Dark Matter in the Dwarf Galaxy NGC 247

Abstract

Dwarf galaxies are dominated by dark matter even in the innermost regions and, therefore, provide excellent probes for the investigation of dark halos. To that purpose, we analyse ROSAT PSPC-data of the dwarf galaxy NGC 247. We focus in particular on the diffuse X-ray emission in the $1/4 $keV band. Assuming an isothermal density profile, we find that the mass of the hot emitting gas is about $10^8 {\rm M_{\odot}}$, corresponding to $\lesssim 0.5%$ of the total dynamical mass of the galaxy. The total mass of NGC 247, as derived from the X-ray data agrees quite well with the value obtained from the measured rotation curve (Burlak). The X-ray profile in the $3/4 $keV and $1.5 $keV band shows an excess at a radial distance of about $15 $ arcmin from the center. Such a ``hump'' in the radial X-ray profile can be explained by the presence of a cluster of young low mass stars or brown dwarfs. Therefore, NGC 247 offers the possibility to observe the formation of a halo of MACHOs.