Do Galaxies Change in Size ? An Angular Size Test at Low Redshift with SDSS Data

Based on magnitudes and Petrosian radii from the Sloan Digital Sky Survey (SDSS, data release 7) at low redhift (z <0.2), we developed a test of galaxy-size evolution. For this first quantitative size analysis using SDSS data, several possible sources of systematic errors had to be considered. The Malmquist bias is excluded by volume-limited samples. A correction for seeing has been developed and applied. We compare different methods to perform the K-correction, and avoid selection effects due to different filters. It is found that apparent average galaxy size slightly decreases with redshift z, corresponding to a growth in time. The effect is smaller for a lower H0, and at the same time less pronounced at higher redshifts, but persists in both cases. Although there is no systematic variation with galaxy luminosity, we took into account the recently discovered luminosity evolution with redshift. Assuming this effect of unknown origin to be real, we find a slight increase of galaxy size with z. The relative change of average size with z usually amounts to less than one half of the respective increase of wavelengths due to the cosmological redshift, making a cosmological interpretation difficult. While the effect shows clear statistical significance, unknown systematics cannot be excluded. In any case, the enigmatic observations of size and luminosity evolution need to be analysed together. To facilitate further investigations, a complete Mathematica code and instructions for data download are provided.