Through the LEGA-C survey we aim to understand the evolution of galaxies through cosmic time. The novel aspect of the project lies in the uniqueness (in terms of both quality and quantity) of the observational dataset that has been collected from 2015 to 2018 at the European Southern Observatory, with an instrument called VIMOS on the Very Large Telescope (VLT) in Chile. A team of observers, led by the PI of this project, spent a total of approximately 250 nights at the observatory, collecting the data: spectra of thousands of distant galaxies that reveal the motions, ages and chemical compositions of the stars that make up the content of these galaxies. Due to the large distances (that is, light travel times) of the target galaxies this survey marks the first time that we have access to the stellar content of the universe when it was much younger than it is now: half its current age, roughly 7 billion years ago. So far, the astronomical community had only broad knowledge about the numbers of stars and how they are distributed over different galaxies, while their formation history had remained largely unknown. The immediate goal of the LEGA-C project is to infer the formation history of galaxies by comparing the younger universe with the present day, and through directly measuring the star formation histories of individual galaxies. This reveals when the majority of stars in different types of galaxies formed, why star formation largely stopped in the many old and inactive galaxies we see in the present-day universe, and how so-called mergers between galaxies continue to scramble and mix the stellar populations of smaller progenitors into ever larger galactic systems.
The questions addressed through this project may not be of immediate practical use to society, but do contribute to satisfying the general desire to understand the Universe, how it took its current form, and our place in it. These largest of questions start with an examination of how a Universe consisting of a featureless plasma evolved into a Universe teeming with structure in the form of galaxies, planetary systems and, perhaps, life. Understanding the formation and evolution galaxies holds key answers to these questions.
In reality projects often don't start when funding arrives and finish when the funding runs out. We are actively continuing the project and expanding the international collaboration to include prominent researchers across the world. Its scope is expanding to further comparisons with simulations, and follow-up observational programs, notably with the James Webb Space Telescope. The nature of the LEGA-C spectroscopic dataset is such that it cannot be superseded on the timescale of at least a decade, and probably more, as no further surveys of this kind are being (or indeed can be) planned. Even the newly launched James Webb Space Telescope and giant Earth-based telescopes (e.g. ESO's E-ELT) will not be able to deliver similar data for such a large sample. its legacy is assured and its impact will continue to grow over time.
