Formation and evolution of galaxies

Galaxy evolution in the cosmic web

The matter in the Universe is distributed in a complex filamentary network known as cosmic web. Galaxy clusters, in particular, form at the intersection of cosmic filaments and are the most massive gravitationally bound structures known in the Universe. Both filaments and clusters of galaxies are therefore excellent laboratories to study galaxy evolution in extreme environments.

Stars form in dense and cold molecular hydrogen, which is called gas. One of the main goals of our research at EPFL-LASTRO is  to understand how  large scale environments impact the galaxy gas content and influence their associated star formation on their way to the cluster cores.  To this end we are leading several observing  campaigns at mm wavelength, to detect carbon monoxide (CO) in cluster and filament galaxies from the local universe up to z~3, corresponding a wide range, that is 11.5 Gyr, of cosmic time.  In most conditions, CO traces the molecular hydrogen.Carbon and oxygen are among the most common elements in the Universe and are also the building blocks of all known life on Earth. CO is  also ubiquitous in star forming regions within galaxies. It is characterized by a strong dipole and a corresponding rotational spectrum in the mm domain (~100-1000 GHz). The recent advancements of mm interferometric observatories such as  the Atacama Large Millimeter Array (ALMA) in Chile and the NOrthern Extended Millimeter Array (NOEMA) in the French alps now allow unprecedented studies of molecular gas in distant cluster galaxies in both hemispheres. They help our understanding of the coeval evolution of galaxies and the cosmic web.

Extremely metal-poor stars

Low-mass, long-lived extremely metal-poor stars (EMP, with metal content 1000 times less than that of our Sun, [Fe/H] ≤ -3 dex –  in logarithmic scale)  in the Local Group offer a unique window to investigate the initial stages of the cosmic history. Given that the early Universe was largely devoid of metals, their very low metal content (metallicity) hints that they are exceptionally old. By measuring the surface composition of EMP stars, one has indeed the opportunity to look back in time and learn about the nature of the early Universe. However, our ability to use them to investigate the early stages of star formation is hampered by the very low statistics. Only one in ∼ 80, 000 stars has [Fe/H] ≤ -4 dex and only 12 stars with [Fe/H]≤ – 4.5 are presently known.

We are currently contributing to the international collaboration Pristine  a large imaging and spectroscopic survey to identify extremely metal-poor stars in a very efficient way. Pristine builds on the unique combination of the Ca H&K filter and the wide field of MegaCam on the Canada-France-Hawaii telescope to cover a large footprint and yield a pre-selection of EMP stars that is about 5 times better than previous surveys. At full completion, Pristine will revolutionise the observational landscape of near-field cosmology, delivering thousands of stars with [Fe/H]≤ -3 and a few tens below ≤ -4. The ultimate characterisation of the EMP stars identified by Pristine is given by their chemical patterns. This is achieved with high resolution spectroscopy, deriving a large set of chemical abundances such as C, O, Na, Mg, Al, Si, Ca, O, Ti, Sr, Ba, and Eu. The massive Pristine spectroscopic follow-up campaigns will allow to address a broad range of open issues. What are the physical conditions of the formation of the first stars? What is their initial mass function? When do low-mass stars form and what is the dominant cooling process responsible for cloud fragmentation in the early Universe? What are the relevant nucleosynthesis processes participating to the onset of primordial chemical enrichment? What are the effects of first stars on reionization?

Multiple Stellar Populations in Globular Clusters

Globular clusters (GCs) are among the first objects formed and host some of the oldest stars known. As such, they represent a fossil record of the cosmic history of the host galaxy. Nevertheless, GC formation and evolution still remain two major unsolved problems of modern astrophysics and cosmology. The discovery that GCs host stars with peculiar chemical pattern (called multiple populations, MPs) further challenges our understanding of such complex stellar systems. 

Intrinsic spreads are observed for all elements up to Mg, whereas Fe and heavier elements generally do not vary. GC stars can either exhibit an overabundance of He, N, and Na (sometimes Al) that is associated to a depletion of O and C (sometimes Mg), or show the same chemical composition of field stars at the same metallicity. Stars with field-like abundances are indicated as first population (1P), whereas stars showing some N and Na over-abundance are second population (2P). 

The observed abundance trends have been firstly interpreted as the signatures of hot CNO processing. However, since the CNO-cycle cannot be activated in the stellar interiors of low-mass stars we observe today in GCs, such variations must have originated elsewhere. Many of the scenarios for MPs indeed require the occurrence of several star formation events during GC infancy. Within such a self-enrichment framework, 1P stars are the first that formed in the same star formation event. 2P stars were formed some tens/hundreds of million years later from the enriched ashes of 1P more massive stars. However, even if such self-pollution models were able to explain a number of observational properties, they fail to re-produce some key observations, especially when a global view of the phenomenon is taken. Despite the impressive amount of observations collected in the last two decades, we still lack a satisfactory explanation for MPs and it is unlikely that most of the questions about GC formation and evolution will be answered without a new insight on this puzzling phenomenon.

  Properties of dwarf galaxies in a LCDM universe

In a LCDM universe, dwarf galaxies are building block of larger systems. Relying on numerical simulations, we study the formation and evolution of those small galaxies over a Hubble time and compare their final properties with accurate observations.

The following movie shows the formation and evolution of a dwarf galaxies with an extended star formation history. It ends up with similar properties compared to the Andromeda II local group galaxy.