Recent Talks

The determination of chemical composition and distances of galaxies is crucial for constraining the theory of galaxy formation and evolution in a dark energy and cold dark matter dominated universe. However, the standard technique using HII regions to determine the metallicity of star forming galaxies, nearby and at high redshift, is subject to large systematic uncertainties that are poorly understood and the determinination of accurate distances using Cepheids suffers from uncertainties caused by the metallicity dependence of the period luminosity relationship and extinction and crowding corrections. Multi-object spectroscopy of blue and red supergiant stars - the brightest stars in the universe at visual and NIR wavelengths - provides an attractive alternative. I will present results accumulated over recent years for galaxies in the Local Group and beyond out to a distance of 8 Mpc and will discuss the potential of future work with TMT and E-ELT. Combining the photon collecting power of these next generation telescopes with Adaptive Optics we will be able to study individual supergiant stars in galaxies as distant as the Coma cluster. With spectroscopy of the integrated light of young very massive Star Super Clusters and simple population synthesis techniques we can reach out ten times further.

Clusters grow by mergers, events which release huge quantities of energy and can produce massive outward-travelling shock waves that can have an important effect on cluster gas and galaxies. Giant radio relics form at these shock fronts, where accelerated electrons emit synchrotron radiation. Despite the great interest in relics, candidates with simple geometry, undisturbed morphology and high surface brightness are scarce. The complex interaction between the merger, the shock wave and gas is likely a fundamental driver of galaxy evolution. The effects of dense environments have been previously investigated for relaxed clusters, but never before in highly disturbed, merging clusters hosting a relic. The Sausage and the Toothbrush clusters are providing us with the chance to study this phenomenon and its effects on the relativistic particles and the cluster galaxies. In order to address many of the unanswered questions, we use a unique combination of facilities (GMRT, WSRT, INT) to obtain the first cluster-wide, multi-wavelength, multi-method analysis aimed at giving a complete picture of merging clusters hosting relics. We derive physical parameters such as the Mach number and injection spectral index for the diffuse sources in the field. We present index and curvature maps pinpointing spectral trends conclusive for shock acceleration of relativistic particles and test injection models such as the Jaffe-Perola and Kardashev-Pacholczyk. This analysis is fully complemented by an Halpha mapping of the cluster volume and outskirts. We provide the first direct test whether the shock drives or prohibits star formation to decipher the role of the merger in shaping the Halpha luminosity function.