1998/02/28 by J. Bland-Hawthorn, Joss Bland‐Hawthorn, Sylvain Veilleux +9 · 4 citations
Chemistry · Physics and Astronomy · #Astrophysics #Astrophysics and Star Formation Studies #Atomic and Molecular Physics #Geometry #Line (geometry) #Physics #Spectroscopy and Laser Applications #astro-ph
paper · pdf · doi:10.1046/j.1365-8711.1998.01902.x
18 pages, 14 figures, mn.sty. Our first application of a new method for establishing distances to high velocity clouds. This version matches paper to appear in MNRAS, 299, 611-624 (Sept. 11 issue)
arxiv created 1998/07/22 · openalex publication_date 1998/09/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
The Smith high-velocity cloud (VLSR = 98 km s−1) has been observed at two locations in the emission lines [O iii]λ5007, [N ii]λ6548 and Hα. Both the [N ii] and Hα profiles show bright cores due to the Reynolds layer, and red wings with emission extending to VLSR ≈ 130 km s−1. 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]/Hα 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 3σ) was derived from our data. If the emission arises from dilute photoionization from hot young stars, the highly enhanced [N ii]/Hα ratio, the [O iii] non-detection and weak Hα emission (0.24–0.30R) suggest that the Smith cloud 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 Galactic disc, or (b) such as to maximize the received flux from the disc. The estimated mass and size of the cloud are 4 × 106 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° (∼12 kpc) of the Smith cloud.