151 research outputs found
Development and Implementation of In-Focus Phase Contrast TEM for Materials and Life Sciences
Transmission electron microscopy (TEM) allows the imaging of objects from materials sciences and biology with a resolution of a few nm to a few Ă
. Biological systems can be reconstructed in 3D under physiological conditions using cryo TEM. However, the low signal-to-noise ratio (SNR) of individual images hampers resolution. Weak-phase objects such as native biological samples can be visualized only by phase contrast, which is generated in conventional TEM by defocusing. The defocus technique yields weak contrast and incomplete transfer of object information, which makes reconstruction difficult. Therefore, generating contrast by placing a phase plate in the back focal plane of the objective lens is desirable. This allows for artefact-free imaging of weak-phase objects with strong contrast. For TEM, phase plates have been realised only in the form of a thin carbon film which causes loss of signal and resolution. This work presents the first technical implementation of an electrostatic (Boersch) phase plate for TEM and gives the experimental proof-of-principle for this device. The Boersch phase plate generates maximum phase contrast while avoiding resolution loss. It consists of a miniaturised electrostatic einzel lens that shifts the phase of the unscattered wave. Obstruction effects are minimised by optically magnifying the focal plane. The advancement to an entirely obstruction-free phase plate is outlined which generates pseudo-topographic (Hilbert) contrast. The enhancement of electron-microscopic reconstruction by such a Hilbert phase plate is demonstrated for electron tomography of unstained cell sections
Metallicity Gradients and Gas Flows in Galaxy Pairs
We present the first systematic investigation into the metallicity gradients
in galaxy close pairs. We determine the metallicity gradients for 8 galaxies in
close pairs using HII region metallicities obtained with high signal-to-noise
multi-slit observations with the Keck LRIS Spectrograph. We show that the
metallicity gradients in close pairs are significantly shallower than gradients
in isolated spiral galaxies such as the Milky Way, M83, and M101. These
observations provide the first solid evidence that metallicity gradients in
interacting galaxies are systematically different from metallicity gradients in
isolated spiral galaxies. Our results suggest that there is a strong
relationship between metallicity gradients and the gas dynamics in galaxy
interactions and mergers.Comment: Accepted for publication in ApJL. 6 pages, 3 figures, 1 table.
Article with full resolution figures can be obtained from
http://www.ifa.hawaii.edu/~kewley/Gradients.pd
Type Ibc supernovae in disturbed galaxies: evidence for a top-heavy IMF
We compare the radial locations of 178 core-collapse supernovae to the R-band
and H alpha light distributions of their host galaxies. When the galaxies are
split into `disturbed' and `undisturbed' categories, a striking difference
emerges. The disturbed galaxies have a central excess of core-collapse
supernovae, and this excess is almost completely dominated by supernovae of
types Ib, Ic and Ib/c, whereas type II supernovae dominate in all other
environments. The difference cannot easily be explained by metallicity or
extinction effects, and thus we propose that this is direct evidence for a
stellar initial mass function that is strongly weighted towards high mass
stars, specifically in the central regions of disturbed galaxies.Comment: 22 pages, 5 figures, accepted for publication in Ap
Accounting for Stochastic Fluctuations when Analysing Integrated Light of Star Clusters. I: First Systematics
Star clusters are studied widely both as benchmarks for stellar evolution
models and in their own right. Cluster age and mass distributions within
galaxies are probes of star formation histories, and of cluster formation and
disruption processes. The vast majority of clusters in the Universe is small,
and it is well known that the integrated fluxes and colors have broad
probability distributions, due to small numbers of bright stars. This paper
goes beyond the description of predicted probability distributions, and
presents results of the analysis of cluster energy distributions in an
explicitly stochastic context. The method developed is Bayesian. It provides
posterior probability distributions in the age-mass-extinction space, using
multi-wavelength photometric observations and a large collection of Monte-Carlo
simulations of clusters of finite stellar masses. Both UBVI and UBVIK datasets
are considered, and the study conducted in this paper is restricted to the
solar metallicity. We first reassess and explain errors arising from the use of
standard analysis methods, which are based on continuous population synthesis
models: systematic errors on ages and random errors on masses are large, while
systematic errors on masses tend to be smaller. The age-mass distributions
obtained after analysis of a synthetic sample are very similar to those found
for real galaxies in the literature. The Bayesian approach on the other hand,
is very successful in recovering the input ages and masses. Taking stochastic
effects into account is important, more important for instance than the choice
of adding or removing near-IR data in many cases. We found no immediately
obvious reason to reject priors inspired by previous (standard) analyses of
cluster populations in galaxies, i.e. cluster distributions that scale with
mass as M^-2 and are uniform on a logarithmic age scale.Comment: 17 pages, 13 figures, Accepted for publication in A&A
Fully compressive tides in galaxy mergers
The disruptive effect of galactic tides is a textbook example of
gravitational dynamics. However, depending on the shape of the potential, tides
can also become fully compressive. When that is the case, they might trigger or
strengthen the formation of galactic substructures (star clusters, tidal dwarf
galaxies), instead of destroying them. We perform N-body simulations of
interacting galaxies to quantify this effect. We demonstrate that tidal
compression occurs repeatedly during a galaxy merger, independently of the
specific choice of parameterization. With a model tailored to the Antennae
galaxies, we show that the distribution of compressive tides matches the
locations and timescales of observed substructures. After extending our study
to a broad range of parameters, we conclude that neither the importance of the
compressive tides (~15% of the stellar mass) nor their duration (~ 10 Myr) are
strongly affected by changes in the progenitors' configurations and orbits.
Moreover, we show that individual clumps of matter can enter compressive
regions several times in the course of a simulation. We speculate that this may
spawn multiple star formation episodes in some star clusters, through e.g.,
enhanced gas retention.Comment: 17 pages, 16 figures, accepted for publication in Ap
A Multiwavelength View of Star Formation in Interacting Galaxies in the Pavo Group
We combine Spitzer IRAC mid-infrared (MIR) and Chandra X-ray observations of
the dominant galaxies NGC6872 and NGC6876 in the Pavo group with archival
optical and HI data to study interaction-induced star formation. In spiral
galaxy NGC6872, 8.0 and 5.8 micron nonstellar emission having colors consistent
with polycyclic aromatic hydrocarbons (PAHs) is concentrated in clumps in three
regions: in a 5 kpc radius outer ring about the center of the spiral galaxy, in
a bridge of emission connecting NGC6872's northern spiral arm to IC4970, and
along the full extent of NGC6872's tidal arms. PAH emission is correlated with
young star clusters and dense HI regions. We find no strong differences in the
MIR colors of star-forming regions in the spiral galaxy NGC6872 as a function
of position relative to the tidally interacting companion galaxy IC4970. We
find 11 very luminous X-ray sources (>~ (0.5 - 5) x 10^{39} ergs/s) clustered
to the southwest in NGC6872, near bright star-forming regions. In NGC6872's
tidal features, young star clusters form at the boundaries of diffuse X-ray
gas, suggesting that stars form as gas stripped by the interactions cools. The
nucleus of NGC6872 is a weak X-ray point source (0.5-8 keV luminosity of 8.5 x
10^{39} ergs/s), but there is little evidence in the inner 1 kpc of NGC6872 for
PAH emission from recent star formation or nuclear activity. However, a 4 kpc
`stream', leading from the outer ring of NGC6872 to the nucleus, may signal
transport of interstellar matter into NGC6872's nuclear region. Nonstellar
emission, consistent with PAH emission, is also found in the central region of
elliptical galaxy NGC6877, companion to dominant Pavo group elliptical NGC6876.
However, in the central region of NGC6876, the dust emission is more likely due
to silicate emission from old AGB stars.Comment: 17 pages, 24 figures, ApJ, accepte
A comprehensive HST BVI catalogue of star clusters in five Hickson compact groups of galaxies
We present a photometric catalogue of star cluster candidates in Hickson compact groups (HCGs) 7, 31, 42, 59, and 92, based on observations with the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope. The catalogue contains precise cluster positions (right ascension and declination), magnitudes, and colours in the BVI filters. The number of detected sources ranges from 2200 to 5600 per group, from which we construct the high-confidence sample by applying a number of criteria designed to reduce foreground and background contaminants. Furthermore, the high-confidence cluster candidates for each of the 16 galaxies in our sample are split into two subpopulations: one that may contain young star clusters and one that is dominated by globular older clusters. The ratio of young star cluster to globular cluster candidates varies from group to group, from equal numbers to the extreme of HCG 31 which has a ratio of 8 to 1, due to a recent starburst induced by interactions in the group. We find that the number of blue clusters with MV < â9 correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with MV < â7.8 correlates well (though with large scatter) with the stellar mass. Analyses of the high-confidence sample presented in this paper show that star clusters can be successfully used to infer the gross star formation history of the host groups and therefore determine their placement in a proposed evolutionary sequence for compact galaxy groups
Solid State Fluorination on the Minute Scale: Synthesis of WOâââFx with Photocatalytic Activity
Solid state reactions are notoriously slow, because the rateâlimiting step is diffusion of atoms or ions through reactant, intermediate, and product crystalline phases. This requires days or even weeks of high temperature treatment, consuming large amounts of energy. Metal oxides are particularly difficult to react, because they have high melting points. The study reports a highâspeed solid state fluorination of WOâ with Teflon to the oxyfluorides WOâââFx on a minute (<10 min) scale by spark plasma sintering, a technique that is used typically for a highâspeed consolidation of powders. Automated electron diffraction analysis reveals an orthorhombic ReOââtype structure of WOâââFx with F atom disorder as demonstrated by ÂčâčF magic angle spinning nuclear magnetic resonance spectroscopy. The potential of this new approach is demonstrated by the following results. i) Mixedâ valent tungsten oxide fluorides WOâââFx with high F content (0 < x < 0.65) are obtained as metastable products in copious amounts within minutes. ii) The spark plasma sintering technique yields WOâââFx nanoparticles with high photocatalytic activity, whereas the corresponding bulk phases obtained by conventional solidâstate (ampoule) reactions have no photocatalytic activity. iii) The catalytic activity is caused by the microstructure originating from the processing by spark plasma sintering
Galaxy Collisions - Dawn of a New Era
The study of colliding galaxies has progressed rapidly in the last few years,
driven by observations with powerful new ground and space-based instruments.
These instruments have used for detailed studies of specific nearby systems,
statistical studies of large samples of relatively nearby systems, and
increasingly large samples of high redshift systems. Following a brief summary
of the historical context, this review attempts to integrate these studies to
address the following key issues. What role do collisions play in galaxy
evolution, and how can recently discovered processes like downsizing resolve
some apparently contradictory results of high redshift studies? What is the
role of environment in galaxy collisions? How is star formation and nuclear
activity orchestrated by the large scale dynamics, before and during merger?
Are novel modes of star formation involved? What are we to make of the
association of ultraluminous X-ray sources with colliding galaxies? To what do
degree do mergers and feedback trigger long-term secular effects? How far can
we push the archaeology of individual systems to determine the nature of
precursor systems and the precise effect of the interaction? Tentative answers
to many of these questions have been suggested, and the prospects for answering
most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update
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