National Astronomy Meeting Poster Exhibition

Using the largest mosaic of Hubble Space Telescope images around a galaxy cluster, we map the distribution of dark matter throughout a ∼6×6 Mpc2 area centred on the cluster MS 0451-03 (z=0.54, M200=1.65×1015M⊙). We preform a joint strong- and weak-lensing analysis. Our result shows three possible filaments extending from the cluster, encompassing six group-scale substructures. The dark-matter distribution in the cluster core is elongated, consists of two distinct components, and is characterized by a concentration parameter of c200=3.79±0.36. By contrast, XMM-Newton observations show the gas distribution to be more spherical, with excess entropy near the core, and a lower concentration of c200=2.35+0.89−0.70 (assuming hydrostatic equilibrium). Such a configuration is predicted in simulations of major mergers 2-7Gyr after the first core passage, when the two dark-matter halos approach second turnaround, and before their gas has relaxed. MS 0451-03 will be an ideal target for future studies of the growth of structure along filaments, star-formation processes after a major merger, and the late-stage evolution of cluster collisions.

The plasma in the solar corona originates from the photosphere and would, therefore, be expected to have similar elemental composition. However, elements with a low first ionisation potential (FIP) have been observed to have an increased abundance in certain regions of the corona. This phenomenon is known as the FIP effect and the degree of enhancement is measured using the FIP bias parameter. The increased elemental abundance is typically observed in active regions.

In this statistical study, we analyse how the degree of enhancement varies in active regions of different sizes, ages and level of complexity. We explore whether the average FIP bias is linked to the evolution of active regions and the photospheric magnetic field at the scale of an active region. First, by investigating whether there is a correlation between average FIP bias and the total magnetic flux of the active region. Second, if there is an average FIP bias dependence on magnetic flux density. Third, if the average FIP bias varies depending on whether the plasma is above the leading or following polarity of an active region.

Submillimetre galaxies (SMGs) are thought to play a significant role in the evolution of the cosmic star formation rate density, contributing as much as ∼20–30% at z ∼ 2–3. It has been hypothesised that these extreme star-forming systems are the progenitors of local early-type cluster galaxies. If true, this would imply that SMGs should reside in galaxy cluster progenitors at high redshift. Whilst there are well-known examples of SMGs residing in protoclusters, these systems were selected for follow-up because of their high galaxy or SMG density. To explore the environments of SMGs in an unbiased way we have undertaken a narrowband VLT/HAWK-I study of H-alpha and [OIII] emitters around three ALMA-identified and spectroscopically-confirmed SMGs at z ∼ 2.3 and z ∼ 3.3, which were selected with no prior knowledge of their environments. On average, these SMGs reside in environments which are ∼2–4x overdense compared to the field. Our results suggest that SMGs do tend to reside in protocluster-like environments, supporting the claim that they likely evolve into the passive early-type galaxies observed in local clusters.

In recent years, many studies have highlighted the potential for the Kelvin-Helmholtz instability to form in transversely oscillating coronal flux tubes. The development of the instability may enhance the rate of wave heating by initiating an energy cascade to small spatial scales. As the instability develops, large gradients form in both the perturbed velocity and magnetic fields, leading to increased viscous and Ohmic dissipation and, consequently, enhanced wave heating. The compressive flows that form as a result of the instability force misaligned magnetic field lines together and may therefore trigger magnetic reconnection.

We present results of numerical MHD simulations of driven, transverse waves in a simple geometry. The instability is triggered by a velocity shear that forms across a resonant layer of field lines. We discuss the implications of the KHI on magnetic reconnection rates and explore the effects of field line length and non-potential equilibrium fields. In the latter case, we discuss whether the instability will enhance the rate of reconnection of the background magnetic field.

The Large and Small Magellanic Cloud (LMC and SMC) are the most luminous dwarf galaxy satellites of the Milky Way. Thanks to their proximity (50-60 kpc), they provide one of the best opportunities to study in detail the kinematics of resolved stellar populations in an interacting pair of galaxies. Extensive photometric surveys like the ongoing Gaia mission and the near-infrared VISTA survey of the Magellanic Clouds system (VMC) have significantly impacted our insight into the Magellanic system. However, full-scale proper motion measurements and their analyses across the Magellanic Clouds are still a major challenge. In Schmidt et al. (2020), I highlighted the benefit of combining VMC and Gaia DR2 data to study the stellar kinematic in the Magellanic Bridge, connecting the LMC and the SMC, accounting for the influence of Milky Way stars. Subsequently, I have used bayesian distance estimates provided by StarHorse (Queiroz et al. 2018) to train a machine learning algorithm, which can separate Magellanic and Milky Way foreground stars more reliably than using parallax and proper motion selection criteria. With this method, it is possible to study the sparsely populated outer region of the LMC with a significantly increased sample of stars reaching below the red clump of the LMC. In this contribution, I will present results from my ongoing project dedicated to measure and analyse the proper motions of samples of stars across Magellanic regions characterised by a low stellar density and how debris from the last interaction between LMC and SMC is visible within the outer LMC.

Coronal loops form the basic building blocks of the magnetically closed solar corona yet much is still to be determined concerning their possible fine-scale structuring and the rate of heat deposition within them. Using an improved multi-stranded loop model to better approximate the numerically challenging transition region, this paper examines synthetic NASA Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) emission simulated in response to a series of prescribed spatially and temporally random, impulsive and localised heating events across numerous sub-loop elements with a strong weighting towards the base of the structure; the nanoflare heating scenario. The total number of strands and nanoflare repetition times are varied systematically in such a way that the total energy content remains approximately constant across all the cases analysed. Repeated time lag detection during an emission time series provides a good approximation for the nanoflare repetition time for low-frequency heating. Furthermore, using a combination of AIA 171/193 and 193/211 channel ratios in combination with spectroscopic determination of the standard deviation of the loop apex temperature over several hours alongside simulations from the outlined multi-stranded loop model, it is demonstrated that both the imposed heating rate and number of strands can be realised.

It is clear that the solar corona is maintained at (apparent) thermal equilibrium by a delicate balance between radiative losses and some unknown heating mechanism. It is also clear that the effect of these heating and cooling mechanisms varies with the plasma parameters, such as density and temperature. Slow magnetoacoustic waves - which are common in the corona - are able to perturb these heating and cooling mechanisms enough to be themselves affected in a measurable way. An interesting avenue of research is therefore using these slow waves as probes of the local thermal equilibrium, since by measuring their properties through observations we may infer some information about the enigmatic coronal heating function.
This research direction, considering the effect of heating/cooling misbalance upon slow waves, has received renewed attention in recent years. In this talk I will summarise recent progress, with a focus on the effect of the magnetic field. Crucially I will show that for a sufficiently strong magnetic strength, the slow wave dynamics is insensitive to any dependence of the heating function on the magnetic field. This approximation is found to be valid in the corona so long as the magnetic field strength is greater than approximately 10G for quiescent loops and plumes, and 100G for hot and dense loops. Finally, I will discuss the implications of this result for the seismological inference of the coronal heating function via slow waves.

How does a bar affect its host? We investigate the effect of strong and weak bars in star forming galaxies in the context of galaxy quenching and galaxy evolution. The strong and weak bars in this study were identified using Galaxy Zoo DECaLS (GZD), the newest version of Galaxy Zoo, which uses imaging from the DECaLS survey. Our sample was also cross-matched against ALFALFA for gas mass measurements. In the end, we have a volume-limited sample (0.01 z 0.05, Mr −18.96) of 2,071 galaxies with reliable volunteer classifications.

We find a weak bar fraction of 28.3% and a strong bar fraction of 14.9%, resulting in a total barred fraction of 43.2%, consistent with the literature. Interestingly, we observed that the strong bar fraction is typically higher in quiescent galaxies than in star forming galaxies, whereas the weak bar fraction is similar in both groups. Additionally, we found that star forming galaxies with strong bars have higher fibre SFRs, lower gas masses and shorter depletion timescales, compared to unbarred star forming galaxies. The increase of SFR in the fibre is in agreement with previous theories stating that bars funnel gas to the centre, where it is used for star formation. This was not found for star forming galaxies with a weak bar. This shows that strong bars facilitate the quenching process and suggests that bar morphology plays an important role in the evolution of a galaxy.

Finally, we have also found that the differences between strong and weak bars that we observed, disappear when we control for bar length. We conclude that strong and weak bars are not fundamentally different phenomena, but instead we propose that there is a continuum of bar types, which varies from `weakest' to `strongest'.

Once every two decades, Jupiter, Saturn and the Sun move into close alignment with each other, so that Saturn moves periodically into Jupiter's extended magnetotail. Observations from Voyager 2 show that this rare alignment results in dramatic changes to Saturn's aurora, with the radio emission decreasing by two orders of magnitude. However, since that observation, we have no measurements of this event and no real understanding of how the auroral region changes in brightness or currents, or how the thermosphere might change as a result of changing auroral inputs. Here. we present a series of ground-based observations of the H3+ auroral emission from Saturn across a period of two months, hoping to observe the auroral changes during a jovian magnetotail crossing. Our preliminary results show some tantilizing results - during the final observations of the sequence, Saturn's aurora appears to drop away significantly in brightness, resulting in apparently dramatic changes in the ion winds observed. On one night the entire polar cap appears to super rotate. On subsequent nights, very dim aurora appear to match with strange ion flows that, in some cases, appear to be in anti-phase with the planetary period - perhaps hinting at an ionosphere controlled solely by the atmosphere, within the usual middle magnetosphre breakdown. These results are highly preliminary. We intend to re-analyse them and ensure that these very unusual dynamics are not an artefact of calibration - if they remain, this presents the first strong evidence of what processes occur on a planet when removed from the influence of the solar wind.

Numerical simulations have revealed a new type of turbulence of unidirectional
waves in a plasma that is perpendicularly structured (Magyar et al. 2017), named
uniturbulence. For this new type of turbulence, the transverse structuring modifies the upward propagating wave to have both Elsasser variables, leading to the well-known perpendicular cascade.
In this presentation, we show an analytical description of the non-linear evolution of kink waves in a cylindrical flux tube, which are prone to uniturbulence. We calculate explicit expressions for the wave pressure and energy cascade rate. The computed damping rate $\tau/P$ depends on the density contrast of the flux tube and the background plasma and is inversely proportional to the amplitude of the kink wave. The dependence on the density contrast shows that it plays a role especially in the lower solar corona.
We compute the damping for both standing and propagating kink waves. The heating by propagating kink waves may be important in global coronal models, such as AWSOM. The damping of the standing kink waves is important for seismology. We compare the analytical results with numerical simulations and
observations. In both cases we find a reasonably good match. The comparison
with the simulations show that the non-linear damping dominates in the high
amplitude regime, while the low amplitude regime shows damping by resonant
absorption. In the comparison with the observations, we find a power law
inversely proportional to the amplitude $\eta^{-1}$ as an outer envelope for
our Monte Carlo data points.