Key Project 5

Key Project 5

Structure of the Milky Way and the Local Group

Visualization of the orbit of the Ophiuchus tidal stream. The stream crossed the Milky Way plane twice in the past 370 million years. Its present location and direction are indicated by the arrow.

Our galaxy, the Milky Way, is located in a relatively small group of about 50 galaxies known as the Local Group. The Milky Way is one of the two most massive galaxies in it, the other one being the Andromeda Galaxy. Most of the Local Group galaxies are dwarf irregulars and the majority are gravitationally associated (i.e. as satellites) with the largest members of the group. The structure of the Local Group is dynamic, where each member moves in response to the gravitational pull from all other matter (luminous or dark). Small galaxies close to larger ones are more susceptible to the effects of gravity—they are in danger of being stretched, torn apart, or even engulfed by their larger counterparts (known as hierarchical merging). Therefore, the study of the structure and dynamical history of the Local Group, in particular of the Milky Way and its satellites, is a crucial in determining the shape of our galaxy's gravitational potential and the nearby dark matter distribution, which is otherwise undetectable. In addition, understanding the evolution of the Milky Way and its neighbors provide a local testbed for the emergent general picture of galaxy formation in a ΛCDM cosmology. 

Pan-STARRS1 mapped our galaxy to a level of detail never achieved before. The survey provides, for the first time, a deep and global view of a significant fraction of the Milky Way plane and disk—an area usually avoided by surveys given the complexity of mapping these dense and dusty regions. In this context, MPIA leads ground-breaking research in a number of sub-projects within the Pan-STARRS1 Key Project 5:

  • A census of Local Group and free-floating satellites by means of resolved populations
  • (Sub-)Structure of the faint outer stellar envelope of the Milky Way and those of other galaxies
  • The global structure of the Galaxy
  • Globular clusters
  • Open cluster and stellar association census
  • 3-D dust map of the Milky Way
  • Dynamics of the 1 kpc around the Sun
  • Structure of the bulge
  • Galactic structure and substructure via RR Lyrae stars as primary tracers
A volume rendering of the interstellar dust of the Milky Way, based on real data. The camera revolves around the Sun in a 50 parsec (160 light-year) orbit, and remains pointed directly away from the center of the Galaxy. Famous dust clouds, such as Taurus, Perseus and Orion, are labeled. The Sun is labeled "Sol," for its Latin name.

The 3D model of Milky Way dust used in this movie is based on photometric data on 800 million stars observed by Pan-STARRS1 and data from the 2MASS survey.

Among the top achievements of Key Project 5 by MPIA researchers is the first complete mapping of the Monoceros stream—a tidal stellar stream that (almost fully) wraps around the Milky Way, the discovery of the Ophiuchus tidal stellar stream, the creation of a 3D dust map of the Milky Way, the detection of a dust ring in the Orion star-forming region, the discovery of two low-luminosity Milky Way satellites and three distant dwarf galaxies satellites of Andromeda, and a catalog of 25.8 x 106 variable point sources and likely QSOs.

The extent of the Sagittarius tidal stream from the distribution of RR Lyrae candidates within ± 9 degrees off the Sagittarius plane. The leading and trailing arm of Sagittarius stream can be identified, as well as several substructures up to more than 100 kpc. The longitudes where the Galactic plane intersects the Sagittarius plane are marked.

Pan-STARRS1 has an enormous potential for the all-sky identification of variable sources, such as QSOs and RR Lyrae stars.

Catalogs of RR Lyrae stars are very helpful, as RR Lyrae are used as tracers of Galactic substructures, stellar streams and dwarf satellite galaxies.

Using RR Lyrae periods and metallicities allows for a precise measurement of distances. Also, RR Lyrae candidates make the search for halo streams and dwarf satellite galaxies possible, constraining galaxy formation and evolution models and providing data for mapping the Galactic potential.

In further applications, e.g. to reconstruct the history of the Milky Way, identified stars can be used as tracers of underlying stellar populations. Additional properties like period and detailed shape of lightcurves are helpful in this case.

To identify RR Lyrae stars and quasars among about 400 million sources, we used parameters that characterize the variability of sources and an automated classifier, trained on sample of known quasars and RR Lyrae stars. Among the sources, there are more than a million QSO candidates and an unprecedentedly large and deep sample of about 250,000 RR Lyrae candidates, at distances from ~10 kpc to ~120 kpc.

This analysis enables a detailed view on the structure in the Milky Way halo. For example, we show the Sagittarius leading and trailing arms and the Virgo Overdensity in more detail than existing work.

Further reading:

catalog of variable sources, containing 25.8 x 106 variable point sources and likely QSOs, is available.

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