NGC 247 is a dwarf galaxy and a member of the Sculptor group which contains several other prominent members such as NGC 253, NGC 300, NGC 7793 and NGC 55. Dwarf galaxies are defined by their small size and mass and are the most numerous type of galaxies within groups and cluster, exceeding the number of spirals by a factor of ten. In the standard scenario of cosmic evolution, galaxies are built up via hierarchical merging of smaller galaxies. Dwarf galaxies being the most numerous type in the universe are thought to represent simple primordial galaxies unchanged over billions of years. As simple systems they are thought to play a major role in galaxy formation as essential building blocks. Recent observations show that contrary to previous notions about dwarf galaxies, they appear to possess complex structures that on a smaller scale rival larger disk galaxies like the Milky Way. Their stellar and gas content and extended halo structures suggest that dwarf galaxies have had their own complex evolution and formation histories and in turn have formed from lower mass precursors.
A vexing problem in astronomy today is the elucidation of the huge mass present in galactic halos which cannot be explained by visible matter alone. So called "dark matter" may account for up to 10 times the visible matter that holds galaxies together. The additional invisible matter was initially proposed to explain the flat rotation curves of galactic disks at large galactocentric radii. The rotation curve of a galaxy is a plot of the rotational velocity of an object in the galactic plane versus distance to the center. From observations of starlight alone, the rotational velocity would be expected to fall at distances further from the center of the galaxy if the gas and stars represent the total mass of the galaxy. In fact the curve flattens, suggesting that the galaxy has much more mass than can be explained by luminous matter and must be surrounded by a halo of unseen, dark matter.
Dwarf galaxies show rotation curves that rather than being flat like ordinary spirals, seem to rise steadily at greater distances from the core. The conclusion is that higher than normal concentrations of dark matter must be driving them. Dwarfs are fascinating subjects for astronomers as they appear to be dark matter dominated. Compared to ordinary spiral galaxies, dwarf galaxies have at least two fold greater concentrations of dark matter in their halos. They are also richer in gas content as well which probably accounts for some of the unexpected mass of these galaxies. The nature and origin of extended dark matter halos in Dwarf galaxies remains a mystery at present although several interesting theories have been offered as an explanation such as the presence of massive black holes.