Distributions of dwarf galaxies around giant elliptical galaxies NGC 3923 and NGC4636

抄録

To obtain more information on the environment around elliptical galaxies, we observed the properties of dwarf galaxies around two giant elliptical galaxies, NGC 4636 and NGC 3923, using optical wide-field deep imaging. These galaxies have similar optical luminosities but have widely varying X-ray luminosities. We applied a convolution technique to detect low surface brightness objects, such as dwarf galaxies and used detection algorithms to identify groups of connected pixels exceeding a predetermined threshold above the background level. The wellobserved radial profiles around NGC 4636 reveal that its dwarf galaxies are highly concentrated, especially its red dwarf galaxies. Although both NGC 4636 and NGC 3923 are similarly massive elliptical galaxies (M > 1011Mo), based on gravitational mass profiles, NGC 4636 has a larger-scale potential structure than NGC 3923, with its dark matter surroundings becoming dominant. At a large distance of 300 kpc, NGC 4636 seems to have a deeper dark matter halo than NGC 3923, with a slightly higher number density of dwarf galaxies. In this work, our analyses obtained complete optical number density of dwarf galaxies around elliptical galaxies down to an absolute magnitude of MV = −15.5 within 300 kpc, which is the region of dark matter dominance. Given that elliptical galaxies typically have an absolute magnitude of around MV = −22 mag, the faint end corresponded to a stellar mass ∼ 1/1000 that of a giant elliptical galaxy, with a typical mass of ∼ 108M⊙. The number density of galaxies at MV = −15.5 is 112.2 ± 65.21 Mpc−2 around the X-ray-bright NGC 4636 and 38.7 ± 36.15 Mpc−2 around the X-ray-faint NGC 3923. These values are lower than that of rich clusters of galaxies but higher than that of our local galaxy group that includes M31.