In recent years a dozen small 'dwarf' galaxies that surround our Milky Way have been discovered. A KIPAC team shows how these tiny galaxies are great places to look for the signatures of dark matter and determine its properties.
The most natural theories of dark matter posit it to be a particle that interacts weakly with ordinary matter, and surrounds the ordinary matter of galaxies with an overdense 'halo'. In the centers of galaxies, where the dark matter density is greatest, one could expect to see the visible signatures of dark matter annihilating into ordinary matter. The ordinary matter carries away the energy of the dark matter particles that annihilated and radiates light. The light, largely in the form of Gamma-rays, would be then be seen on Earth coming from the regions of high dark matter density.
Many people have focused on investigating whether we see the signatures of dark matter annihilation in Gamma-ray observations of our Galaxy's plane. However a major obstacle is separating any Gamma-ray distribution due to the products of dark matter annihilation from the radiation resulting from more 'ordinary' astrophysical processes in the Galaxy, such as the emission from pulsars and high energy cosmic rays. This considerable uncertainty in models of the astrophysical signal has thus far been a major impediment to identifying dark matter annihilation in space.
According to a team of KIPAC researchers, however, the small dwarf satellite galaxies that surround the Milky Way provide an alternative. In recent years dozens of these dwarf galaxies have been discovered by observing overdensities of stars in optical sky surveys, and there estimated to be hundreds more that have yet to be identified. They are differentiated from the similar appearing globular clusters of stars in that most of the dwarfs' mass is dark matter. The dwarfs' dark matter is in the form of a locally overdense 'sub-halo' concentration, hence the appropriate application of the term 'galaxy' to these systems.
KIPAC researchers Louie Strigari and Neelima Sehgal, along with SLAC particle theorist Rouven Essig, have investigated the potential for observations of dwarf galaxies to constrain dark matter models. Gamma-ray observations of dwarf galaxies are not subject to the same astrophysical systematics as observations of the Milky Way. Dwarf galaxies are systems with mostly old, quiet stars, and therefore few pulsars or supernova sources of cosmic rays, the common astrophysical signals. Also, dwarfs have an even higher ratio of dark matter to ordinary matter than regular galaxies, making them the most dark matter dominated systems available. Furthermore, for dwarfs that we see outside of our Galactic plane, the uncertainty due to propagation of products through dense interstellar space is greatly reduced.
In order to show the constraints on dark matter that can be achieved with observations of dwarf satellite galaxies, Strigari, Sehgal, and Essig first estimate the dark matter density and spatial distribution within the dwarfs. This is done by determining the mass profile needed to achieve the observed orbital motions of the dwarf's stars. They can then predict the expected Gamma-ray signal from a dwarf galaxy as a function of the dark matter particle's mass and annihilation cross section.
The researchers have determined constraints on the dark matter particle using existing Gamma-ray observations of four dwarf galaxies with space and ground-based Gamma-ray instruments. These constraints are competitive with the other currently available astrophysical and direct detection constraints. They also show that a recently discovered dwarf, Segue 1, is particularly promising for constraining dark matter properties with data that will come from the Fermi Gamma Ray Space Telescope. They conclude that given one year of data collection, Fermi observations of Segue 1 have excellent prospects to detect dark matter particles with properties that have been proposed to explain the signal observed by the PAMELA instrument.
This work is based in part on a paper published in Physical Review Letters D (2009, 80:032506), and is available from astro-ph at http://arxiv.org/abs/0902.4750.
Dr. Louie Strigari