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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - LEGA-C (The Physics of Galaxies 7 Gyr Ago)

Teaser

Summary

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 novelty of the observational dataset that has been collected over the past years at the European Southern Observatory, with an instrument called VIMOS on the Very Large Telescope (VLT). A team of observers led by the PI of this project spent a total of approximately 250 nights at Paranal, the mountain in Chile that is the site of the VLT. The collected data consist of 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 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 hidden from view. The immediate goal of the LEGA-C project is to infer the formation history of galaxies not only by comparing the younger universe with the present day, but also to infer the star formation histories of individual galaxies. This will reveal 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.

Work performed

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 global properties. These include the strength of absorption features that arise in the atmospheres of stars, emission line luminosities from plasma clouds and the Doppler shifts and broadening of these features due to motions within the galaxies. This information was used in a first generation of peer-reviewed publications that describe the age distribution and internal dynamical structure of the distant galaxies and how this compares, in a qualitative sense, with galaxies in the present-day universe. We described the ageing of the galaxy population in the context of the ageing of the Universe; a follow-up study revealed that as galaxies follow a variety of evolutionary pathways as they age and stop forming new stars; we also showed that the rotation of galaxies slows down over time, which we interpret for evidence of significant merging among galaxies.

The second step was to go beyond the simple qualitative picture that the new spectra reveals and develop and perform advanced data modelling techniques. We highlight the two main examples. 1) Via comparing the observed spectra with state-of-the-art stellar population model spectra we inferred the star formation histories of galaxies; we found that the most massive galaxies generally contain the oldest stars and, in fact, stopped any significant star formation activity several billion years prior to the observed epoch when the Universe was still young. Moreover, we found that even among currently active star-forming galaxies the total mass is a good indicator of its age. Finally, we showed that galaxies have a `memory\': star-formation activity at any given moment is correlated with star-formation activity as as much as 3 billion years earlier. 2) We built dynamical models that reproduce the observed motions (Doppler broadening and rotation), and thus infer the mass and angular momentum of the galaxies. That vast majority of galaxies show significant degrees of ordered rotation, but the exact degree of rotation (as quantified by the net angular momentum) strongly correlates with star-formation activity: star-forming galaxies rotate faster than passive galaxies. This result provides a new and powerful constraint on state-of-the-art galaxy formation models.

Several spin-off projects and results that already materialised include the following: 1) the fraction of galaxies with strong emission on radio wavelengths is tightly correlated with the absence of young stars and the galaxies\' dynamical state (radio-loud galaxies do not rotate); 2) part of the reason that distant galaxies stop forming stars is that they contain less molecular gas -- the material out of which stars form.

Final results

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 will shift focus from analysing and presenting the data to in-depth interpretation and comparison with state-of-the art models of galaxy formation. We already started exploring the dust attenuation of galaxies, the chemical composition of gas and stars, the internal motions of gas, radial gradients in stellar populations, the connection between galaxy properties and their large-scale environment, and the connection between the galaxies under study here and those at even earlier cosmic times. These projects will come to full fruition over the next few years.