J. Blandhawthorn et al., THE SMITH CLOUD - H-I ASSOCIATED WITH THE SGR DWARF, Monthly Notices of the Royal Astronomical Society, 299(2), 1998, pp. 611-624
The Smith high-velocity cloud (V-LSR = 98 km s(-1)) has been observed
at two locations in the emission lines [O III] lambda 5007, [N II] lam
bda 6548 and H alpha. Both the [N II] and H alpha profiles show bright
cores due to the Reynolds layer, and red wings with emission extendin
g to V-LSR approximate to 130 km s(-1). This is the first simultaneous
detection of two emission lines towards a high-velocity cloud, allowi
ng 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 cle
ar distinction between emission at the cloud velocity, and the Reynold
s layer emission (V-LSR approximate to 0). The [N II]/H alpha ratio (a
pproximate to 0.25) for the Reynolds layer is typical of the warm ioni
zed 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 moderat
ely deep upper limit at EO mi (0.12R at 3 sigma) was derived from our
data. If the emission arises from dilute photoionization from hot youn
g stars, the highly enhanced [N II]/H alpha ratio, the [O III] non-det
ection and weak H alpha emission (0.24-0.30R) suggest that the Smith c
loud is 26 +/- 4 kpc from the Sun, at a Galactocentric radius of 20 +/
- 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 Galac
tic disc, or (b) such as to maximize the received flux from the disc.
The estimated mass and size of the cloud are 4 X 10(6) M-. and 6 kpc.
We discuss a possible association with the much larger Sgr dwarf, at a
Galactocentric radius of 16 +/- 2 kpc, which lies within 35 degrees (
similar to 12 kpc) of the Smith cloud.