Dwarf galaxies and the Galactic halo

Dwarf galaxies play a major role in the formation and evolution of the Galactic halo. These systems, with respect to the Milky Way, illustrate typical examples of minor merging events, the collision of galaxies that differ in mass by at least a factor of ten. In a minor merger, tidal forces from the larger galaxy are capable of gradually stripping material (dark matter, stars and gas) from the satellite galaxy, that after several orbital periods gets fully disrupted. The best methodology to detect the tidal debris of disrupted satellites combines information on the position, motion, and chemical composition of individual tracers. Minor mergers occur frequently in the Universe and a more comprehensive knowledge of such mergers will allow us to better understand hierarchical structure formation in a cosmological context.

The research in the “Dwarf galaxies and the Galactic halo” section is focused on the study of the Magellanic Clouds, as an example of an early phase of a minor merger event, and on the study of the earliest history of the Milky Way galaxy by uncovering and analysing its oldest stars, combining dedicated observations with predictions from cosmological simulations. The properties of the most pristine stars in the Milky Way compared to those in its surrounding satellite dwarf galaxies provide valuable insight on early galaxy evolution in different environments.

Contact: Prof. Dr. Maria-Rosa Cioni, +49-331-7499-651, mcioni@aip.de

The people of this group are presented here: Team


Magellanic Clouds

[Magellanic clouds] The Magellanic Clouds are the largest and most luminous dwarf satellite galaxies of the Milky Way. In spite of their important role in supplementing material to the Milky Way halo and the numerousinvestigations made in the last decade, there remain several uncertainties. Our research is focused on understanding the links between the star formation history and the chemical enrichment history with the dynamical history of the system.

Our group is leading the VISTA survey of the Magellanic Clouds system (VMC) and has been awarded an ERC consolidator grant on “Using the Magellanic Clouds to study the interaction of galaxies” (INTERCLOUDS). It leads also a spectroscopic 4MOST consortium survey to obtain radial velocities and chemical abundances for half a million stars that sample stellar populations across the structural components of the Magellanic Clouds. Our group is involved in several photometric projects that observe the Magellanic Clouds at optical wavelengths and across wide areas, these aim to characterise and trace stellar populations in external regions and to uncover substructures, e.g. new satellites.

Cepheids are very bright pulsating stars which are important standard candles through the period-luminosity relation. They are instrumental to calibrate the extra-galactic distance scale. We are studying the effect of metallicity on these calibrations using samples of low metallicity Cepheids in the Magellanic Clouds combined with similar samples of more metal rich Galactic Cepheids.


The Early Milky Way

[early milky way]

What was the Universe like just after the Big Bang when the first stars were forming to shine their light? One avenue to study this question is to look carefully at our own Galaxy where the oldest stars alive today still carry information from the early Universe. Because stars keep a chemical fingerprint during their lives, and also preserve kinematical information for long times, studying the present-day stars can teach us about the past. The main aim of our group is to unravel the information contained within the oldest and most pristine stellar populations in our Galaxy and use it to make significant progress in our understanding in the early formation of galaxies like our Milky Way and their satellites.

Our group is based on an DFG Emmy Noether Group grant and we are co-leading the "Pristine" survey especially designed to uncover the most metal-deficient stars in the Galaxy. We are furthermore involved in several international observational projects to study the most metal-deficient stars in the Galaxy - including the future generations of multi-object spectrographs WEAVE and 4MOST - as well as a careful comparison of observational results with cosmological models of Milky Way formation.

Contact: Dr. Else Starkenburg, +49-331-7499-213, estarkenburg@aip.de


The VMC survey

[Tarantula nebula] The VISTA survey of the Magellanic Clouds system (VMC) is the most sensitive near-infrared imaging survey of the Magellanic Clouds to date. It aims to uncover the detailed spatially resolved star formation history and the three-dimensional geometry of the system from different age indicators. The VMC survey is an imaging programme in three filters (Y, J, and Ks) sampling an area of ~170 deg2 and stars as faint as ~22 mag (5σ Vega) in each waveband. It also includes a monitoring component with at least 3 epochs in Y and J filters and 12 epochs in the Ks filter. The VMC survey is a Public Survey project of the European Southern Observatory. The VISTA telescope is a 4-m near-infrared optimised telescope equipped with a large array of 16 infrared detectors that fill a 1.5 square degree field. VISTA observations for the VMC survey started in 2009 and will be completed in 2018. The VMC team has produced many articles that address scientific questions that span a wide range of topics.




This is a European Research Council consolidator grant project that aims to offer a firm perspective of the Magellanic Clouds system. By providing a comprehensive census of the orbits, ages and compositions of the stars and a robust understanding of the history, structure and dynamics of the Magellanic Clouds this study will have immediate implications for knowledge of the Milky Way halo, on the understanding of minor merging events, and will also strengthen our analysis techniques, that in the future may be applied to more distant systems.





The Pristine Survey

[first stars]

Studying extremely metal-deficient stars is greatly hampered by the difficulty in finding them. Stars with less than 1/1000 of the solar metal content - of which some will belong to the second generation of stars formed - constitute 0.1% of the population even if one observes towards the metal-poor Galactic halo. The Pristine survey greatly helps uncovering these stars very efficiently by using a narrow-band color filter on the wide-field imager of the 4-meter Canada-France-Hawaii Telescope. This color filter is centered on very strong calcium absorption lines in stellar spectra. By measuring the light in this wavelength region through the narrow-band filter in comparison with broad-band filters we isolate very metal-deficient candidates with small calcium absorption lines.