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The End of Dark Galaxies

A few years ago, the discovery of a galaxy without stars caused great consternation amongst researchers. Until then, a galaxy was thought to include stars, gases and dust, all surrounded by an invisible halo of dark matter. Starless VirgoHI21, however, defied the predictions of theoretical models, and soon gave rise to the term “dark galaxy.” But using numerical simulations, a French team from the AIM laboratory1 in Paris has just shown that VirgoHI21 is probably just debris from a high-speed collision between two massive galaxies,2 thus overturning the dark galaxy prototype. While it is predictable for galaxies with a very small mass to contain no stars, it seemed inconceivable that VirgoHI21, roughly the mass of the Milky Way, had never managed to form stars, or had somehow lost them all. In fact, at the time of its discovery, VirgoHI21 appeared to be only an isolated gas cloud in the Virgo galaxy cluster. Even deep optical images from the Hubble Space Telescope had revealed no trace of stars. Yet the rapid rotation of this cloud seemed to suggest it was being driven by an enormous invisible mass akin to a dark matter halo, qualifying it for galaxy status.
The hypothesis that VirgoHI21 was just a gas cloud pulling away after a high velocity collision between galaxies had originally been evoked. But its discoverers dismissed this scenario because no neighboring galaxies appeared disturbed, as would have been expected after a massive collision.
The AIM team chose to revisit this scenario primarily because VirgoHI21’s oblong shape so closely resembled the “tidal tails”–debris in the form of long filaments–that result from a collision between two galaxies. Moreover, researchers had noticed a diffuse bridge between VirgoHI21 and a spiral galaxy to the south called Messier 99, which suggested a possible tidal interaction.
Finally, after performing numerical simulation at the CEA’s CCRT,3 the AIM team discovered that, contrary to previous assumptions, when gas-rich spirals collide at speeds as high as 1000 km/s, purely gaseous matter may be ejected at large distances, leaving the parent galaxies undisturbed. The last remaining argument against VirgoHI21 being the result of colliding galaxies was gone.

Lucille Hagège

Notes :

1. Laboratoire d'Astrophysique des interactions multi-échelle (CNRS / CEA / Université Paris Diderot).
2. P.-A. Duc and F. Bournaud, “Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21,” The Astrophysical Journal, 2008. 673: 787-797.
3. Centre de calcul recherche et technologie.

Contacts :

Pierre Alain Duc,
CEA, Saclay.


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