Star clusters in the Whirlpool Galaxy

Abstract

This thesis presents the results of observational studies of the star cluster population in the interacting spiral galaxy M51, also known as the Whirlpool galaxy. Observations taken by the Hubble Space Telescope in the optical and the near-UV are used to determine fundamental properties of the star clusters, such as their ages, masses, radii and their spatial distribution. We study how these properties are related and how they depend on different environmental conditions in the galaxy, such as galactocentric radius and the distance from the spiral arms. By comparing the properties of the young star clusters to the properties of the giant molecular clouds from which they form, we study the process of star formation indirectly. We determine the radius distribution of 1284 young star clusters, which is different compared to the radius distribution of the giant molecular clouds. This suggests that during the formation of star clusters their radii change in a non-uniform way. The majority of the youngest star clusters are found in the spiral arms and these clusters are slightly more compact compared to older star clusters in the interarm regions. We discover a peculiar, fuzzy object with a projected position close to the nucleus of M51. After considering different scenarios for this object, we conclude that this object is most likely a fuzzy star cluster in front of the disc, with an age of 1.4 Gyr. The spatial distribution of the young star clusters is analysed using two-point autocorrelation functions. From this we find that the positions of the star clusters show a hierarchy with a fractal dimension similar to that of the turbulent interstellar medium in other galaxies, suggesting that star formation is hierarchical with a universal fractal dimension. Exploiting different multi-wavelength datasets we compare the positions of current star formation sites and recently formed star clusters younger than 10 Myr. A quantitative comparison between star and cluster formation is used to study the rapid dispersion, also called infant mortality, of young star clusters. Both star and cluster formation peak in the spiral arms and in the centre of the galaxy, but also at a galactocentric radius of 2.5 and 5 kpc, which is likely caused by the presence of the 4:1 resonance and the corotation radius, respectively. We derive the star cluster formation efficiency, which is the fraction of star formation that takes place in the star clusters we observe. We correct this fraction for selection effects by use of the cluster initial mass function, which we derive from our new data. We conclude that 20% of the star formation takes place in the form of star clusters. The remaining 80% takes place in a dispersed way, suggesting that the infant mortality can be as high as 80% and occuring on timescales of less than 10 Myr.