Scientific Rationale

Garching, Germany, 23-27 February, 2015

Baryons at low densities: the stellar halos around galaxies


Stellar halos are ubiquitous in luminous galaxies, but because of their faint surface brightness the detailed study of their physical properties has been difficult and, until recently, confined largely to the Milky Way (MW) and Andromeda. Since the advent of large cameras and surveys, both from ground and space, our knowledge of stellar halos is increasing. Several late and early-type galaxies had stellar halo properties traced out to hundred kiloparsecs or beyond, revealing very low luminosity extended stellar structures similar to halos of our own MW and our closest neighbor, the Andromeda galaxy.

Observations show that these halos have complex morphologies with multiple stellar components, complex kinematics and substructures that indicate past history of mergers. These morphologies resemble the density maps from cosmological simulations of galaxy formation in a hierarchical universe and a large amount of effort is invested to try to understand how we go from a qualitative resemblance to quantitative measurements and frequencies of these substructures.

This workshop will bring together theorists and observers to discuss the results from the space and ground-based surveys of stellar halos in disks and ellipticals as well as from simulations. The properties of the MW halo will also be discussed as a representative example of halos in spiral galaxies, with the main focus of the workshop being extragalactic. The aim is to review our knowledge (or lack thereof) about the physical properties of stellar halos and their origin in the context of cosmological predictions.

Main Science Topics include

  • What is a stellar halo?
  • Physical properties of stellar halo tracers from across the Universe
  • Kinematics and dynamical evolution of stellar halos
  • Origin and assembly of stellar halos


Setting the scene: defining a stellar halo

What is assumed by a galaxy halo? While the stellar halos appear to be relatively easy to distinguish from the disks and bulges in spiral galaxies, this may not be so easy for halos in elliptical galaxies. Are the halo stars those found in the outskirts and how is the location of the stars in the halo linked to the formation of the halo and galaxy as a whole?

In the first session we start  with the theoretical and observational definition of the halo component in galaxies.


Physical properties of stellar halo tracers  across the Universe

Bright red giant stars as well as planetary nebulae and globular clusters are used to trace stellar halos in nearby galaxies. In more distant galaxies very deep integrated photometry and stacking techniques are adopted to investigate the low luminosity outskirts of galaxies and infer the properties of the underlying stellar populations. These challenging observations are revealing the ubiquity of stellar halos, their extent, density and metallicity gradients and presence of substructures and multiple components.  During the workshop recent observational advances will be reviewed from wide area deep observations of individual galaxies using different stellar halo tracers as well as from surveys that aim to establish global properties of stellar halos such as stellar density structure, amount of substructure, the extent of the halos, their metallicity, age and density gradients.


Kinematics and dynamical evolution of stellar halos

Since dynamical times are of the order of Gyrs at a distance of 50 kpc or larger, the stellar halos may contain the fossil records of the events that led to their formation. By combining information on the amount of substructures and the dynamical status of the stellar halos as a function of distance from the bright galaxy host with the morphology of halos in different environments and redshifts, we can set constraints on the epoch of most likely assembly of these components of galaxies.


Origin and assembly of stellar halos

The next challenge is combining the observational properties, kinematics, stellar populations content and structure, with the predictions from the simulations. This is important in order to answer the questions about how galaxy halos and galaxies in general form as well as whether halos (or parts of) formed together with the bright regions of galaxies (the in-situ stars) or were acquired at a  later epoch (the accreted stars). More will come with the future observing facilities as well as ever more detailed and extended simulations. 


Other recent and upcoming meetings on related topics

Stellar halos was one of the topics to be covered in the MPA/MPE/ESO/EC Joint Conference on The Physical Link between Galaxies and their Halos in 2013. This workshop will be concentrating on stellar halos only, and on the study and the comparison of their physical properties across morphological galaxy types, host masses, environments and redshifts, and on the predictions from cosmological simulations of galaxy formation. It will set the stage for many meetings that will take place later in 2015, like the MIAPP workshop in Garching on the MW spectroscopic surveys in May, “Satellites and Streams” workshop in Santiago in April, and the IAU symposium 317 “The General Assembly of Galaxy Halos: Structure, Origin and Evolution” at the IAU General Assembly in August. The MIAPP workshop is fully dedicated to the MW and the IAUS 317 has a strong component about the Milky Way stellar halo, which is complementary to the present workshop that will have a stronger emphasis on the properties of other galaxies in comparison with the MW. The “Satellites and Streams” workshop will cover additionally the substructures that are found far beyond the stellar halos: their satellites and tidal structures.   

The timing for this workshop is appropriate to discuss the results of the space telescopes’ (HST, Spitzer, etc.) as well as ground based telescopes' (VLT, VISTA, Subaru, ING, Keck, Magellan, CFHT, etc.) surveys of stellar halos in disks and ellipticals in the nearby Universe as well as properties of more distant galaxy halos observed through stacking techniques of many thousands of galaxies. This ESO workshop will provide a forum for a discussion on stellar halos and their implications for galaxy formation as well as other important cosmological problems.