Stellar Cartography: Mapping the Local Group using Variable Stars

This project will use observations of variable stars from a range of ground and space-based telescopes including Gaia, 4MOST, OGLE, and JWST to investigate the structure and evolution of our nearest neighbour galaxies.

This output from this work will inform studies of the evolution of the LMC, including the nature and history of the Magellanic Bridge structure, will allow comparison to simulations of the merger history of the Magellanic Clouds.

This work will also provide an independent distance measure for local galaxies which will allow the construction of a Cepheid-independent astronomical distance. Importantly, as RR-Lyrae are found in galactic haloes, their use as a distance measure will enable the calibration of distances to SNe host galaxies that are edge-on to our viewpoint, widening the sample of SNe galactic hosts that can be used in the determination of the Hubble Constant at the same time as potentially removing a further source of systematic error in the distance scale.

There is scope for the project to reach out to examine galaxies at the outskirts of the Local Group using JWST (subject to proposal approval). The project can be broken down as follows:

An apparent infrared magnitude Period-Luminosity Relation for RR Lyrae in the Magellanic Clouds will be determined using data from Southern Hemisphere sky surveys.

Precision astrometric distances from the GAIA survey will then be used to calibrate this P-L relation with local galactic RR-Lyrae in order to produce an absolute P-L relation. Metallicity effects will be accounted for using metallicity data from the 4MOST survey.

Using this derived absolute P-L relation, accurate distances to individual RR Lyrae in the Magellanic Clouds can be determined.

Coupling RR Lyrae distances of stars in the Magellanic Clouds to GAIA 2-D astrometry of these stars will enable the construction of detailed 3-D map of the LMC.

RR Lyrae are older stars, hence the derived LMC 3-D map will trace the older stellar population. This can then be compared to recent work on Cepheids in the LMC which trace the younger stellar population, with comparisons possible in several regions of parameter space including metallicity, age, and spatial.

Comparison with theoretical models of the merger history and evolution of the Magellanic Clouds.

One third of the project time will be spent on activities directly related to 4MOST. The exact details of this aspect will be confirmed through discussion with the 4MOST consortium, but this is most likely to be pipeline development activity related to variable star identification. The benefit from this to the project is membership of the 4MOST consortium, with a guarantee of proprietary access to 4MOST data including spectroscopic metallicities.

Beyond the PhD, this proprietary access will be retained for the lifetime of 4MOST, and this status is transferable between institutions and will significantly benefit my career as a researcher.

There are around 100,000 sources identified as galactic RR Lyrae in the GAIA catalogue. From previous work on Cepheids there is an expectation that many of these sources will have been incorrectly classified. In order to clean this data set, machine learning will be used to accurately recognise RR Lyrae light-curves. This step may require the use of High-Performance Computing resources.