Segue 1 is one of the tiny satellite galaxies orbiting the Milky Way whose dark matter component has caused great astronomical interest. As we saw in this post a couple of weeks ago, these ultra-faint objects have been turning up in Sloan Digital Sky Survey data, surprising astronomers by their mass, which indicates they’re dominated by dark matter.
Consider them top-heavy with the stuff: Segue 1 turns out to be a billion times fainter than the Milky Way, yet a study by members of the same team shows that it is a thousand times more massive than would be expected by its visible stars. The new regime of faint galaxies offers intriguing observational clues to galaxy formation while putting dark matter’s properties on display. Thus Marla Geha (Yale University):
“These dwarf galaxies tell us a great deal about galaxy formation. For example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant.”
Supercomputer simulations are also put to work to study how dark matter interacts with galaxies. A new paper shows that while most early clumps of dark matter eventually merged to form a halo around the Milky Way, the largest would have been torn apart to form a disk of dark matter within the galaxy itself. If that’s the case, the dark matter disk would be less dense than the halo. Because the dark matter halo does not rotate around galactic center like the Sun, dark matter should be flowing toward us at considerable speed. The disk, on the other hand, rotates along with the stars and thus produces little of this dark matter ‘wind.’
Image: A composite image of the dark matter disk (red contours) and the Atlas image mosaic of the Milky Way obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Credit: J. Read & O. Agertz.
This has interesting implications for detection, according to Laura Baudis (University of Zurich), who is one of the lead investigators for the XENON direct detection experiment that is looking for dark matter at the Gran Sasso Underground Laboratory in Italy. Baudis is quoted in this Royal Astronomical Society news release:
“Current detectors cannot distinguish these slow moving particles from other background ‘noise.’ But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”
Well, we’ll see. Dark matter’s presence in and around our galaxy seems increasingly clear but nailing down what it consists of has been an elusive challenge, to say the least. Doing so would be hugely important because cold dark matter (CDM) is part of the overall model being continuously refined by such studies. That model, called ΛCDM or Lambda-CDM, includes a cosmological constant Λ that makes up 72 percent of the energy density of the universe, yet another area of immense scientific interest. And the development of a dark matter disk is, the authors of the new study believe, inevitable under this model. From the paper:
In this paper, we study how the Milky Way disc affects the accretion of satellite galaxies in a ΛCDM cosmology, and how these satellites in turn affect the Milky Way disc. The Milky Way disc is the dominant mass component of the Milky Way interior to the solar circle. It is important because dynamical friction against the disc causes satellites to be preferentially dragged into the disc plane… As satellites are torn apart by tidal forces, they deposit both their stars and their dark matter into a thick disc. The latter point is the key new idea presented in this work: a dark matter disc must form in a ΛCDM cosmology and we set out to quantify its mass and kinematic properties.
While we are seeing the dark matter puzzle examined through simulation and observation, we are a long way from fully integrating its effects into theories of galaxy formation. The work is knotty, highly theoretical and carries the almost surreal excitement of making sense out of something we cannot see. The simulation paper is Read et al., “Thin, Thick and Dark discs in ΛCDM,” Monthly Notices of the Royal Astronomical Society 389 (2008), pp. 1041-1057 (abstract). The paper on Segue 1 is Geha et al., “The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1,” accepted by the Astrophysical Journal and available online.