37,513 research outputs found
First-principles Calculations of Engineered Surface Spin Structures
The engineered spin structures recently built and measured in scanning
tunneling microscope experiments are calculated using density functional
theory. By determining the precise local structure around the surface
impurities, we find the Mn atoms can form molecular structures with the binding
surface, behaving like surface molecular magnets. The spin structures are
confirmed to be antiferromagnetic, and the exchange couplings are calculated
within 8% of the experimental values simply by collinear-spin GGA+U
calculations. We can also explain why the exchange couplings significantly
change with different impurity binding sites from the determined local
structure. The bond polarity is studied by calculating the atomic charges with
and without the Mn adatoms
Dust-to-gas ratio, factor and CO-dark gas in the Galactic anticentre: an observational study
We investigate the correlation between extinction and H~{\sc i} and CO
emission at intermediate and high Galactic latitudes (|b|>10\degr) within the
footprint of the Xuyi Schmidt Telescope Photometric Survey of the Galactic
anticentre (XSTPS-GAC) on small and large scales. In Paper I (Chen et al.
2014), we present a three-dimensional dust extinction map within the footprint
of XSTPS-GAC, covering a sky area of over 6,000\,deg at a spatial angular
resolution of 6\,arcmin. In the current work, the map is combined with data
from gas tracers, including H~{\sc i} data from the Galactic Arecibo L-band
Feed Array H~{\sc i} survey and CO data from the Planck mission, to constrain
the values of dust-to-gas ratio and CO-to-
conversion factor for the entire GAC
footprint excluding the Galactic plane, as well as for selected star-forming
regions (such as the Orion, Taurus and Perseus clouds) and a region of diffuse
gas in the northern Galactic hemisphere. For the whole GAC footprint, we find
\, and \,. We have also
investigated the distribution of "CO-dark" gas (DG) within the footprint of GAC
and found a linear correlation between the DG column density and the -band
extinction: . The mass fraction of DG is found to be toward
the Galactic anticentre, which is respectively about 23 and 124 per cent of the
atomic and CO-traced molecular gas in the same region. This result is
consistent with the theoretical work of Papadopoulos et al. but much larger
than that expected in the cloud models by Wolfire et al.Comment: 11 pages, 7 figures, accepted for publication in MNRA
XMM-Newton Observations of High Redshift Quasars
We report on our XMM observations of the high redshift quasars BR 2237--0607
(z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found
in the XMM public archive. Contrary to former reports, we do not find high
redshift radio-loud quasars to be more absorbed than their radio-quiet
counterparts. We find that the optical to X-ray spectral index alpha-ox is
correlated with the luminosity density at 2500 A, but does not show a
correlation with redshift. The mean 2-10 keV power-law slope of the 9 high
redshift radio-quiet quasars in our sample for which a spectral analysis can be
performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA
observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and
significantly different from alpha-x=0.78 found for low redshift Broad-Line
Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars
look remarkably similar, we find a first indication of a difference in their
X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios
L/L_Edd. These observations give credence to the hypothesis of Mathur (2000)
that NLS1s are low luminosity cousins of high redshift quasars, both likely to
be in their early evolutionary stage.Comment: 25 pages, AJ, in press (Jan 2006
Redshifted 21cm Signatures Around the Highest Redshift Quasars
The Ly-alpha absorption spectrum of the highest redshift quasars indicates
that they are surrounded by giant HII regions, a few Mpc in size. The neutral
gas around these HII regions should emit 21cm radiation in excess of the Cosmic
Microwave Background, and enable future radio telescopes to measure the
transverse extent of these HII regions. At early times, the HII regions expand
with a relativistic speed. Consequently, their measured sizes along the
line-of-sight (via Ly-alpha absorption) and transverse to it (via 21 cm
emission) should have different observed values due to relativistic time-delay.
We show that the combined measurement of these sizes would directly constrain
the neutral fraction of the surrounding intergalactic medium (IGM) as well as
the quasar lifetime. Based on current number counts of luminous quasars at z>6,
an instrument like LOFAR should detect >2 redshifted 21cm shells per field
(with a radius of 11 degrees) around active quasars as bright as those already
discovered by SDSS, and >200 relic shells of inactive quasars per field. We
show that Ly-alpha photons from the quasar are unable to heat the IGM or to
couple the spin and kinetic temperatures of atomic hydrogen beyond the edge of
the HII region. The detection of the IGM in 21cm emission around high redshift
quasars would therefore gauge the presence of a cosmic Ly-alpha background
during the reionization epoch.Comment: 11 pages, 6 figures. Submitted to Ap
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