What is a galaxy ? Stars, gas, some dust and surrounding them an invisible dark matter halo. The discovery a few years ago of a so-called « dark galaxy » devoid of its most famous component - the stars - raised a lot of interest in the public and among astrophysicists puzzled by a specimen which had not been predicted by classical models. A team from the Service d'Astrophysique (SAp) and laboratory AIM (CNRS, CEA-Saclay, Université Paris Diderot has used numerical simulations to show that VirgoHI21, believed to be a massive dark galaxy, was in fact most probably collisional debris from a past encounter between two massive galaxies in the Virgo Cluster. The prototypical dark galaxy is no longer one ! These results will be published in the February 2008 issue of the Astrophysical Journal.
As indicated by its name, VirgoHI21 is a galaxy member of the Virgo cluster first detected by a radio antenna that was carrying out a deep survey of the atomic hydrogen (HI). At first sight, VirgoHI21 appears as an isolated HI cloud containing no stars : even on deep images from the Hubble Space Telescope, none of them are visible. However, the apparent rapid rotation of this gas cloud seem to indicate that it is driven by an invisible mass equivalent to billions of solar masses ; it would hence be a dark matter halo, with small quantities of gas, but no stars, in other words, a « dark galaxy ».
Theoretical models predict that galaxies with a small mass might not contain any stars. Many of them could actually orbit around the Milky Way ; several surveys are being carried out to detect these satellites. But that a galaxy as massive as the Milky Way might never have managed to form stars or have lost all of them is much more puzzling. Since this discovery, several teams have tried to detect other similar ghost galaxies.
However, a French team from the laboratory AIM (CNRS, CEA-Saclay and University Paris Diderot) has just shown with the help of numerical simulations done with the computer of the CEA/CCRT2 that VirgoHI21 is in fact most probably just collisional debris from a past collision between two massive galaxies of the Virgo Cluster. When two galaxies encounter, tidal forces pull out part of the galactic disk. This debris appears as long filaments named « tidal tails » that are made of both gas and stars. The team has shown that under specific circumstances purely gaseous matter might be ejected at large distances and condense in the intergalactic medium ; they look as isolated HI clouds in rotation. This especially occurs when gas-rich spirals encounter at speeds as high as 1000 km/s. High velocity collisions are common in clusters of galaxies like Virgo. Until now, they were not believed to be able to produce long tidal tails. In fact, a close up look at the surroundings of VirgoHI21 reveals the presence of a diffuse bridge between the cloud and a spiral galaxy to the South, Messier 99, which already tells about a possible tidal interaction. However the companion galaxy was missing in the neighborhood. Using a realistic numerical model of this structure, the astrophysicists of AIM have figured out that the companion was now very far away in the cluster. It could be the spiral galaxy Messier 98.
All these arguments support the following scenario : VirgoHI21 is just a gas cloud, pulled from a larger parent galaxy when the latter was hit at high velocity by a companion. The prototypical dark galaxy is no longer one : good news for the theoreticians ; bad ones for the imagination of observers...
" Tidal debris from high-velocity collisions as fake dark galaxies: A numerical model of VirgoHI21 "
P.A. Duc et F. Bournaud; to be published in Astrophysical Journal Februray 2008
for an electronic version (see arxiv:0710.3867 and file PDF- 372 Ko)