1,398 research outputs found
Silver Nanoparticle Aggregates as Highly Efficient Plasmonic Antennas for Fluorescence Enhancement
The enhanced local fields around plasmonic structures can lead to enhancement of the excitation and modification of the emission quantum yield of fluorophores. So far, high enhancement of fluorescence intensity from dye molecules was demonstrated using bow-tie gap antenna made by e-beam lithography. However, the high manufacturing cost and the fact that currently there are no effective ways to place fluorophores only at the gap prevent the use of these structures for enhancing fluorescence-based biochemical assays. We report on the simultaneous modification of fluorescence intensity and lifetime of dye-labeled DNA in the presence of aggregated silver nanoparticles. The nanoparticle aggregates act as efficient plasmonic antennas, leading to more than 2 orders of magnitude enhancement of the average fluorescence. This is comparable to the best-reported fluorescence enhancement for a single molecule but here applies to the average signal detected from all fluorophores in the system. This highlights the remarkable efficiency of this system for surface-enhanced fluorescence. Moreover, we show that the fluorescence intensity enhancement varies with the plasmon resonance position and measure a significant reduction (300×) of the fluorescence lifetime. Both observations are shown to be in agreement with the electromagnetic model of surface-enhanced fluorescence
Star formation in mergers with cosmologically motivated initial conditions
We use semi-analytic models and cosmological merger trees to provide the
initial conditions for multi-merger numerical hydrodynamic simulations, and
exploit these simulations to explore the effect of galaxy interaction and
merging on star formation (SF). We compute numerical realisations of twelve
merger trees from z=1.5 to z=0. We include the effects of the large hot gaseous
halo around all galaxies, following recent obervations and predictions of
galaxy formation models. We find that including the hot gaseous halo has a
number of important effects. Firstly, as expected, the star formation rate on
long timescales is increased due to cooling of the hot halo and refuelling of
the cold gas reservoir. Secondly, we find that interactions do not always
increase the SF in the long term. This is partially due to the orbiting
galaxies transferring gravitational energy to the hot gaseous haloes and
raising their temperature. Finally we find that the relative size of the
starburst, when including the hot halo, is much smaller than previous studies
showed. Our simulations also show that the order and timing of interactions are
important for the evolution of a galaxy. When multiple galaxies interact at the
same time, the SF enhancement is less than when galaxies interact in series.
All these effects show the importance of including hot gas and cosmologically
motivated merger trees in galaxy evolution models.Comment: 19 pages, 15 figures, 6 tables. Accepted for publication in MNRA
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
From Discs to Bulges: effect of mergers on the morphology of galaxies
We study the effect of mergers on the morphology of galaxies by means of the
simulated merger tree approach first proposed by Moster et al. This method
combines N-body cosmological simulations and semi-analytic techniques to
extract realistic initial conditions for galaxy mergers. These are then evolved
using high resolution hydrodynamical simulations, which include dark matter,
stars, cold gas in the disc and hot gas in the halo. We show that the satellite
mass accretion is not as effective as previously thought, as there is
substantial stellar stripping before the final merger. The fraction of stellar
disc mass transferred to the bulge is quite low, even in the case of a major
merger, mainly due to the dispersion of part of the stellar disc mass into the
halo. We confirm the findings of Hopkins et al., that a gas rich disc is able
to survive major mergers more efficiently. The enhanced star formation
associated with the merger is not localised to the bulge of galaxy, but a
substantial fraction takes place in the disc too. The inclusion of the hot gas
reservoir in the galaxy model contributes to reducing the efficiency of bulge
formation. Overall, our findings suggest that mergers are not as efficient as
previously thought in transforming discs into bulges. This possibly alleviates
some of the tensions between observations of bulgeless galaxies and the
hierarchical scenario for structure formation.Comment: MNRAS Accepted, 17 pages, 11 figures, 3 Table
On the dependence of galaxy morphologies on galaxy mergers
The distribution of galaxy morphological types is a key test for models of
galaxy formation and evolution, providing strong constraints on the relative
contribution of different physical processes responsible for the growth of the
spheroidal components. In this paper, we make use of a suite of semi-analytic
models to study the efficiency of galaxy mergers in disrupting galaxy discs and
building galaxy bulges. In particular, we compare standard prescriptions
usually adopted in semi-analytic models, with new prescriptions proposed by
Kannan et al., based on results from high-resolution hydrodynamical
simulations, and we show that these new implementations reduce the efficiency
of bulge formation through mergers. In addition, we compare our model results
with a variety of observational measurements of the fraction of
spheroid-dominated galaxies as a function of stellar and halo mass, showing
that the present uncertainties in the data represent an important limitation to
our understanding of spheroid formation. Our results indicate that the main
tension between theoretical models and observations does not stem from the
survival of purely disc structures (i.e. bulgeless galaxies), rather from the
distribution of galaxies of different morphological types, as a function of
their stellar mass.Comment: MNRAS in press, 11 pages, 5 figure
Arabidopsis PEN3/PDR8, an ATP binding cassette transporter, contributes to nonhost resistance to inappropriate pathogens that enter by direct penetration
Strong Evolution in the Luminosity-Velocity Relation at z>1?
We present a method for constraining the evolution of the galaxy
luminosity-velocity (LV) relation in hierarchical scenarios of structure
formation. The comoving number density of dark-matter halos with circular
velocity of 200 km/s is predicted in favored CDM cosmologies to be nearly
constant over the redshift range 0<z<5. Any observed evolution in the density
of bright galaxies implies in turn a corresponding evolution in the LV
relation. We consider several possible forms of evolution for the zero-point of
the LV relation and predict the corresponding evolution in galaxy number
density. The Hubble Deep Field suggests a large deficit of bright (M_V < -19)
galaxies at 1.4 < z < 2. If taken at face value, this implies a dimming of the
LV zero-point by roughly 2 magnitudes. Deep, wide-field, near-IR selected
surveys will provide more secure measurements to compare with our predictions.Comment: 4 pages, 2 figures. Submitted to ApJ Letter
Second harmonic generation with zero phase velocity waves
We design a dual-band nonlinear composite right-left handed transmission line with phase-matching achieved between the fundamental frequency and second harmonic when both interacting waves have zero phase velocity. Additionally, we show that such a transmission line supports a new regime where the generation of backward second harmonic waves is achieved from a backward fundamental frequency wave
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