The HiEnvironment of the Sculptor Dwarf Spheroidal Galaxy

Abstract

New observations of the neutral hydrogen (H I) in and around the line of sight of the Sculptor dwarf spheroidal galaxy (dSph) are presented. The data obtained with the single-dish Parkes telescope cover a large area of 7° × 7° in the direction of the dwarf and have resolutions of 155 × 1.12 km s-1. The Australia Telescope Compact Array was used to map a smaller area of 22 × 22 centered on the direction of the dwarf with higher resolutions (350'' × 140'' × 1.65 km s-1). Many H I structures having velocities outside the range of the normal Galactic disk velocities were detected, including the two Sculptor clouds (northeast and southwest) of Carignan et al. (C98). The present study shows the total extent of the C98 clouds. We derived heliocentric radial velocities for the northeast and southwest clouds of 100.2 ± 0.9 and 105.1 ± 0.3 km s-1, respectively. The intensity-weighted mean H I velocity for both clouds is 104.1 ± 0.4 km s-1. Three different hypotheses concerning the association of the C98 Sculptor clouds were considered. The case for the clouds belonging to the Sculptor group of galaxies is found to be inconsistent with the observational data. The probability of the C98 Sculptor clouds to be Milky Way features at anomalous velocities (HVCs) superposed by chance on the Sculptor dSph is estimated to be less than 2%. The third hypothesis assumes that the clouds are physically associated with the Sculptor dSph and is supported by the following evidence: (1) the radial velocities for both clouds are very close to the optical velocity of the Sculptor dSph (ΔV = 4 ± 3 km s-1), (2) 88% of the total H I flux is contained within the optical radius of the galaxy, and (3) the clouds are located symmetrically relative to the center of the Sculptor dSph. Arguments are presented that the C98 Sculptor clouds are still gravitationally bound to the dwarf galaxy and are part of its interstellar medium. The mass of each cloud is (4.1 ± 0.2) × 104 M⊙ (northeast cloud) and (1.93 ± 0.02) × 105 M⊙ (southwest cloud) at the Sculptor dSph distance (79 kpc).