Most galaxies have a large disc. This is the case for the Milky Way galaxy and for the Andromeda galaxy, its closest neighbor. Spiral galaxies make up roughly two thirds of the galaxies in the present Universe, so that understanding their origin constitutes one of the central questions in astrophysics.
When and how these discs were created is still unknown : did they grow by accreting gas from outside, or via the coalescence of smaller entities leading to the creation of more massive galaxies ? This latter scenario is known as the « hierarchical model » of galaxy formation.
A team of astronomers from the Paris Observatory, working with a scientist from the Marseille Laboratory for Astrophysics (CNRS/Aix-Marseille University) has furnished fresh and decisive information on how galactic discs are formed.
Their work is based on the observation of about one hundred distant galaxies which emitted their light 8 billion years ago : a heretofore unprecedented variety of instrumental techniques were enrolled for this work.
Firstly, observations made using the KMOS multi-integral field spectrograph, recently installed on ESO’s Very Large Telescope (VLT), have enabled these galaxies to be studied, and their internal motions to be analyzed.
They have shown that 8 billion years ago, many galaxies were in rotation. This first observation would seem to support a formation scenario involving gas accretion. . However, the spatial resolution of KMOS was not good enough to determine unambiguously the origin of the motion of these distant galaxies.
Consequently, the team of scientists re-analyzed these cinematic observations, by combining them with deep imagery furnished by the Hubble Space Telescope. From this new analysis, it has emerged that their cinematics is far more perturbed than had been thought before.
At the same time, the team has launched the first systematic study of the companion galaxies physically linked to these distant galaxies , using the Hubble telescope’s close infra-red spectroscopic survey coordinated by Yale University.
Almost two thirds of distant galaxies are in the process of combining. The interaction with a massive companion galaxy explains the strange structures seen on the Hubble Space Telescope images, such as the tidal streams and the matter bridges.
Since these distant galaxies are the ancestors of present-day spiral galaxies, the observations show that spiral galaxies recreated a new disc after combining.
These results provide further clear proof in favor of the hierarchical scenario for galaxy formation, in which small galaxies combine to make larger ones.
Reference
This work was published under the title « Morpho-kinematics of z 1 galaxies probe the hierarchical scenario », by M. Rodrigues et al., in the December 13th 2016 issue of Monthly Notices of the Royal Astronomical Society (arXiv:1611.03499).
Collaboration
The team is made up of :
- L’Observatoire de Paris / CNRS / Univ. Paris Diderot, France
- M. Rodrigues (GEPI)
- F. Hammer (GEPI)
- H. Flores (GEPI)
- M. Puech (GEPI)
- Le Laboratoire d’astrophysique de Marseille (CNRS/Aix-Marseille
Université).- E. Athanassoula
