The Smith cloud: H I associated with the Sgr dwarf?

Abstract

ABSTRA C T The Smith high-velocity cloud (VLSR o 98 km s π1 ) has been observed at two locations in the emission lines [O III]l5007, [N II]l6548 and Ha. Both the [N II] and Ha profiles show bright cores due to the Reynolds layer, and red wings with emission extending to VLSR < 130 km s π1 . This is the first simultaneous detection of two emission lines towards a high-velocity cloud, allowing us to form the ratio of these line profiles as a function of local standard of rest (LSR) velocity. At both cloud positions, we see a clear distinction between emission at the cloud velocity, and the Reynolds layer emission (VLSR < 0). The [N II]/Ha ratio (<0:25) for the Reynolds layer is typical of the warm ionized medium. At the cloud velocity, this ratio is enhanced by a factor of 3‐4 compared to emission at rest with respect to the LSR. A moderately deep upper limit at [O III] (0.12R at 3j) was derived from our data. If the emission arises from dilute photoionization from hot young stars, the highly enhanced [N II]/Ha ratio, the [O III] non-detection and weak Ha emission (0.24‐0.30R) suggest that the Smith cloud is 26 6 4 kpc from the Sun, at a Galactocentric radius of 20 6 4 kpc. This value assumes that the emission arises from an optically thick slab, with a covering fraction of unity as seen by the ionizing photons, the orientation of which is either (a) parallel to the Galactic disc, or (b) such as to maximize the received flux from the disc. The estimated mass and size of the cloud are 4 〈 10 6 M( and 6 kpc. We discuss a possible association with the much larger Sgr dwarf, at a Galactocentric radius of 16 6 2 kpc, which lies within 358 (,12 kpc) of the Smith cloud.