Periodic Reporting for period 5 - LEGA-C (The Physics of Galaxies 7 Gyr Ago)
Période du rapport: 2021-02-01 au 2022-09-30
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.
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.
The main practical task we aimed to achieve was to translate the spectra -- that describe the intensity of the light as a function of its wavelength -- into physical information about the galaxies. The first step was to translate each galaxy spectrum into a set of measurements that summarise its properties. These include the strength of absorption features that arise in the atmospheres of the stars in the target galaxies, emission line luminosities from plasma clouds and the Doppler shifts and broadening of these features due to motions within the galaxies. Building on this, we created more advanced data products to reveal the physical global properties of the galaxies: total mass, angular momentum, age, chemical composition, to name a few. All spectra, illustrated in the figure, and derived data products are now publicly available to the community.
The main scientific highlights are 1) the revelation of a very complex interdependency between the ages and structures of galaxies that cannot yet be reproduced by even the most advanced simulations of galaxy formation; 2) the establishment of a consistent mass scale of galaxies across half of cosmic time with a precision and accuracy of 20%; 3) the finding that galaxies of all types obey the same fundamental scaling relation between structure and velocity dispersion (a measure of mass); and 4) the first assessment of the stellar dynamical structure of galaxies in the early Universe.
The main scientific highlights are 1) the revelation of a very complex interdependency between the ages and structures of galaxies that cannot yet be reproduced by even the most advanced simulations of galaxy formation; 2) the establishment of a consistent mass scale of galaxies across half of cosmic time with a precision and accuracy of 20%; 3) the finding that galaxies of all types obey the same fundamental scaling relation between structure and velocity dispersion (a measure of mass); and 4) the first assessment of the stellar dynamical structure of galaxies in the early Universe.
The spectra themselves represent a vast improvement over the state-of-the-art as defined at the beginning of the project. We compared for the first time the stellar dynamical state of galaxies in the young universe and the present-day universe, and we inferred for the first time the star-formation histories of individual distant galaxies. These achievements were simply impossible before our observations. During the 2nd half of the project we shifted focus from analysing and presenting the data to in-depth interpretation and comparison with state-of-the art models of galaxy formation. Numerous publications on the stellar content, dynamical properties and formation history of the galaxies testify to the ground-braking results of the project.