Ruprecht-Karls-Universität Heidelberg


Stellar Cosmogony: Using Stars to Decipher Galaxy Evolution

The bimodal colour distribution of galaxies, brought in the limelight by the SDSS survey is, perhaps, the most direct proof of the role played by star formation in the cosmic life of galaxies. The blue "cloud" of this distribution is populated by star-forming galaxies, the majority of the local galaxy population. They experience a prolonged star formation activity, with a rate that depends on the amount of available cold gas. This rate becomes virtually zero only in early-type galaxies (inhabiting the red sequence of the galaxy colour distribution), which are characterised by old stellar populations formed at high redshift.
Both the blue-cloud and the red-sequence galaxies exhibit important scaling laws, which link luminosity, surface brightness and mass to metallicity. They tell us that, once born, stars change the ionization  state, the kinematics and chemistry of the interstellar medium, and, thus, change the initial conditions of the next episode of star formation. Substantially, star formation is a loop mechanism which drives the luminosity and  metallicity evolution of a galaxy. The details of such a complex mechanism are still observationally not well established and theoretically not well developed, but set the basis of our understanding of galaxy evolution from the early Universe to the present day.

In what follows, I address some of these open questions which are  central theme of my research.


Star formation modes. What are the properties of young, massive star clusters? Do they build up the field stellar population?
Star formation preferentially occurs in stellar aggregates spanning a wide range of morfologies and total masses. We use optical/NIR, HST/LBT/VLT imaging and spectroscopic data of Galactic star-forming regions and clusters to study their resolved stellar populations, their detailed internal structure and their stellar feedback, i.e. how they affect the local Insterstellar Medium, its kinematics, ionization level, dust properties and chemical composition. This analysis is extended to young massive clusters in starburst galaxies, mergers and ULIRGS imaged with HST, in order to characterize their age and mass distributions.
We also use Galactic open clusters and the chemical abundances of their stars as derived by the Gaia/ESO and APOGEE surveys to test stellar evolution models, and their predictions regarding first dredge-up, extra-mixing and diffusion.


The star-formation history of early-type galaxies. When did early-type form their stellar populations?
Early-type galaxies in the local Universe form a remarkably homogeneous class of objects with a tight colour-magnitude relation and a well-defined fundamental plane. The small scatter in their colours and mass-to-light ratios suggests that they formed the bulk of their stars at high redshift and, since then, evolved passively. These properties fit reasonably well the hierarchical paradigm of galaxy formation. In order to fine-tune  the current theory of galaxy formation and evolution, we need to extend the study of early-type galaxies to intermediate and high redshifts, and hence trace their mass assembly and star formation histories in great detail. 
We use the grism mode of HST/ACS and HST/WFC3 to acquire spectra of early-type galaxies in the GOODS fields in the redshift range 0.5 < z < 1, in order to study their stellar populations as a function of galaxy stellar mass and redshift. We also study the stellar population properties and the star formation activity in early-type galaxies at z~0 and as a function of galaxy environment using SDSS, CAHA/PPAK and VLT/XShooter data.


The Initial Mass function of early-type galaxies: Kroupa-like or  bottom-heavy?
In recent years the analysis of the kinematics and gravity-sensitive spectral features of early-type galaxies has pointed to a non-universal IMF, whose slope is consistent with a Kroupa's IMF value in low velocity-dispersion  (low stellar mass) galaxies and compatible with a bottom-heavy IMF in massive early-type galaxies. We use VLT/XShooter spectra to measure a large number of optical/NIR spectral indices and single out the effect of the IMF against other stellar population parameters (in particular abundance ratios), as a function of galacto-centric distance.
In the case of a massive, high velocity-dispersion galaxy we find a bottom-heavy IMF in the galaxy core changing to a standard Milky Way IMF at about half the effective radius. This result is in line with the predictions of hierarchical structure growth. We also use SDSS spectra of early-type galaxies to investigate the dependence of their IMF slope on their host environment and their hierarchy (centrals vs satellites).


Star formation and galaxy environment. How does environment affect the star formation activity of galaxies?
Galaxy evolution is driven by intrinsic and environmental processes, both contributing to shape the observed properties of galaxies. A number of early studies, both observational and theoretical, have shown that the star formation activity of galaxies depends on their environmental local density and also on galaxy hierarchy, i.e. centrals vs. satellites.
We use the SDSS group catalogue of Yang et al. (2007) to study the properties of satellite and central galaxies over a large range of stellar mass and a wide interval of dark matter mass of their host environments, in order to constrain the environmental effects at play, such as strangulation, ram-pressure and tidal stripping, harassment, and ¨cannibalism¨. 

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