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The Evolution of Structure

Studies of the mass-size relation of galaxies in the nearby (z < 0.1) and distant Universe (z > 1) show a x5 growth in galaxy sizes at fixed stellar mass and in number-density. This extraordinary result, confirmed by numerous groups, implies a dramatic physical change occurring in the galaxy population over the redshift range 0.2 − 1.0. Possible explanations include dynamical relaxation, major mergers, minor mergers, and disc growth.

However, this result is only clearly established for high stellar mass systems (i.e., > 1011M), found in extreme dense cluster environments. Whether this growth is endemic or confined to a specific mass or environment remains unclear. A deep spectroscopic survey with HST-resolution imaging over a sustained area is needed to extend the measurements to fainter mass limits and to distinguish between the competing hypotheses.

WAVES-Deep/UltraDeep in combination with Euclid imaging, provides exactly the dataset required to study this extraordinary growth. A key starting point will be to address whether the fundamental nature of galaxies is its bimodality (red v blue), or its evident duality (bulges plus discs). At very low redshift (z < 0.1) this issue is clear-cut with multi-component decomposition a routine part of the analysis toolkit. At high redshift (z > 1.5) the case is less clear as galaxies no-longer appear to adhere to the simple idea of bulge plus disc systems, but exhibit highly asymmetrical and irregular shapes. As such the language of high redshift galaxy work is typically focused on turbulence, distortions, and the global colour (red v blue).


To date the largest contiguous survey by HST covers 1.8 deg2 (COSMOS). Euclid will transform this by imaging upto 8000 deg2 of sky at 0.200 resolution. Surveying this entire area spectroscopically is unrealistic, however WAVES-Deep will provide over 0.5-million galaxies in the range 0.2 < z < 0.8, with imaging resolution sufficient to discern and measure bulge, bar, and disc components to 1kpc resolution, and WAVES-UltraDeep extending this further to 0.5-million galaxies to z < 1.5 in a select number of single pointing fields.

This will open the door for direct measurements of the mass and size evolution of the distinct structural components (bulges, bars, discs) within a sufficiently comprehensive survey to dissect trends by halo mass, star-formation rate or a multitude of other indicators. If tracing the duality of galaxies is critical for understanding galaxy formation, as we suspect, then WAVES-Deep/UltraDeep + Euclid will provide more than an order of magnitude advancement over COSMOS/zCOSMOS, and in doing so firmly bridge the near and far Universe.

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Fig. 1: Left-top: Comparison of various imaging datasets, with median seeing as indicated. Left-bottom: Structural decomposition fitting of galaxies. Right: Angular-size versus lookback time with the low-z limitations of the SDSS and VST shown.