by the filamentary structure in atomic hydrogen emission
Funding: J.D.S., R.K.S., and S.C.O.G. are funded by the European Research Council via the ERC Synergy Grant “ECOGAL – Understanding our Galactic ecosystem: From the disk of the Milky Way to the formation sites of stars and planets” (project ID 855130). R.J.S. acknowledges funding from an STFC ERF (grant ST/N00485X/1) and HPC from the DiRAC facility (ST/P002293/1).We present a study of the filamentary structure in the atomic hydrogen (HI) emission at the 21 cm wavelength toward the Galactic plane using the observations in the HI4PI survey. Using the Hessian matrix method across radial velocity channels, we identified the filamentary structures and quantified their orientations using circular statistics. We found that the regions of the Milky Way's disk beyond 10 kpc and up to roughly 18 kpc from the Galactic center display HI filamentary structures predominantly parallel to the Galactic plane. For regions at lower Galactocentric radii, we found that the HI filaments are mostly perpendicular or do not have a preferred orientation with respect to the Galactic plane. We interpret these results as the imprint of supernova feedback in the inner Galaxy and Galactic rotation in the outer Milky Way. We found that the HI filamentary structures follow the Galactic warp and that they highlight some of the variations interpreted as the effect of the gravitational interaction with satellite galaxies. In addition, the mean scale height of the filamentary structures is lower than that sampled by the bulk of the HI emission, thus indicating that the cold and warm atomic hydrogen phases have different scale heights in the outer galaxy. Finally, we found that the fraction of the column density in HI filaments is almost constant up to approximately 18 kpc from the Galactic center. This is possibly a result of the roughly constant ratio between the cold and warm atomic hydrogen phases inferred from the HI absorption studies. Our results indicate that the HI filamentary structures provide insight into the dynamical processes shaping the Galactic disk. Their orientations record how and where the stellar energy input, the Galactic fountain process, the cosmic ray diffusion, and the gas accretion have molded the diffuse interstellar medium in the Galactic plane.Peer reviewe
Observations of H1426+428 with HEGRA -- Observations in 2002 and reanalysis of 1999&2000 data
The HEGRA system of imaging air Cherenkov telescopes has been used to observe
the BL Lac object H1426+428 () for 217.5 hours in 2002. In this data
set alone, the source is detected at a confidence level of ,
confirming this object as a TeV source. The overall flux level during the
observations in 2002 is found to be a factor of lower than during
the previous observations by HEGRA in 1999&2000. A new spectral analysis has
been carried out, improving the signal-to-noise ratio at the expense of a
slightly increased systematic uncertainty and reducing the relative energy
resolution to over a wide range of energies. The new
method has also been applied to the previously published data set taken in 1999
and 2000, confirming the earlier claim of a flattening of the energy spectrum
between 1 and 5 TeV. The data set taken in 2002 shows again a signal at
energies above 1 TeV. We combine the energy spectra as determined by the CAT
and VERITAS groups with our reanalyzed result of the 1999&2000 data set and
apply a correction to account for effects of absorption of high energy photons
on extragalactic background light in the optical to mid infrared band. The
shape of the inferred source spectrum is mostly sensitive to the
characteristics of the extragalactic background light between wavelengths of 1
and 15~mComment: 12 pages, 4 Figures, submitted to A&
BLAST: Correlations in the Cosmic Far-Infrared Background at 250, 350, and 500 microns Reveal Clustering of Star-Forming Galaxies
We detect correlations in the cosmic far-infrared background due to the
clustering of star-forming galaxies in observations made with the Balloon-borne
Large Aperture Submillimeter Telescope, BLAST, at 250, 350, and 500 microns. We
perform jackknife and other tests to confirm the reality of the signal. The
measured correlations are well fit by a power law over scales of 5-25
arcminutes, with Delta I/I = 15.1 +/- 1.7%. We adopt a specific model for
submillimeter sources in which the contribution to clustering comes from
sources in the redshift ranges 1.3 <= z <= 2.2, 1.5 <= z <= 2.7, and 1.7 <= z
<= 3.2, at 250, 350, and 500 microns, respectively. With these distributions,
our measurement of the power spectrum, P(k_theta), corresponds to linear bias
parameters, b = 3.8 +/- 0.6, 3.9 +/- 0.6 and 4.4 +/- 0.7, respectively. We
further interpret the results in terms of the halo model, and find that at the
smaller scales, the simplest halo model fails to fit our results. One way to
improve the fit is to increase the radius at which dark matter halos are
artificially truncated in the model, which is equivalent to having some
star-forming galaxies at z >= 1 located in the outskirts of groups and
clusters. In the context of this model we find a minimum halo mass required to
host a galaxy is log (M_min / M_sun) = 11.5 (+0.4/-0.1), and we derive
effective biases $b_eff = 2.2 +/- 0.2, 2.4 +/- 0.2, and 2.6 +/- 0.2, and
effective masses log (M_eff / M_sun) = 12.9 +/- 0.3, 12.8 +/- 0.2, and 12.7 +/-
0.2, at 250, 350, and 500 microns, corresponding to spatial correlation lengths
of r_0 = 4.9, 5.0, and 5.2 +/- 0.7 h^-1 Mpc, respectively. Finally, we discuss
implications for clustering measurement strategies with Herschel and Planck.Comment: Accepted for publication in the Astrophysical Journal. Maps and other
results available at http://blastexperiment.info
Angle-dependent radiative grain alignment; Confirmation of a magnetic field - radiation anisotropy angle dependence on the efficiency of interstellar grain alignment
Interstellar grain alignment studies are currently experiencing a renaissance
due to the development of a new quantitative theory based on Radiative
Alignment Torques (RAT). One of the distinguishing predictions of this theory
is a dependence of the grain alignment efficiency on the relative angle
() between the magnetic field and the anisotropy direction of the
radiation field. In an earlier study we found observational evidence for such
an effect from observations of the polarization around the star HD 97300 in the
Chamaeleon I cloud. However, due to the large uncertainties in the measured
visual extinctions, the result was uncertain.
By acquiring explicit spectral classification of the polarization targets, we
have sought to perform a more precise reanalysis of the existing polarimetry
data.
We have obtained new spectral types for the stars in our for our polarization
sample, which we combine with photometric data from the literature to derive
accurate visual extinctions for our sample of background field stars. This
allows a high accuracy test of the grain alignment efficiency as a function of
.
We confirm and improve the measured accuracy of the variability of the grain
alignment efficiency with , seen in the earlier study. We note that the
grain temperature (heating) also shows a dependence on which we
interpret as a natural effect of the projection of the grain surface to the
illuminating radiation source. This dependence also allows us to derive an
estimate of the fraction of aligned grains in the cloud.Comment: 7 pages, 4 figures Accepted for publication in A&
Planck Intermediate Results. IV. The XMM-Newton validation programme for new Planck galaxy clusters
We present the final results from the XMM-Newton validation follow-up of new
Planck galaxy cluster candidates. We observed 15 new candidates, detected with
signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck
survey. The candidates were selected using ancillary data flags derived from
the ROSAT All Sky Survey (RASS) and Digitized Sky Survey all-sky maps, with the
aim of pushing into the low SZ flux, high-z regime and testing RASS flags as
indicators of candidate reliability. 14 new clusters were detected by XMM,
including 2 double systems. Redshifts lie in the range 0.2 to 0.9, with 6
clusters at z>0.5. Estimated M500 range from 2.5 10^14 to 8 10^14 Msun. We
discuss our results in the context of the full XMM validation programme, in
which 51 new clusters have been detected. This includes 4 double and 2 triple
systems, some of which are chance projections on the sky of clusters at
different z. We find that association with a RASS-BSC source is a robust
indicator of the reliability of a candidate, whereas association with a FSC
source does not guarantee that the SZ candidate is a bona fide cluster.
Nevertheless, most Planck clusters appear in RASS maps, with a significance
greater than 2 sigma being a good indication that the candidate is a real
cluster. The full sample gives a Planck sensitivity threshold of Y500 ~ 4 10^-4
arcmin^2, with indication for Malmquist bias in the YX-Y500 relation below this
level. The corresponding mass threshold depends on z. Systems with M500 > 5
10^14 Msun at z > 0.5 are easily detectable with Planck. The newly-detected
clusters follow the YX-Y500 relation derived from X-ray selected samples.
Compared to X-ray selected clusters, the new SZ clusters have a lower X-ray
luminosity on average for their mass. There is no indication of departure from
standard self-similar evolution in the X-ray versus SZ scaling properties.
(abridged)Comment: accepted by A&
Planck 2015 results. XXIII. The thermal Sunyaev-Zeldovich effect--cosmic infrared background correlation
We use Planck data to detect the cross-correlation between the thermal
Sunyaev-Zeldovich (tSZ) effect and the infrared emission from the galaxies that
make up the the cosmic infrared background (CIB). We first perform a stacking
analysis towards Planck-confirmed galaxy clusters. We detect infrared emission
produced by dusty galaxies inside these clusters and demonstrate that the
infrared emission is about 50% more extended than the tSZ effect. Modelling the
emission with a Navarro--Frenk--White profile, we find that the radial profile
concentration parameter is . This indicates
that infrared galaxies in the outskirts of clusters have higher infrared flux
than cluster-core galaxies. We also study the cross-correlation between tSZ and
CIB anisotropies, following three alternative approaches based on power
spectrum analyses: (i) using a catalogue of confirmed clusters detected in
Planck data; (ii) using an all-sky tSZ map built from Planck frequency maps;
and (iii) using cross-spectra between Planck frequency maps. With the three
different methods, we detect the tSZ-CIB cross-power spectrum at significance
levels of (i) 6 , (ii) 3 , and (iii) 4 . We model the
tSZ-CIB cross-correlation signature and compare predictions with the
measurements. The amplitude of the cross-correlation relative to the fiducial
model is . This result is consistent with
predictions for the tSZ-CIB cross-correlation assuming the best-fit
cosmological model from Planck 2015 results along with the tSZ and CIB scaling
relations.Comment: 18 pages, 16 figure
Evidence for Environmental Changes in the Submillimeter Dust Opacity
The submillimeter opacity of dust in the diffuse Galactic interstellar medium
(ISM) has been quantified using a pixel-by-pixel correlation of images of
continuum emission with a proxy for column density. We used three BLAST bands
at 250, 350, and 500 \mu m and one IRAS at 100 \mu m. The proxy is the
near-infrared color excess, E(J-Ks), obtained from 2MASS. Based on observations
of stars, we show how well this color excess is correlated with the total
hydrogen column density for regions of moderate extinction. The ratio of
emission to column density, the emissivity, is then known from the
correlations, as a function of frequency. The spectral distribution of this
emissivity can be fit by a modified blackbody, whence the characteristic dust
temperature T and the desired opacity \sigma_e(1200) at 1200 GHz can be
obtained. We have analyzed 14 regions near the Galactic plane toward the Vela
molecular cloud, mostly selected to avoid regions of high column density (N_H >
10^{22} cm^-2) and small enough to ensure a uniform T. We find \sigma_e(1200)
is typically 2 to 4 x 10^{-25} cm^2/H and thus about 2 to 4 times larger than
the average value in the local high Galactic latitude diffuse atomic ISM. This
is strong evidence for grain evolution. There is a range in total power per H
nucleon absorbed (re-radiated) by the dust, reflecting changes in the
interstellar radiation field and/or the dust absorption opacity. These changes
affect the equilibrium T, which is typically 15 K, colder than at high
latitudes. Our analysis extends, to higher opacity and lower T, the trend of
increasing opacity with decreasing T that was found at high latitudes. The
recognition of changes in the emission opacity raises a cautionary flag because
all column densities deduced from dust emission maps, and the masses of compact
structures within them, depend inversely on the value adopted.Comment: Original version (22 Dec 2011): 14 pages, 8 figures. Revised version
(24 February 2012) accepted for publication in the Astrophysical Journal (14
March 2012): elaborated details of analysis, extended discussion including
new Appendix; abstract, results, conclusions unchanged. 16 pages, 9 figure
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