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 Å. 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 < −9 correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with MV < −7.8 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 WO₃₋ₓFx with Photocatalytic Activity

Solid state reactions are notoriously slow, because the rate‐limiting 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 high‐speed solid state fluorination of WO₃ with Teflon to the oxyfluorides WO₃₋ₓFx on a minute (<10 min) scale by spark plasma sintering, a technique that is used typically for a high‐speed consolidation of powders. Automated electron diffraction analysis reveals an orthorhombic ReO₃‐type structure of WO₃₋ₓFx with F atom disorder as demonstrated by ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy. The potential of this new approach is demonstrated by the following results. i) Mixed‐ valent tungsten oxide fluorides WO₃₋ₓFx 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 WO₃₋ₓFx nanoparticles with high photocatalytic activity, whereas the corresponding bulk phases obtained by conventional solid‐state (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 Vol.