Origin and evolution of galaxies
Chemical evolution and kinematics of the Milky Way
The Milky Way constitutes a testbed for studies of the physics and evolution of galaxies. The analysis of the chemical composition of its various stellar components (bulge, disc, halo) constrains the properties of the stars that progressively enriched the galactic gas with chemical elements heavier than helium ; it also helps to reconstruct the history of star formation and gas accretion from the intergalactic medium into the dark matter halo of the Milky Way. Along with those “traditional” studies, our group recently started to investigate the role of the various dynamical processes leading to the vertical heating of the disk and the radial motions of stars and gas, while they interact with the inhomogeneities in the gravitational field. The new generation of models that account for these dynamical effects will serve to intepret the large amount of data from forthcoming observational surveys, in particular those expected from the european space mission GAIA.
Permanent researcher: Nikos Prantzos
Interstellar medium and star formation
The interstellar medium (ISM) comprises gas in several phases (ionized, atomic and molecular), dust, and cosmic rays. It plays a key role in galaxy evolution, through star formation and feedback processes. New stars form from the local gravitational collapse of the ISM, return metals to it at the end of their lives, and constantly inject energy into the ISM in the form of radiation, winds and supernova ejecta and shocks. Other sources of excitation include tidal interactions and active galactic nuclei. When the gas cools down, it produces spectral signatures allowing to characterize its physical properties and the processes at play.
The following topics are addressed by exploiting observations of various ISM tracers, from the far-ultraviolet to centimeter radio waves : chemical evolution mechanisms in galaxies (comparing the abundances of the neutral and ionized ISM); dust properties, and radiative transfer effects (explaining that in some classes of starbursts, the radiated energy is almost entirely converted to infrared emission); the circulation of matter and energy through different galactic components; and the star formation activity in different types of galaxies and intergalactic environments.
Permanent researchers: Michel Dennefeld, Daniel Kunth
Turbulence and regulation of galaxy formation
Galaxies form by gravitational collapse of gas in dark matter haloes having reached a dynamical equilibrium (virialized). Gas must cool and condense for stars to form, but several processes limits this cooling. The gravitational energy released by the formation of large-scale structures (the hierarchical clustering of dark and baryonic matter in the current paradigm of CDM cosmology) and the mechanical + radiative energy released by mergers and feedback by stars and supermassive black holes is transferred to the intergalactic and interstellar gas as heat, turbulence, and cosmic rays. This energy must be dissipated for star formation to proceed. How does this energy input impact the star formation efficiency? This question, which puts the physics of the interstellar matter into the broad context of galaxy evolution, is addressed by researchers in this group, as it is central to our understanding of galaxy formation. They study the physics of gas and dust in nearby galaxies exhibiting suppressed star formation and in haloes of galaxies, both where turbulence is much higher than in the Milky Way.
Comparison of observations and models allow the researchers in this group to characterize the physical and dynamical state of the molecular gas not associated with star formation, which is present in active phases of galaxy evolution where large amounts of kinetic energy is being dissipated (Active Galactic nuclei, galaxy mergers, galactic winds, gas accretion onto haloes and cluster of galaxies). Characterizing the injection of turbulence into gas accreted onto haloes of galaxies is crucial to understand how star formation is regulated and why galaxy formation is such an inefficient process.
Permanent researchers: Pierre Guillard
Starburst galaxies
Starburst galaxies are fundamental to understand the formation of stars and galaxies, and the enrichment of the interstellar medium in heavy elements. Local galaxies also serve as laboratories for understanding the distant Universe. In this context, the so-called "Wolf-Rayet" galaxies are examined in order to elucidate their excess in some galaxies deficient in heavy elements. The formation of massive stars in starburst galaxies is accompanied by an intense emission of Lyman α photons, much of which is diffuse. The work of the group's researchers relates to the quantification of the parameters that regulate this emission in galaxies, and serves as a bridge to the search for galaxies at high redshift.
Permanent researchers: Hakim Atek, Daniel Kunth
Internal kinematics of galaxies and clusters
Permanent researchers: Gary Mamon
Multiwavelength study of galaxy clusters
The coupled observations at visible, infrared and X-ray wavelengths of galaxy clusters allows one to better understand not only their present properties, but also how they have evolved with time. In X-rays, temperature maps of the hot gas provide researchers with insight into recent mergers, whereas metallicity maps, coupled with the results of numerical simulations, allow them to understand how the intergalactic medium has been enriched with metals. At optical wavelengths, galaxy luminosity functions lead to estimates of the proportions of galaxies at various masses, while far infrared imaging reveals how cluster mergers can in some cases increase the star formation rate in galaxies. The properties and formation mechanisms of compact groups of galaxies are also analysed within cosmological simulations.
Permanent researchers: Florence Durret, Gary Mamon
Ancestors and descendants of galaxies
The great diversity of galaxies can be described by a reduced number of classes, characterizing their shape, their brigthness, their colors: elliptical, lenticular, spiral, irregular, dwarf spheroidal. These properties are the result of the growth of galaxies by fusions between them, as well as by the accretion of gases during the migration of galaxies along the sheets and filaments of the cosmic web, and the ejections of gas at the end of star evolution.
In order to decipher how galaxies have changed over time, it is necessary to characterize the different types of galaxie at different epochs of the history of the Universe. This requires the analysis of large imaging surveys containing hundreds of thousands of galaxies with sophisticated software, based on modelling galaxies. Bayesian inference is often used for exploring parameter spaces, as well as deep learning using neural networks. One can then measure, for example, the evolution with time of the proportion of high and low mass galaxies for each class, and therefore understand how their mutual interactions and those with their intergalactic environment have determined their star formation.
Permanent researchers:Valérie de Lapparent, Damien Le Borgne
Models of galaxy evolution
Modeling the observed properties of galaxies is a necessary step towards understanding
their formation and evolution. Two multiple-purpose, public codes are being developed
at the IAP: Galaxev and
PÉGASE. By combining models of stellar
evolution with stellar spectral libraries, these codes allow one to follow, for
different types of galaxies, the evolution of the stellar, gas and dust content
and the associated global spectral energy distribution. The user can define the
law according to which stars form. Therefore, the evolution of stellar populations
and of the interstellar medium can be followed in a coherent way. The two codes
developed at the IAP are both intended to reproduce in the best possible way the
observed properties of nearby galaxies, but they differ in their detailed treatments
of, e.g., chemical enrichment and the emission from ionized gas, dust and compact X-ray
sources in galaxies. These codes are widely used at the IAP and by astronomers worldwide
to interpret the predictions from cosmological N-body simulations and to measure physical
parameters, such as stellar mass, metallicity, age and redshift in galaxies observed with
modern large telescopes. Analytic models of the formation of galaxies in the local Universe
are also being developed by this research group.
Permanent researchers: Michel Fioc, Damien Le Borgne, Brigitte Rocca-Volmerange
High redshift galaxies
Permanent researchers: Hakim Atek, Daniel Kunth, Brigitte Rocca-Volmerange, Jacques Roland
Messier 99
