4,772 research outputs found
Level repulsion in hybrid photonic-plasmonic microresonators for enhanced biodetection
We theoretically analyse photonic-plasmonic coupling between a high Q
whispering gallery mode (WGM) resonator and a core-shell nanoparticle. Blue and
red shifts of WGM resonances are shown to arise from crossing of the photonic
and plasmonic modes. Level repulsion in the hybrid system is further seen to
enable sensitivity enhancements in WGM sensors: maximal when the two resonators
are detuned by half the plasmon linewidth. Approximate bounds are given to
quantify possible enhancements. Criteria for reactive vs. resistive coupling
are also established
The quenching of star formation in accretion-driven clumpy turbulent tori of active galactic nuclei
Galactic gas-gas collisions involving a turbulent multiphase ISM share common
ISM properties: dense extraplanar gas visible in CO, large linewidths (>= 50
km/s), strong mid-infrared H_2 line emission, low star formation activity, and
strong radio continuum emission. Gas-gas collisions can occur in the form of
ICM ram pressure stripping, galaxy head-on collisions, compression of the
intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the
galaxy group at a high velocity, or external gas accretion on an existing gas
torus in a galactic center. We suggest that the common theme of all these
gas-gas interactions is adiabatic compression of the ISM leading to an increase
of the turbulent velocity dispersion of the gas. The turbulent gas clouds are
then overpressured and star formation is quenched. Within this scenario we
developed a model for turbulent clumpy gas disks where the energy to drive
turbulence is supplied by external infall or the gain of potential energy by
radial gas accretion within the disk. The cloud size is determined by the size
of a C-type shock propagating in dense molecular clouds with a low ionization
fraction at a given velocity dispersion. We give expressions for the expected
volume and area filling factors, mass, density, column density, and velocity
dispersion of the clouds. The latter is based on scaling relations of
intermittent turbulence whose open parameters are estimated for the CND in the
Galactic Center. The properties of the model gas clouds and the external mass
accretion rate necessary for the quenching of the star formation rate due to
adiabatic compression are consistent with those derived from high-resolution
H_2 line observations. Based on these findings, a scenario for the evolution of
gas tori in galactic centers is proposed and the implications for star
formation in the Galactic Center are discussed.Comment: 13 pages, 1 figure, accepted for publication by A&
Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438
Ram pressure stripping of the multiphase ISM is studied in the perturbed
Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction
~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA
radio continuum observations at 6 and 20 cm are presented. We detect prominent
extraplanar emission to the west of the galactic center, which extends twice as
far as the other tracers of extraplanar material. The spectral index of the
extraplanar emission does not steepen with increasing distance from the galaxy.
This implies in situ re-acceleration of relativistic electrons. The comparison
with multiwavelength observations shows that the magnetic field and the warm
ionized interstellar medium traced by Halpha emission are closely linked. The
kinematics of the northern extraplanar Halpha emission, which is ascribed to
star formation, follow those of the extraplanar CO emission. In the western and
southern extraplanar regions, the Halpha measured velocities are greater than
those of the CO lines. We suggest that the ionized gas of this region is
excited by ram pressure. The spatial and velocity offsets are consistent with a
scenario where the diffuse ionized gas is more efficiently pushed by ram
pressure stripping than the neutral gas. We suggest that the recently found
radio-deficient regions compared to 24 mum emission are due to this difference
in stripping efficiency.Comment: 8 pages, 6 figures, A&A, accepted for publicatio
The flaring HI disk of the nearby spiral galaxy NGC 2683
New deep VLA D array HI observations of the highly inclined nearby spiral
galaxy NGC 2683 are presented. Archival C array data were processed and added
to the new observations. To investigate the 3D structure of the atomic gas
disk, we made different 3D models for which we produced model HI data cubes.
The main ingredients of our best-fit model are (i) a thin disk inclined by 80
degrees; (ii) a crude approximation of a spiral and/or bar structure by an
elliptical surface density distribution of the gas disk; (iii) a slight warp in
inclination; (iv) an exponential flare; and (v) a low surface-density gas ring.
The slope of NGC 2683's flare is comparable, but somewhat steeper than those of
other spiral galaxies. NGC 2683's maximum height of the flare is also
comparable to those of other galaxies. On the other hand, a saturation of the
flare is only observed in NGC 2683. Based on the comparison between the high
resolution model and observations, we exclude the existence of an extended
atomic gas halo around the optical and thin gas disk. Under the assumption of
vertical hydrostatic equilibrium we derive the vertical velocity dispersion of
the gas. The high turbulent velocity dispersion in the flare can be explained
by energy injection by (i) supernovae, (ii) magneto-rotational instabilities,
(iii) ISM stirring by dark matter substructure, or (iv) external gas accretion.
The existence of the complex large-scale warping and asymmetries favors
external gas accretion as one of the major energy sources that drives
turbulence in the outer gas disk. We propose a scenario where this external
accretion leads to turbulent adiabatic compression that enhances the turbulent
velocity dispersion and might quench star formation in the outer gas disk of
NGC 2683.Comment: accepted for publication in A&
Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501
VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The
HI disk is sharply truncated to the southwest, well within the stellar disk. A
region of low surface-density gas, which is more extended than the main HI
disk, is discovered northeast of the galaxy center. These data are compared to
existing 6cm polarized radio continuum emission, Halpha, and optical broad band
images. We observe a coincidence between the western HI and polarized emission
edges, on the one hand, and a faint Halpha emission ridge, on the other. The
polarized emission maxima are located within the gaps between the spiral arms
and the faint Halpha ridge. Based on the comparison of these observations with
a sample of dynamical simulations with different values for maximum ram
pressure and different inclination angles between the disk and the orbital
plane,we conclude that ram pressure stripping can account for the main observed
characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and
is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The
southwestern ridge of enhanced gas surface density and enhanced polarized
radio-continuum emission is due to ram pressure compression. It is argued that
the faint western Halpha emission ridge is induced by nearly edge-on ram
pressure stripping. NGC 4501 represents an especially clear example of early
stage ram pressure stripping of a large cluster-spiral galaxy.Comment: 22 pages, 25 figures, accepted for publication in A&
The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
The influence of the environment on the polarized and total power radio
continuum emission of cluster spiral galaxies is investigated. We present deep
scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo
spiral galaxies. These data are combined with existing optical, HI, and Halpha
data. Ram pressure compression leads to sharp edges of the total power
distribution at one side of the galactic disk. These edges coincide with HI
edges. In edge-on galaxies the extraplanar radio emission can extend further
than the HI emission. In the same galaxies asymmetric gradients in the degree
of polarization give additional information on the ram pressure wind direction.
The local total power emission is not sensitive to the effects of ram pressure.
The radio continuum spectrum might flatten in the compressed region only for
very strong ram pressure. This implies that neither the local star formation
rate nor the turbulent small-scale magnetic field are significantly affected by
ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc)
and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&
Genebank ‐ in vitro propagation of potato and sweetpotato. CIP‐SOP056 V 3.0
This procedure describes the in vitro multiplication of potato and sweetpotato germplasm for international and national germplasm distribution, as well as, in vitro conservation, phytosanitary, and cryopreservation activities
Hot gas in Mach cones around Virgo Cluster spiral galaxies
The detailed comparison between observations and simulations of ram pressure
stripped spiral galaxies in the Virgo cluster has led to a three dimensional
view of the galaxy orbits within the hot intracluster medium. The 3D velocities
and Mach numbers derived from simulations can be used to derive simple Mach
cone geometries for Virgo spiral galaxies. We search for indications of hot gas
within Mach cones in X-ray observations of selected Virgo Cluster spiral
galaxies (NGC 4569, NGC 4388, and NGC 4501). We find extraplanar diffuse X-ray
emission in all galaxies. Based on the 3D velocity vectors from dynamical
modelling a simple Mach cone is fitted to the triangular shape of NGC 4569's
diffuse X-ray emission. Assuming that all extraplanar diffuse X-ray emission
has to be located inside the Mach cone, we also fit Mach cones to NGC 4388's
and NGC 4501's extraplanar X-ray emission. For NGC 4569 it is hard to reconcile
the derived Mach cone opening angle with a Mach number based on the sound speed
alone. Instead, a Mach number involving the Alfv\'enic speed seems to be more
appropriate, yielding a magnetic field strength of -6 G for a
intracluster medium density of cm. Whereas the
temperature of the hot component of NGC 4569's X-ray halo (0.5 keV) is at the
high end but typical for a galactic outflow, the temperature of the hot gas
tails of NGC 4388 and NGC 4501 are significantly hotter (0.7-0.9 keV). In NGC
4569 we find direct evidence for a Mach cone which is filled with hot gas from
a galactic superwind. We suggest that the high gas temperatures in the X-ray
tails of NGC 4388 and NGC 4501 are due to the mixing of the stripped ISM into
the hot intracluster medium of the Virgo cluster.Comment: 11 pages, 10 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysic
Dislocation-mediated growth of bacterial cell walls
Recent experiments have illuminated a remarkable growth mechanism of
rod-shaped bacteria: proteins associated with cell wall extension move at
constant velocity in circles oriented approximately along the cell
circumference (Garner et al., Science (2011), Dominguez-Escobar et al. Science
(2011), van Teeffelen et al. PNAS (2011). We view these as dislocations in the
partially ordered peptidoglycan structure, activated by glycan strand extension
machinery, and study theoretically the dynamics of these interacting defects on
the surface of a cylinder. Generation and motion of these interacting defects
lead to surprising effects arising from the cylindrical geometry, with
important implications for growth. We also discuss how long range elastic
interactions and turgor pressure affect the dynamics of the fraction of
actively moving dislocations in the bacterial cell wall.Comment: to appear in PNA
- …