My research interests concentrate on galaxy formation and evolution, probed by the stellar populations and structural properties of their compact stellar systems (young, old and nuclear star clusters). One of the most efficient ways to reconstruct galaxy star formation history (SFH) is by studying their most readily observable stellar population. They represent snapshots of the formation and chemical enrichment history of their host galaxies, and have been recognized as a powerful tool to reconstruct galaxy assembly, merging and SFH.
Old globular clusters, whose properties reflect the physical conditions at the time of their early formation, and comparison with GCs in massive galaxies can be used to quantify the contribution of dwarf galaxies to the assembly of today's massive galaxies and their globular cluster system.
An excellent example is the Milky Way globular cluster system. It harbors a sub-population of GCs whose properties (colors, luminosities, chemical compositions, horizontal-branch morphologies, structural parameters and orbital kinematics) suggest that they might have formed in lower-mass satellites and were later incorporated into the Milky Way. In particular, the GC properties in combination with the specific frequencies (the number of GCs per unit galaxy luminosity/mass, i.e. the specific GC formation efficiency) are very useful tools to perform the aforementioned study.
The horizontal branch (HB) morphology of old GCs is an important parameter because it can significantly affect the age of the GC estimated from integrated light optical absorption line indices spectroscopy. Hot, blue HB stars enhance high-order Balmer lines which render younger GC age. To tackle this GCs' age-HB degeneracy, I am working on a method that utilizes a full spectrum fitting analysis.
A robust estimate of the GCs' properties (age, metallicity, mass, structural parameters) allows one to draw conclusions on the host galaxy star formation history, driven by accretions, mergers and galaxy-galaxy interactions in galaxy groups and clusters.
Top: Hubble Space
Telescope (HST/ACS) image (from V and I filters) of the dwarf spheroidal
galaxy KK 197. A zoom at its nuclear GC is shown in the bottom left
corner.
Bottom: Spectroscopic
age and metallicity estimate using full spectrum fitting analysis. Two
highly probable solutions for GC age at same metallicity reveals the
effect of hot, blue HB stars (Georgiev et al. in prep.).