Discovery of Neutral Hydrogen Associated with the Diffuse Shells of NGC 5128 (Centaurus A)

Abstract

In this Letter, we present for the first time images and kinematics of neutral hydrogen, which is unambiguously associated with the optical shells of an E/S0 galaxy, NGC 5128. Using the VLA, we have mapped diffuse H I (21 cm) emission out to ~15 kpc (15') from the nucleus of the galaxy to a limiting column density of 10^20^ cm^-2^ These observations reveal the presence of 1.5 x 10^8^ M_sun_ of H I, 10'-15' NE, N, and SW of the galaxy. The H I distribution follows the curvature of the outermost "diffuse" shells, and is slightly displaced, shifted 1' to the outside of the shells. The H I shows a smooth gradient in velocity over ~450 km s^-1^, nearly the same line-of-sight velocity range observed in the dust lane. Our results suggest that the H I might be a partial ring, rotating with the same sense as the main body of the galaxy. We discuss merger, accretion, and internal shell formation models and describe how the observed properties of the H I challenge existing theories of shell formation. We also find a morphological and kinematical connection between the NE cloud of H I and the outer optical filaments of Cen A, identifying the H I as the source of gas being ionized. Finally, we discuss the implications of these results on the study of high-z radio galaxies with extended emission-line regions.