Observation of the halo of the edge-on galaxy IC5249

Abstract

We report optical photometry and HI synthesis observations of the southern edge-on S_c,d galaxy IC5249. The observations were carried out primarily to determine if IC5249 has a red halo surrounding it similar to the halo that has been recently found surrounding the northern edge-on galaxy NGC5907. The galaxy IC5249 is almost perfectly edge-on, and it shows no direct sign of warping or flaring out to its optical radius. It has a low luminosity and a slow rotation velocity. We find that it has a remarkably similar halo to NGC5907. The halo is visible at a distance of 3kpc from the disk of the galaxy, and at 6kpc the surface brightness is ~ 28 mag arcsec^-2. The measured distribution of halo light is similar to the distribution expected for dark matter. Red dwarfs and red giants are possible sources of the halo light. If red dwarfs are the source, and if they are distributed like dark matter, then their density at galactocentric radius r = 8.5 kpc is (7+/-3) times the the local Galactic density of halo red dwarfs. Their total contribution to the mass of IC5249 would be about 10% of the mass of the galaxy. If red giants are the dominant source of the observed halo light, and if they are distributed like Galactic gobular clusters, then their density at r = 8.5 kpc is 1.0 +1.0-0.5 times the local Galactic density of halo giants, and they provide a gravitationally insignificant fraction of the mass of the galaxy. Further optical measurements are required to identify the source of the halo light in IC5249, and also to determine more precisely its distribution. Further radio measurements are required to examine the HI distribution and rotation velocity beyond the optical radius of the galaxy.