Katja Fahrion
Globular clusters (GCs) are dense, massive star clusters found in all massive galaxies and due to their old ages, they are often regarded as fossil records of galaxy assembly that inherit the chemodynamical properties of their birthplace. In the Milky Way, the study of individual stars can reveal detailed information of the chemical and orbital properties of individual GCs, but in distant galaxies, GCs appear as barely resolved point sources. Photometric surveys of GC systems in the last decades have shown that most massive galaxies exhibit bimodal GC colour distributions with a red and blue population. These have been interpreted as a red, metal-rich, in-situ born and a blue, metal-poor, accreted population, linking GC populations directly to a two-stage formation of galaxies. However, to effectively use GCs as tracers of galaxy assembly, spectroscopy is required. My poster presents a novel sample of 722 GCs in 32 galaxies of the Fornax cluster observed with the MUSE instrument that allows us to extract GC spectra even in the inner regions of galaxies which are usually unexplored with multi-object spectrographs. With this spectroscopic sample, we test how well GCs trace velocities and metallicities of their host, establishing their value as bright tracers in distant galaxies. Further, we study the relationship between photometric colours and spectroscopic metallicities and derive a non-linear colour-metallicity relation that challenges the simple division of GCs solely based on their colour and has implications for merger histories as traced by GCs.
with the red, metal-rich GCs having formed in-situ and the blue, metal-poor GCs having their origin in low-mass dwarf galaxies that were accreted. This makes the red, metal-rich GCs potentially powerful tracers of host galaxy properties, while the blue GCs should hold information about the assembly history of their host.
However, to effectively use GCs as tracers, spectroscopy is required that reveals their kinematic and stellar population properties.
My poster presents a novel sample of 722 GCs in 32 galaxies of the Fornax cluster (at 20 Mpc) observed with the MUSE instrument as part of the Fornax3D survey (Sarzi et al. 2018). MUSE is an integral-field spectrograph mounted at the Very Large Telescope in Chile, that provides three-dimensional data cube where each pixel contains a spectrum in the optical wavelength range (470 - 930 nm), thus combining imaging and spectroscopy. This data is ideally suited to study the properties of GCs and their host galaxies with full spectral fitting. However, the galaxies outshine the GCs in the central regions, making their detection difficult. Therefore, we first build a model of the galaxy light to subtract from the image. In the residual, GCs can be picked up and cross-referenced with photometric catalogues (Jordan et al. 2015). When extracting the spectra, it is important to remove the galaxy background. The cleaned GC spectra is then fit to get velocities and stellar population properties (age and metallicity).
For each galaxy with sufficient numbers of GCs, we model the rotation velocity and velocity dispersion of the full GC system and directly compare it to the host galaxy. We find that some galaxies have GC systems that clearly rotate, but in general the GCs follow the rotation of the galaxy spheroid and not the rotation of the disk. The velocity dispersion of the GC systems close follow the dispersion seen in the stellar main body and they trace the enclosed mass, establishing their value as bright kinematic tracers (Fahrion et al. 2020a).
By combining the GC metallicities of the full sample, we study the relation between photometric colours and metallicities. We find a non-linear colour-metallicity relation that has implications for the interpretation of colour distributions because their shape is not preserved in metallicity. This challenges the simple division of GC origins into in-situ and accreted based on a single colour cut (Fahrion et al. 2020b).