Dr. Johnston and her team investigate the formation and evolution of dwarf galaxies, the most common type of galaxies in our local universe, and the formation of the Milky Way galaxy through detailed hydrodynamic and N-body simulations. The observed chemical composition of stars and their distribution today together with simulations is used to learn about ancient accretion events in the stellar halo, and to recover the properties of progenitor objects (e.g. dwarf galaxies and star clusters) that contributed to our galaxy's formation. The research aims to reconstruct the galaxy's past through comparing simulations to the observed distribution of the abundances of the chemical elements in large samples of stellar populations in all components of our Galaxy, which will become available from ongoing and planned surveys (e.g. with the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the HERMES spectrometer on the Anglo-Australian telescope). The chemo-dynamical formation models of the Milky Way's halo that are produced here also provide guidance on the interpretation of large data sets from stellar surveys. The models include physics on star formation, feedback and mixing, and take into account the recurrent cycle of star formation and the chemical enrichment of new star-forming regions through ejecta from dying stars. The predicted abundance distributions of the chemical elements in different stellar populations might be detected in the spectroscopic surveys of stars. The plans for the semi-analytic chemo-dynamic models include the development of multi-zoned descriptions of the galaxy with collisionless stellar orbits allowing for star formation and enrichment of gas in chemical elements. In order to derive a more complete picture about the formation and evolution of the Galactic halo, results are combined from the semi-analytic chemo-dynamical models of galaxies, the fully self-consistent cosmological N-body and hydrodynamic simulations; and the statistical analyses of structures in high dimensional spaces. The self-consistent simulations will be used to refine the descriptions of internal baryonic physics and external environmental influences that are implemented in the simpler chemo-dynamical models. This research program provides research topics for two graduate students.
How is it possible to tell where a star came from? If we could do this for all the stars that are close enough to observe individually - most of which reside in our own, Milky Way Galaxy - we could learn much about how our Galaxy came to be. Luckily for us, stars do carry a fingerprint of their birth-sites with them in the elements from which they are made. The chemical composition that we can measure for a star corresponds to the chemical composition of the gas cloud in which it was originally born. Hence if we observe stars with exactly the same composition there is some chance that they were born at the same time and in the same place - we can chemically-tag them with their origin. This is just one of the ideas that underpins the field of Galactic Archaeology, whose purpose is to reconstruct how the Milky Way formed and evolved from the way it looks today. Our NSF-funded research explores the feasibility of chemically-tagging stars in several ways. For example - many of the stars that now reside in the diffuse stellar halo that surrounds our Galaxy actually formed in many distinct dwarf galaxies in the early in the Universe. One interesting question is to what extent the stars in the stellar halo might be similar to stars that still live in surviving dwarf galaxies, and to what extent they might be different. In order to answer this question we looked at a simulation which traced where matter was expected to clump and form stars in dwarf galaxies in the early Universe, contrasting those which went on to be disrupted by the (simulated) Milky Way and accreted into the stellar halo, with those which survived to the end of the simulation as separate objects. We found that the surviving dwarfs were more isolated, spread out over a much larger volume of space than those that went on to form the stellar halo. The gas in all these dwarfs is enriched (contains more complex elements) over time by supernovae explosions, which can also blow gas entirely outside their original hosts. However, the gas in the isolated dwarfs was much less likely to become mixed during these processes with gas form other dwarfs. We conclude that the chemical make-up of the ancient stars in surviving dwarfs should be quite different from those that make the stellar halo. In another project, we asked if we might be able to find the signatures of the many different dwarf galaxies that the Milky Way might have cannibalized in the past by looking at the chemical compositions of stars that make up the stellar halo today. We found that this is possible in principle - in particular, that in near-future surveys we might find signatures of some of the very smallest dwarf galaxies that formed quite early in the history of the Universe. This is a very exciting prospect for the next decade in this field!
