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We present the first fully and uniformly sampled, spatially complete $\ion{H}{i}$ survey of the entire Magellanic System with high velocity resolution ($\Delta v = 1.0$ km s$^{-1}$), performed with the Parkes Telescope. Approximately 24 percent of the southern sky was covered by this survey on a $\approx$5´ grid with an angular resolution of ${\it HPBW} = 14\farcm1$. A fully automated data-reduction scheme was developed for this survey to handle the large number of $\ion{H}{i}$ spectra ($1.5\times10^6$). The individual Hanning smoothed and polarization averaged spectra have an rms brightness temperature noise of $\sigma$ = 0.12 K. The final data-cubes have an rms noise of $\sigma_{\rm rms} \approx 0.05$ K and an effective angular resolution of $\approx$16´. In this paper we describe the survey parameters, the data-reduction and the general distribution of the $\ion{H}{i}$ gas. The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are associated with huge gaseous features – the Magellanic Bridge, the Interface Region, the Magellanic Stream, and the Leading Arm – with a total $\ion{H}{i}$ mass of $M$($\ion{H}{i}$) = $4.87\times10^8~M_\odot \left[d/55~{\rm kpc}\right]^2$, if all $\ion{H}{i}$ gas is at the same distance of 55 kpc. Approximately two thirds of this $\ion{H}{i}$ gas is located close to the Magellanic Clouds (Magellanic Bridge and Interface Region), and 25% of the $\ion{H}{i}$ gas is associated with the Magellanic Stream. The Leading Arm has a four times lower $\ion{H}{i}$ mass than the Magellanic Stream, corresponding to 6% of the total $\ion{H}{i}$ mass of the gaseous features. We have analyzed the velocity field of the Magellanic Clouds and their neighborhood introducing a LMC-standard-of-rest frame. The $\ion{H}{i}$ in the Magellanic Bridge shows low velocities relative to the Magellanic Clouds suggesting an almost parallel motion, while the gas in the Interface Region has significantly higher relative velocities indicating that this gas is leaving the Magellanic Bridge building up a new section of the Magellanic Stream. The Leading Arm is connected to the Magellanic Bridge close to an extended arm of the LMC. The clouds in the Magellanic Stream and the Leading Arm show significant differences, both in the column density distribution and in the shapes of the line profiles. The $\ion{H}{i}$ gas in the Magellanic Stream is more smoothly distributed than the gas in the Leading Arm. These morphological differences can be explained if the Leading Arm is at considerably lower $z$-heights and embedded in a higher pressure ambient medium.

AbstractThe interaction between the Galaxy and the Magellanic Clouds has resulted in several high-velocity complexes which are connected to the Clouds. The complexes are known as the Magellanic Bridge, an HI connection between the Large and Small Magellanic Clouds, the Magellanic Stream, a 10° × 100° HI filament which trails the Clouds, and the Leading Arm, a diffuse HI filament which leads the Clouds. The mechanism responsible for these features formation remains under some debate, with the lack of detailed HI observations being one of the limiting factors in resolving the issue. Here I present several large mosaics of HI Parkes All-Sky Survey (HIPASS) data which show the full extent of the three Magellanic complexes at almost twice the resolution of previous observations. These interactive features are connected, but unique in their spatial and velocity distribution. The differences may shed light on their origin and present environment. Dense clumps of HI along the sightline to the Sculptor Group, which may or may not be associated with the Magellanic complexes, are also discussed.

AbstractWith the onset of the HIParkes All-Sky Survey (HIPASS), a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HIsky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately.

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 × 106M⊙ 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.