753 research outputs found
Employment from new firm formation in the Netherlands:Agglomeration economies and the Knowledge Spillover Theory of Entrepreneurship
Employment from new firm formation in the Netherlands:Agglomeration economies and the Knowledge Spillover Theory of Entrepreneurship
Are Compact High-Velocity Clouds Extragalactic Objects?
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the
Local Group model and would have HI volume densities of order 0.0003/cm^3.
Clouds with these volume densities and the observed neutral hydrogen column
densities will be largely ionized, even if exposed only to the extragalactic
ionizing radiation field. Here we examine the implications of this process for
models of CHVCs. We have modeled the ionization structure of spherical clouds
(with and without dark matter halos) for a large range of densities and sizes,
appropriate to CHVCs over the range of suggested distances, exposed to the
extragalactic ionizing photon flux. Constant-density cloud models in which the
CHVCs are at Local Group distances have total (ionized plus neutral) gas masses
roughly 20-30 times larger than the neutral gas masses, implying that the gas
mass alone of the observed population of CHVCs is about 40 billion solar
masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass
in such CHVCs is a significant fraction of the dynamical mass of the Local
Group, and their line widths would exceed the observed FWHM. Models with dark
matter halos fare even more poorly; they must lie within approximately 200 kpc
of the Galaxy. We show that exponential neutral hydrogen column density
profiles are a natural consequence of an external source of ionizing photons,
and argue that these profiles cannot be used to derive model-independent
distances to the CHVCs. These results argue strongly that the CHVCs are not
cosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa
Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond
The nitrogen-vacancy (N-V) center in diamond is a promising atomic-scale
system for solid-state quantum information processing. Its spin-dependent
photoluminescence has enabled sensitive measurements on single N-V centers,
such as: electron spin resonance, Rabi oscillations, single-shot spin readout
and two-qubit operations with a nearby 13C nuclear spin. Furthermore, room
temperature spin coherence times as long as 58 microseconds have been reported
for N-V center ensembles. Here, we have developed an angle-resolved
magneto-photoluminescence microscopy apparatus to investigate the anisotropic
electron spin interactions of single N-V centers at room temperature. We
observe negative peaks in the photoluminescence as a function of both magnetic
field magnitude and angle that are explained by coherent spin precession and
anisotropic relaxation at spin level anti-crossings. In addition, precise field
alignment unmasks the resonant coupling to neighboring dark nitrogen spins that
are not otherwise detected by photoluminescence. The latter results demonstrate
a means of investigating small numbers of dark spins via a single bright spin
under ambient conditions.Comment: 13 pages, 4 figure
Polarization-controlled optimal scatter suppression in transient absorption spectroscopy
Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by 33 52 â. (or 3 compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy
A new, kinematically anomalous HI component in the spiral galaxy NGC 2403
We discuss new, high sensitivity HI observations of the spiral galaxy NGC2403
which show extended emission at anomalous velocities with respect to the `cold'
disk. This `anomalous' gas component (~1/10 of the total HI mass) is probably
located in the region of the halo and rotates more slowly (~20-50 km/s) than
the gas in the disk. Moreover, it shows a distortion in the velocity field that
we interpret as a large-scale radial motion (10-20 km/s inflow) towards the
centre of the galaxy. The most likely explanation for its origin and kinematics
seems to be that of a galactic fountain. There is, however, a significant part
of the anomalous gas which seems to be moving contrary to rotation and is
difficult to understand in such a picture. These anomalous gas complexes
discovered in NGC 2403 may be analogous to the High Velocity Clouds of our
Galaxy. They may be rather common in spiral galaxies and not have been detected
yet for lack of sensitivity.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Bipolar-Hyper-Shell Galactic Center Statrburst Model: Further Evidence from ROSAT Data and New Radio and X-ray Simulations
Using the all-sky ROSAT soft X-ray and 408-MHz radio continuum data, we show
that the North Polar Spur and its western and southern counter-spurs draw a
giant dumbbell-shape necked at the galactic plane. We interpret these features
as due to a shock front originating from a starburst 15 million years ago with
a total energy of the order of ergs or type II
supernovae. We simulate all-sky distributions of radio continuum and soft X-ray
intensities based on the bipolar-hyper-shell galactic center starburst model.
The simulations can well reproduce the radio NPS and related spurs, as well as
radio spurs in the tangential directions of spiral arms. Simulated X-ray maps
in 0.25, 0.75 and 1.5 keV bands reproduce the ROSAT X-ray NPS, its western and
southern counter-spurs, and the absorption layer along the galactic plane. We
propose to use the ROSAT all-sky maps to probe the physics of gas in the
halo-intergalactic interface, and to directly date and measure the energy of a
recent Galactic Center starburst.Comment: To appear in ApJ, Latex MS in ApJ macro, 8 figures in jpg (original
quality ps figs available on request
The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. II. Fluid-Dynamical Models
We report a dynamical determination of the separate contributions of disk and
dark halo masses to the rotation curve of a spiral galaxy. We use
fluid-dynamical models of gas flow in the barred galaxy NGC 4123 to constrain
the dynamical properties of the galaxy: disk M/L, bar pattern speed, and the
central density and scale radius of the dark halo. We derive a realistic barred
potential directly from the light distribution. For each model we assume a
value of the stellar M/L and a bar pattern speed Omega_p and add a dark halo to
fit the rotation curve. We then compute the gas flow velocities with a 2-D gas
dynamical code, and compare the model flow patterns to a 2-D velocity field
derived from Fabry-Perot observations. The strong shocks and non-circular
motions in the observed gas flow require a high stellar M/L and a fast-rotating
bar. Models with I-band disk M/L of 2.0 -- 2.5 h_75, or 80 -- 100% of the
maximum disk value, are highly favored. The corotation radius of the bar must
be <= 1.5 times the bar semi-major axis. These results contradict some recent
claimed ``universal'' galaxy disk/halo relations, since NGC 4123 is of modest
size (rotation curve maximum 145 km/sec, and V_flat = 130 km/sec) yet is quite
disk-dominated. The dark halo of NGC 4123 is less concentrated than favored by
current models of dark halos based on cosmological simulations. Since some 30%
of bright disk galaxies are strongly barred and have dust lanes indicating
shock morphology similar to that of NGC 4123, it is likely that they also have
high stellar M/L and low density halos. We suggest that luminous matter
dominates inside the optical radius R_25 of high surface brightness disk
galaxies.Comment: accepted by ApJ, 20 pages, 11 figures (2 color), uses emulateapj.sty,
onecolfloat.st
Proper Motions in the Galactic Bulge: Plaut's Window
A proper motion study of a field of 20' x 20' inside Plaut's low extinction
window (l,b)=(0 deg,-8 deg), has been completed. Relative proper motions and
photographic BV photometry have been derived for ~21,000 stars reaching to
V~20.5 mag, based on the astrometric reduction of 43 photographic plates,
spanning over 21 years of epoch difference. Proper motion errors are typically
1 mas/yr and field dependent systematics are below 0.2 mas/yr.
Cross-referencing with the 2MASS catalog yielded a sample of ~8,700 stars, from
which predominantly disk and bulge subsamples were selected photometrically
from the JH color-magnitude diagram. The two samples exhibited different
proper-motion distributions, with the disk displaying the expected reflex solar
motion as a function of magnitude. Galactic rotation was also detected for
stars between ~2 and ~3 kpc from us. The bulge sample, represented by red
giants, has an intrinsic proper motion dispersion of (sigma_l,sigma_b)=(3.39,
2.91)+/-(0.11,0.09) mas/yr, which is in good agreement with previous results,
and indicates a velocity anisotropy consistent with either rotational
broadening or tri-axiality. A mean distance of 6.37^{+0.87}_{-0.77} kpc has
been estimated for the bulge sample, based on the observed K magnitude of the
horizontal branch red clump. The metallicity [M/H] distribution was also
obtained for a subsample of 60 bulge giants stars, based on calibrated
photometric indices. The observed [M/H] shows a peak value at [M/H]~-0.1 with
an extended metal poor tail and around 30% of the stars with supersolar
metallicity. No change in proper motion dispersion was observed as a function
of [M/H]. We are currently in the process of obtaining CCD UBVRI photometry for
the entire proper-motion sample of ~21,000 stars.Comment: Submitted to AJ April 17th 2007. Accepted June 8th 2007. 45 pages, 14
figure
Structure, mass and stability of galactic disks
In this review I concentrate on three areas related to structure of disks in
spiral galaxies. First I will review the work on structure, kinematics and
dynamics of stellar disks. Next I will review the progress in the area of
flaring of HI layers. These subjects are relevant for the presence of dark
matter and lead to the conclusion that disk are in general not `maximal', have
lower M/L ratios than previously suspected and are locally stable w.r.t.
Toomre's Q criterion for local stability. I will end with a few words on
`truncations' in stellar disks.Comment: Invited review at "Galaxies and their Masks" for Ken Freeman's 70-th
birthday, Sossusvlei, Namibia, April 2010. A version with high-res. figures
is available at
http://www.astro.rug.nl/~vdkruit/jea3/homepage/Namibiachapter.pd
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