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Measuring galaxy formation and evolution by low surface brightness structures

Final Activity Report Summary - GALAXIES (Measuring Galaxy Formation and Evolution by Low Surface Brightness Structures)

Galaxy formation and evolution is one of the most fascinating topics in astrophysical research today. It comprises all the various aspects of individual astrophysical research areas and therefore represents a wonderful crossover of astrophysics in general. Current knowledge states that galaxies were created by small density fluctuations in the very early Universe, which contracted as a result of gravity and forming stars. After that, they evolved with time, driven by interaction, merging and continuous gas recycling in the interstellar and intracluster medium building the present day observed variety of galaxy types.

Disk galaxies in particular are composed of three main distinct, stellar subcomponents. A rotationally supported disk, often showing recent star formation, beautifully aligned along two or more spiral arms, and a central, slightly flattened, elliptical component, called bulge. They both are situated in a more spherical envelope of stars, called the stellar halo. The detailed formation process is extremely complicated and not well known. The key goal here is to determine the actual history of galaxies like the Milky Way over the last 8 000 million years, or else 8 Gyrs.

One can do this in two ways. On the one hand, studying galaxies can be performed using the so called near-field cosmology, which consists of looking for clues close to home, by observing nearby galaxies in detail. On the other hand one can use the so called far-field cosmology, which is based on looking at distant objects and thus, because of the finite speed of light, looking back in time for the progenitors of modern-day galaxies.

This project combined these two ways by observing very distant galaxies, i.e. at an early stage of their evolution, with very big telescopes using data which is now publicly available and studying in parallel more nearby galaxies. During our research we used one of the key observing techniques to study galaxies, namely the study of the (broadband) surface photometry, to analyse and compare the results for local galaxies with their progenitors at large distances or, equivalently, at large lookback times of about 8 Gyrs ago, when the Universe was 40 % of its present age. Surface photometry, which consists of taking photographs, nowadays digital using charge-coupled devices (CCDs), in certain filters and analysing the measured intensity is almost as old as the great debate about the island universes itself, but remains the principal method for studying the universal structure.

Under the simplified assumption that a galaxy is made up of a few separate components one can model the surface-brightness distribution and determine, relatively easily, its parameters. By doing this one obtains a common ground to measure, compare, and sort large samples of galaxies across time, i.e. from the first galaxies up to now. We concentrated in particular on a special feature, a break, in the radial light distribution. The observed light profiles of many disk galaxies were best described by a broken exponential fit with a shallow inner region, enclosing the spiral arms, and a steeper outer exponential region, separated at a relatively well-defined break radius.

The main achievement of our research was to obtain images for two samples of local and distant galaxies at various lookback times, and analyse the breaks in their surface brightness distribution. According to theory, disk galaxies are thought to form inside-out, i.e. new material is accreted over time always in outskirts of the disk; hence using the radial position of the break as a direct estimator of the size of the stellar disk we could directly constrain galaxy evolution over the last 8 Gyrs.