H [CSC]i[/CSC] Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud

Abstract

We present a 93' × 93' map of the area near the Local Group dwarf galaxy LGS 3, centered on an H I cloud 30' away from the galaxy. Previous authors associated this cloud with LGS 3 but relied on observations made with a 36' beam. Our high-resolution (34), wide-field Arecibo observations of the region reveal that the H I cloud is distinct from the galaxy and suggest an interaction between the two. We point out faint emission features in the map that may be gas that has been tidally removed from the H I cloud by LGS 3. We also derive the rotation curve of the cloud and find that it is in solid-body rotation out to a radius of 10', beyond which the rotation velocity begins to decline. Assuming a spherical geometry for the cloud, the implied mass is 2.8 × 107(d/Mpc) M☉, where d is the distance in Mpc. The observed H I mass is 5.5 × 106(d/Mpc)2 M☉, implying that the cloud is dark matter dominated unless its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity cloud that is undergoing a tidal interaction with LGS 3 and therefore is located roughly 700 kpc away from the Milky Way. The cloud then contains a total mass of ~2.0 × 107 M☉, 82% of which consists of dark matter.