National Astronomy Meeting Poster Exhibition

We investigate the radio properties of a sample of 850 μm-selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS) using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array (LOFAR). Combining these data with additional observations at 324 MHz, 610 MHz, and 1.4 GHz from the Giant Metrewave Radio Telescope and the Jansky Very Large Array, we find a variety of radio spectral shapes and luminosities within our sample despite their similarly bright submillimetre flux densities. We characterise their spectral shapes in terms of multi-band radio spectral indices. Finding strong spectral flattening at low frequencies in ~20% of sources, we investigate the differences between sources with extremely flat low-frequency spectra and those with `normal' radio spectral indices (α > ‒0.25). As there are no other statistically significant differences between the two subgroups of our sample as split by the radio spectral index, we suggest that any differences are undetectable in galaxy-averaged properties that we can observe with our unresolved images, and likely relate to galaxy properties that we cannot resolve, on scales ≲1 kpc. We attribute the observed spectral flattening in the radio to free-free absorption, proposing that those sources with significant low-frequency spectral flattening have a clumpy distribution of star-forming gas. We estimate an average spatial extent of absorbing material of at most several hundred parsecs to produce the levels of absorption observed in the radio spectra. This estimate is consistent with the highest-resolution observations of submillimetre galaxies in the literature, which find examples of non-uniform dust distributions on scales of ~100 pc, with evidence for clumps and knots in the interstellar medium.

Measurements of FUV auroral intensity can be used to derive estimates of Saturn's ionospheric Pedersen conductance which can be enhanced above the solar EUV-driven background by up to 20 times following energetic auroral precipitation [Galand+ 2011]. Local time profiles of the auroral intensity reveal spatial and temporal variation in the enhanced ionospheric conductance in response to magnetospheric dynamics. Seasonal variation in large-scale current flows linking the ionosphere and magnetosphere has been observed in Saturn’s polar regions using Cassini magnetic field measurements, implying stronger conductivity in the summer hemisphere than in winter [Bradley+ 2018]. We first aim to answer the question, ‘How do Saturn’s auroral intensities vary with season?’ by applying statistical auroral boundaries - based on the full Cassini UVIS image set [Bader+ 2019] - to auroral images, in order to obtain local time intensity profiles under different seasons in both hemispheres. We then test for evidence of a modest increase in northern conductance between equinox and summer solstice owing to the increasing solar illumination over 2009-2017. We will also examine whether there is any difference between the average conductances predicted in this way for the northern and southern solstice conditions, and how this compares to the south:north conductance ratio of 1.2 associated with the magnetic field asymmetry.

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Field-aligned currents link the ionosphere to the magnetopause (Region 1) and the ring current (Region 2), and are a key part of the way in which energy is transferred into the ionosphere. We use data from the Active Magnetosphere and Planetary Electrodynamic Response Experiment (AMPERE) to quantify the way in which the field-aligned current densities are distributed in each spatial coordinate, and analyse the implications that this has for the parts of the system likely to see the largest amounts of current. We fit a Tsallis, or q-exponential, distribution to the current densities in spatial coordinate, to generate maps of the probability of field-aligned current densities above certain thresholds in both hemispheres. We discuss this in terms of its ramifications for space weather preparedness.

In the current cold and arid Martian epoch, wind driven processes have likely been the dominant mode of erosion and landscape modification. Sand fluxes comparable to Earth’s have been observed on Mars through the migration of ripples and dunes across the surface [1, 2]. The driving force behind the movement of such features is the impact of saltating sand sized particles. These impacts result in the abrasion of silicate minerals and the generation of dust [3].

Here we report preliminary results from experiments designed to test the dependence of mineral abrasion on temperature. We simulated the saltation of sand sized (125 – 300 µm) olivine, pyroxene, feldspar, opal and quartz, at temperatures between 193 K and 293 K – typical of the surface of Mars. Our 75 day experiment was equivalent to ~ 6 years of continuous sand mobilisation at close to threshold wind speeds (~ 1 ms-1). This resulted in the comminution of between 4.0 ± 0.4 and 13.6 ± 0.8 % by mass of each sample to below 125 µm. Importantly, each of the minerals tested produced significantly (p .05) less fines at 193 K than at 293 K, with a mean decrease of ~ 22 %.

These results suggest that the contribution of dust generated from saltation on Mars is temperature dependant, potentially linking dust generation to obliquity and other Martian temperature controls such as atmospheric composition and solar luminosity. There may also be implications for Martian drilling campaigns; the increase in resistance to the abrasion of minerals at low temperatures highlighted in this study could lead to slower than expected rates of penetration on Mars.



[1] Bridges, N. T. et al. (2012), Nature 485, 339. [2] Silvestro, S. et al. (2020), JOGR: Planets 125(8) e2020JE006446. [3] Merrison, J., (2012), Aeolian Research 4, 1-16.

The Ultraviolet and Visible spectrometer (UVIS)[1], is the ultraviolet and visible channel of the NOMAD spectrometer[2] onboard the ExoMars Trace Gas Orbiter (TGO). UVIS has been in orbit around mars for over two years with near continuous nadir observations through the latter half of MY34, through to MY36.
A retrieval procedure was developed to obtain the ozone and aerosol column abundances in the martian atmosphere. The radiative transfer is performed using the discrete ordinates DISORT package developed by Knut Stamnes and collaborators[3] and the ‘front-end’ routines (DISORT_MULTI) developed by Mike Wolff, for studies of the martian atmosphere [4,5,6].

Seasonally the ozone distribution is consistent, with low ozone abundances in equatorial regions and higher ozone abundances at higher latitudes in the winter season. As the martian atmosphere cools through northern spring, from the reduced solar insolation, we observed a steady increase in equatorial ozone. UVIS observations show significant ozone entrapment in large impact basins, such as Hellas Basin[6]. Ozone abundances measured within Hellas can be 10-20 µm-atm, compared to 2-5 µm-atm outside the crater.

A global dust storm was observed in MY34 which started around solar longitude (Ls) ~ 185º with derived dust optical depths exceeding 6. The dust loading remained enhanced until Ls ~ 270º, before settling back towards opacities between 1 and 2. A second storm, the C storm, was observed at Ls ~ 330º where dust opacities again exceeded 2. Shortly after the C storm the dust settled back to seasonal values between 0.5 and 1.

[1] Patel, M. R. et al., Applied (2017)
[2] Neefs, E, et al. Applied optics (2015)
[3] Stamnes, K, et al. Applied optics (1988).
[4] Wolff, M. J. et al Icarus (2010)
[5] Wolff, M. J. et al. Icarus (2019).
[6] Clancy, R. T, et al. Icarus (2016).

At z > 2, many studies have identified protoclusters - overdensities of galaxies spanning tens of Mpc in size and hosting some of the most vigorous star formation observed in the Universe to date. Conversely, galaxy clusters are typically observed at z 1 and are massive, virialised and abundant in quiescent, elliptical galaxies. Current models of galaxy formation and evolution predict that protoclusters will evolve into z ~ 0 massive galaxy clusters, and so there must be some rapid, environmentally-driven quenching of star formation at z ~ 1 that transforms protoclusters into clusters. In order to understand this evolution, we must search for overdensities of galaxies at the onset of cluster formation at which time we can observe the full web of activity: the assembly of a collapsed cluster core feeding on the star-forming filamentary material that resides in the protocluster web. In this project, we present an analysis of SpARCS J0330, one of the most massive, high-z galaxy clusters discovered to date. By building up SEDs with up to 20-band photometry for the spectroscopically confirmed cluster members, as well as for the potential cluster members, we aim to map out distant, star-forming galaxies over a ~15Mpc region across the cluster, bridging the gap between studies of high-z protoclusters and local galaxy clusters, and providing critical insights into how galaxy clusters form out of the cosmic web.

In preparation for future large surveys such as LSST and Euclid. These expect to find more than 10^5 gravitational lenses, I am interested in making sure the novel systems are discovered as these offer greater constraints on dark matter. I have been developing a CNN model to identify gravitational lenses from simulated Euclid images. This CNN model performs well with an F1 score of 0.98, but why? I have applied several approaches including deep dream, occlusion maps, and class generated images to understand the aspects of the image which influences the model’s classification. Currently I am creating images of compound lenses to understand how well my model performs on data of rare lens configurations.

We are using the high sensitivity and angular resolution of the VISTA Magellanic Clouds (VMC) multi-epoch survey at 1-2.5 micron as a basis for characterising the variability of Active Galactic Nuclei (AGN) out to redshift >1. Two AGN, at redshifts 0.14 and 1 that are dominated in the infrared (IR) by the emission from nuclear dust are studied in-depth, and the prevalence of IR variability is studied in a larger sample. We are using the hot dust (variability) as a tracer of the accretion instabilities, but the dust grains are subject to extreme conditions. The findings are placed in context of optical (OGLE) and mid-IR (Spitzer, WISE) variability studies and new radio data from the Australian SKA Pathfinder to identify the causes for the near-IR variations.

Please note: I will be using hot dust variability as a tracer.

Earth’s magnetosheath is filled with complex, highly nonlinear fluctuations known as turbulence that generate a multitude of small-scale current sheets. Previous studies have shown that these current sheets can be potential locations for magnetic reconnection and recent high resolution, multi-spacecraft measurements from NASA’s Magnetospheric Multiscale (MMS) mission have begun to reveal that in some cases the reconnection events may occur without forming ion jets – so-called electron-only magnetic reconnection. However, the interplay between these reconnection events and the turbulence remains an important open question for understanding the dynamics and dissipation of collisionless plasma turbulence. In this study, we perform a detailed survey of 60 intervals of magnetosheath turbulence observed by MMS. Within each interval, characteristic properties of the turbulence are determined and several hundred individual magnetic reconnection events are systematically identified, including both reconnection events with and without ion jets. It is found that thinner reconnecting current sheets with faster electron outflows, consistent with electron-only reconnection, tend to occur in intervals with shorter magnetic correlation lengths – shorter than a few tens of ion inertial lengths. The results may have implications for how turbulent dissipation partitions energy between ions and electrons in collisionless plasma systems.