Astronomy seminar: Young star clusters and clumps in the local Universe – The effect of galactic environment on star-formation

Stars do not form in isolation, but rather out of a hierarchical structure set by the turbulence of the interstellar medium. At the densest peak of the gas distribution, the star formation process can produce young star clusters (YSCs), gravitationally bound systems of stars with mass between ~100 and 10^5 M_sun and typical size of few parsecs. At larger scales, clusters are themselves arranged into cluster complexes, on scales of hundred parsecs and up to kiloparsec scales, which are usually referred to as star-forming clumps. Observations of local star-forming galaxies show that YSCs form over a wide range of galactic environment. However, it is not yet clear if and how the galactic environment relates to the properties of star clusters. I will present the results obtained by studying the YSC population of the nearby spiral galaxy M51. We find that the cluster mass function can be described by a power-law with a -2 slope and an exponential truncation at 10^5 M_sun, consistently to what observed in similar galaxies in the literature. The shape of the mass function is similar when looking at increasing galactocentric distances. We observe significant differences, however, when comparing clusters located in the spiral arm and the inter-arm environments. On average, more massive clusters are formed in the spiral arms, as also previously found for the YSC progenitors, the giant molecular clouds (GMCs). Finally we see that clusters are more quickly disrupted in denser environment, as expected if their disruption is mainly caused by tidal interaction with dense gas structures like the GMCs. I have also undertaken  the analysis of the interplay between galactic scale properties and larger star forming units, the stellar clumps. The analysis has been conducted in a sample of 14 local starburst galaxies, the Lyman-Alpha Reference Sample (LARS). The elevated star formation rate densities of such galaxies allow to form clumps with densities comparable to clumps at high-redshift, typically more massive and denser than what normally observed in the local universe. The clumps in the LARS galaxies contribute to a large fraction (in many galaxies >50%) to the UV flux of the galaxy itself, resulting in galaxies which appear ‘clumpy’. In agreement with formation theories we observe that clumpiness is higher in galaxies with higher SFR surface density  and dominated by turbulent gas motion.