Galactic Dark Matter Modeled

I don’t spend too much time worrying about the ultimate fate of the Earth as it interacts with a swollen red Sun some five billion years from now. My thought is that if any civilization is still on the planet in a billion years, it will have long since worked out how to exit when necessary (and it will be necessary a lot sooner than five billion years!), or maybe how to tweak planetary orbits to preserve our planet, if only as a choice historical site.

Still less do I worry about the Milky Way being destroyed by a collision with one or more satellite galaxies, like the Large and Small Magellanic Clouds that move around the parent galaxy. So when I read that an Ohio State team led by Stelios Kazantzidis had shown via computer simulations that such a collision would leave the galaxy more or less intact, my real interest was in the implications of this work in terms of one of science’s great mysteries — the nature of dark matter. Have a look at the team’s modeling of the dark matter structure thought to surround galaxies like ours.

Image: This image from a supercomputer simulation shows the density of dark matter in our Milky Way galaxy which is known to contain an ancient thin disk of stars. Brightness (blue-to-violet-to-red-to-yellow) corresponds to increasing concentration of dark matter. The bright central region corresponds roughly to the Milky Way’s luminous matter of gas and stars and the bright clumps indicate dark-matter satellites orbiting our Milky Way galaxy which are known as “substructure”. The simulation predicts that the dark-matter halos of spiral galaxies are lumpy, filled with hundreds of dark matter substructures that pass through the stellar disks of galaxies, leaving their imprint and disturbing them in the process. Credit: Stelios Kazantzidis, Ohio State University.

If galaxies are embedded within huge haloes of dark matter (the Milky Way’s halo is thought to be a million light years across, or ten times larger than the 100,000 light year width of the galaxy), then a filamentary model of dark matter running throughout the universe emerges, with the larger galaxies at the intersection of dark matter filaments. As this Ohio State news release suggests, satellite galaxies like the Magellanics would move along the strands of this web, gradually drawn into orbit around the larger galaxies.

Kazantzidis’ team ran computer simulations of galaxy formation to study this model, setting up collisions between satellite galaxies (and their associated dark matter) and the larger spiral galaxy. During the collision, the dark matter interacts gravitationally with the spiral galaxy. The result: The satellites pass through the galactic disk again and again, losing mass each time. The effect of their gradual dissolution is that the primary galaxy shows a distinctive signature that is consistent with observation.

Says Kazantzidis:

“We can’t know for sure what’s going to happen to the Milky Way, but we can say that our findings apply to a broad class of galaxies similar to our own. Our simulations showed that the satellite galaxy impacts don’t destroy spiral galaxies — they actually drive their evolution, by producing this flared shape and creating stellar rings — spectacular rings of stars that we’ve seen in many spiral galaxies in the universe.”

The next figure shows the formation of this unique flared shape:

Image: Density maps of disk stars illustrating the global morphological transformation of a galactic disk subject to bombardment by dark matter substructures. Brighter colors indicate regions of higher density of disk stars. The left panel shows the initial disk, while the right panel depicts the final disk after the violent gravitational encounters with the orbiting substructures. The edge-on (upper panels) and face-on (bottom panels) views of the disk are displayed in each frame. Satellite-disk interactions of the kind expected in the currently favored cosmological model produce several distinctive signatures in galactic disks including: long-lived, low-density, ring-like features in the outskirts; conspicuous flares; bars; and faint filamentary structures above the disk plane that resemble tidal streams. These morphological features are similar to those being discovered in the Milky Way, the Andromeda galaxy, and in other spiral galaxies. Credit: Stelios Kazantzidis, Ohio State University.

So we have what Kazantzidis calls ‘a wealth of signatures’ that are both consistent with the current cosmological model (including dark matter) and consistent with observations of other galaxies. The flared edges of the above image, in which a disk that is narrow at the center widens toward the edges, are such a signature. The latest report on this work is in Kazantzidis et al., “Cold Dark Matter Substructure and Galactic Disks. II. Dynamical Effects of Hierarchical Satellite Accretion,” Astrophysical Journal 700 (2009), pp. 1896-1920 (abstract). A preprint is also available.