The three-dimensional structure of the Magellanic Clouds as traced by Classical Cepheids

: (Alternative Format Thesis)

Abstract

Classical Cepheids are variable stars whose fluctuations in brightness are due to radial pulsations. Around a century ago, the Cepheid Leavitt Law (LL) was discovered, which relates the duration of these radial pulsations to the brightness of the star. Since its initial discovery, the LL has proven to be an excellent tool for measuring astronomical distances. Moreover, with the ability to measure individual distances to Cepheids, the LL can be used to construct three-dimensional (3D) maps of nearby galaxies and examine their structure. Of particular importance for such a study are the Magellanic Clouds as their 3D maps play a key role in understanding the complex morphologies and interaction history of the system. Several studies have already constructed 3D maps of the Magellanic Clouds using Cepheids. However, these studies suffer from either small sample sizes or from working at shorter wavelengths where distance uncertainties are greater. In this thesis, I combined the precision of working in the mid-infrared with the large sample sizes used in optical studies to construct 3D maps of the Magellanic Clouds using its entire fundamental-mode classical Cepheid population.

To create the 3D maps, a precise mid-infrared LL was required. By combining data from the OGLE-IV catalogue with mid-infrared photometry from the Spitzer Space Telescope, I obtained the 3.6 and 4.5 μm LLs for the Magellanic Clouds using around 5000 fundamental-mode classical Cepheids. Analysis of the LL revealed that ultra-long period (ULP) Cepheids, those with periods greater than 80 days, should be excluded when fitting the LL as they appear to follow a shallower relation. The linearity of the 3.6 μm LL was examined using the Bayesian Information Criterion (BIC). The Large Magellanic Cloud (LMC) 3.6 μm relation was found to be linear but the Small Magellanic Cloud (SMC) LL is best described by a non-linear relation with a break at log(P) = 0.40. The 3D maps of the Magellanic Clouds were constructed using the 3.6 μm LL for each galaxy.

Combined with Cepheid ages from period–age relations and metallicities from Gaia, I examined the 3D geometric, age and chemical structure of the LMC and SMC. This study identified several features of the 3D structure of these galaxies that support the scenario in which the LMC and SMC underwent a direct collision relatively recently. First, the eastern regions of the LMC are closer to us than its western regions, while the eastern SMC Cepheids seem to be trailing off towards the LMC. Second, SMC Cepheids younger than 300 Myr were found to be more centrally located and less dispersed in distance compared to the Cepheids older than 300 Myr. Such a feature could arise from a direct collision around 300 Myr ago in which the already existing Cepheid population became more dispersed and elongated, with the collision triggering star formation and the young Cepheids forming more centrally. Additional evidence for a collision was found, including the existence of stronger small-scale clustering in the LMC compared to the SMC and the presence of a correlated burst of star formation that occurred in both galaxies.

Date of Award	23 Mar 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Vicky Scowcroft (Supervisor) & Stijn Wuyts (Supervisor)