National Astronomy Meeting Poster Exhibition

Limb darkening

Solar limb darkening is a phenomenon observed on the disc of the sun where regions at greater radial distances emit less total radiance than the peak radiance emitted at the centre[1]. This effect is caused by an observer's line of sight passing through deeper, hotter, regions of the Sun's photosphere near the observed centre and only shallower, cooler, regions nearer the limb before the optical depth in each case reaches unity and renders greater depths inaccessible to the observer.

The radial change in observed solar radiance is not uniform as a function of wavelength as a greater degree of darkening occurs for shorter wavelength (higher energy) photons, which are emitted in greater relative proportions from the hotter regions of the photosphere[2].

UVIS Observations

This work presents an analysis of spectral radiance scans across the solar disc, ranging from ultraviolet to visible wavelengths (200-650~nm), taken during multiple observing windows over a period of approximately 2 years by the NOMAD instrument's UVIS spectrometer[3] aboard ESA's Trace Gas Orbiter spacecraft, currently in orbit around Mars.

The individual measurements comprising one of these complete scans form a 2-dimensional grid over the solar surface with parallel lines of closely separated measurements, these grids have a spatial resolution of 2~minutes of arc. Interpolated surface maps over individual observational grids and combined averaged maps are constructed from the observed radiances at various wavelengths representing the normalised surface radiance relative to the central peak intensity.

Surface fits against multiple limb darkening models[4] are performed yielding corresponding sets of limb darkening coefficients which characterise the magnitude of the observed effect, the mean observed spectral surface intensity and the darkening wavelength sensitivity over the full spectral range of the UVIS instrument.

[1] Sanchez-Bajo et al. (2002)
[2] Solanki et al. (2001)
[3] Patel et al. (2017)
[4] Claret et al. (2013)

The Proximal orbits of the Cassini spacecraft during 2017 have given us the opportunity to examine the auroral field-aligned currents in the northern hemisphere dawn sector in relation to wider magnetospheric conditions. Here we will combine the results of three recent studies on the properties of the auroral field-aligned current, magnetospheric ring current and the overall compressional state of Saturn’s magnetosphere due to solar wind conditions. Using these results, we will examine the response of the auroral field-aligned currents in combination with the magnetospheric ring current to compressions and expansions of the Saturnian magnetosphere. We will show that for a compression of Saturn’s magnetosphere the current within in the downward current sheet, located equatorward of the main auroral oval, increases in strength with increasing total ring current. While the inverse relation occurs during an interval of quiet or expanded magnetospheric conditions. We observe within the magnetic field region which carries the downward current that during compression events there is an increase in hot plasma intensity, in particular, in the protons (35-506 keV). This response is akin to an Earth-like ‘region 2’ field aligned current within Saturn’s dawn magnetosphere, with a partial nightside ring current closing via a downward current within the dawn sector. We will further discuss the implications of these observations for Saturn’s magnetospheric current systems and dynamics.

Particle acceleration can occur continuously over active regions, with keV electrons producing radio Type III noise storms from hours to days. What is not clear is whether all the particles escape the solar atmosphere, or whether some are confined to the corona with the MeV energy required to produce gyrosynchrotron emission. The type III noise storm observed by LOFAR on 10th April 2019 was followed by series of microflares detected within the 4-8 GHz range by Siberian Radioheliograph. We present the preliminary results of the study of the relationship between activity in microwave and radio ranges. We carried out the analysis of the quasiperiodicity observed in both frequency ranges and used data in MW and EUV for finding the possible source of the type III bursts.

The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Undergoing wave-particle interactions with Langmuir waves, these beams are the driver for type III radio bursts, the brightest astrophysical radio sources detected by humans. The formation and motion of type III fine frequency structures is a puzzle but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic, wave-particle simulations and high-resolution radio type III observations, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, both significantly expanding the current diagnostic potential of solar radio emission.

Astronomy for development is making great strides in Namibia. As a country renowned for its dark and pristine skies, Namibia offers the perfect landscape for the construction of ground-based observatories. The benefits of developing astrophysics infrastructure are not only limited to solving the mysteries of the Universe, however. Numerous research has shown that astronomy projects around the world lead to many other benefits – societal, cultural, economic, and environmental.

The High Energy Stereoscopic System (H.E.S.S.) is so far the first and only large-scale telescope to arrive in Namibia, in operation since 2002. Plans of building more telescopes in the country, such as the African Millimetre Telescope (AMT) and a part of the African Very Long Baseline Interferometry Network (AVN) of telescopes are currently underway.

The AMT will bring with it many more opportunities for capacity-building, and our multi-disciplinary collaboration is working on a Social Impact Plan to maximise the societal benefits brought about by the future observatory. This Plan takes a multi-disciplinary approach to setting the scene for astronomy and sustainable development in Namibia. Looking at education more closely, we outline plans for the Mobile Planetarium; the materials we will create for schools; teacher training; bringing international astronomy training programmes to Namibia; mentorship, scholarship, and fellowship programmes; and the inclusion of indigenous knowledge. In all, sustainability is our utmost priority and by incorporating these different efforts we hope to inspire a new generation of scientists in Namibia.

The star formation history of the Universe is fundamental to understand when studying the physics of galaxy evolution. Understanding this key property of the Universe requires the statistical study of large samples of galaxies through various redshifts.

Narrowband surveys have done wonders for the observation of line emitting galaxies by providing a blind and unbiased selection. Large samples of Ha, [OIII] and [OII] emitters have been found in this way and recently the Ha, [OIII] and [OII] luminosity functions (LFs) have become well-constrained, but now is the time to constraint the faint and bright ends in a more consistent way: with deep, wide area surveys.

Therefore, I will present the first results from Y-NBS, a large narrowband survey (1.06 um) conducted with HAWK-I/VLT which finds ~2000 line emitters between z~0.6-1.8, in the GOODS-S and COSMOS field. We combine an area of ~1 deg^2 down to a Ha luminosity limit of 10^43.0 erg/s, providing wide and deep coverage in order to explore the full range of the luminosity functions for Ha, [OIII] and [OII] in a consistent way.

We select our fully-corrected samples of Ha (z~0.6), [OIII] (z~1.1) and [OII] (z~1.8) emitting galaxies and present results of luminosity functions, further constraining our knowledge of how line emitters evolve. We put these data in the context of the cosmic star formation rate density evolution, thereby adding to our understanding of the evolution of galaxies and the Universe as a whole and constraining current models of star formation history. These Ha, [OIII] and [OII] are also ideal for future study with the upcoming MOONS instrument.

The morphological properties of galaxies are important tracers of the physical processes, e.g. minor/major mergers, gas accretion and tidal interactions, that have shaped their evolution. Forthcoming ‘Big data’ surveys (e.g. LSST/SKA), which will produce exabyte volumes of data will be the new ‘normal’ in this decade. These volumes will make morphological classification using traditional methods (e.g. direct visual inspection) impractical. Even semi-automated techniques, e.g. supervised machine learning with training sets built via visual inspection, may be difficult, because of the time-consuming nature of creating the training sets. However, unsupervised machine learning, does not require training sets, making it ideal for galaxy morphological classification for large surveys.

We present an unsupervised machine learning algorithm, that utilizes hierarchical clustering and growing neural gas networks to group together survey image patches with similar visual properties, followed by a clustering of objects (e.g. galaxies) that are reconstructed from these patches. We implement the algorithm on the Deep layer of the Hyper Suprime-Cam Subaru-Strategic-Program, to reduce a population of hundreds of thousands of galaxies to a small number (~100) of morphological clusters, which exhibit high purity. These clusters can then be rapidly benchmarked via visual inspection and classified by morphological type. Using these morphological clusters, we successfully reproduce many known trends of galaxy properties (e.g. stellar-mass functions, rest-frame colours) as a function of morphological type, which demonstrates the efficacy of the method.

Due to its excellent ability to produce fine morphological classifications with minimal human intervention, in forthcoming papers we will implement this algorithm on future surveys (e.g. LSST, Euclid, JWST) to explore in detail the morphological evolution of galaxies as a function of redshift, stellar mass and local environment across 80 per cent of cosmic time.

Recent observational and theoretical studies indicate that the damping of solar coronal loop oscillations depends on the oscillation amplitude. We consider the mechanisms of linear resonant absorption and of nonlinear damping due to the development of the Kelvin-Helmholtz instability. We confront theoretical predictions from these models with observed data in the plane of observables defined by the damping ratio and the oscillation amplitude. The structure of the Bayesian evidence in this plane displays a clear separation between the regions where each model is more plausible relative to the other. There is qualitative agreement between the regions of highest marginal likelihood and Bayes factor for the nonlinear damping model and the arrangement of observed data. A quantitative application to 101 loop oscillation cases observed with SDO/AIA results in the marginal likelihood for nonlinear damping being larger in the majority of them. The cases with conclusive evidence for nonlinear damping outnumber considerably those in favor of linear resonant absorption.

Both observations and simulations show that much of the black hole growth in the universe has occurred in galaxies with no major mergers in their history since approximately z = 2, meaning that a significant fraction of active galactic nuclei (AGN) reside in disk-dominated galaxies with calm interaction histories and little or no merger-generated bulge components. Recent studies — both spectroscopic and otherwise — indicate bar-driven black hole growth, although past results lack clarity because of complicating factors such as selection effects and the much longer lifetimes of bars relative to AGN. Simulations also predict that long-lived bars are responsible for the growth of pseudobulges, meaning that “bulgeless” barred galaxies are more likely to have younger bars, which should resolve many of the lifetime complications observed in the full population of barred disk galaxies. We present a sample of AGN hosted in strongly disk-dominated galaxies at redshifts 0.015  z  0.25, with high-resolution HST images permitting secure identification of barred and unbarred morphology and 100% optical longslit spectroscopic completeness of both AGN and galaxies. We combine HST and spatially-resolved spectroscopic data to quantify the galaxy structural parameters, stellar masses, and star formation rates, and determine the bar fraction in the host galaxies of these luminous AGN. We additionally compare our data to a matched sample of disk-dominated galaxies lacking an AGN. By statistically comparing the bar fraction in the two samples and controlling for confounding variables, we can constrain the secular growth of supermassive black holes. These carefully matched samples of merger-free AGN and inactive galaxies are a promising means of quantifying the role of secular black hole growth in galaxy evolution.

A sub-kpc-scale binary AGN candidate at z ~ 0.35 is investigated by combining several radio observations from e-MERLIN. A dual radio source is detected which, when considered with previous optical observations, provides strong evidence for a binary AGN; the most distant pair discovered to date. The angular separation is measured to be 0.21+/-0.11 arcseconds consistent with previous measurements. This corresponds to an approximate physical separation of 1030+/-490pc at the epoch of the source. Further observations are required to definitively classify the target as a sub-kpc-scale AGN pair due to the large error on the separation. This is caused by coincident obscuration by the PSF along the position angle of the two cores. Further study could also look for a core-jet morphology within either radio core, or investigate the variability of the source, as combing radio observations to necessarily increase the sensitivity comes with the loss of chronological information.