The Magellanic System's Interactive Formations

M.E. Putman, PASA, 17 (1), 1.

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Magellanic Complexes

The three high velocity complexes which are classified as Magellanic debris are the Magellanic Bridge, the Magellanic Stream and the Leading Arm (see Putman (2000) for a review). The Magellanic Bridge (see Figure 1) is a $\sim 10^8$ M$_{\odot}$ filament of neutral hydrogen which joins the two Magellanic Clouds and has a velocity gradient which proceeds from 125 km ${\rm s}^{-1}$  at the tail of the SMC or Shapley's Wing, to $\sim 300$ km ${\rm s}^{-1}$  at the extended arm of the LMC. Tidal models predict that the Bridge was pulled from the SMC during a close encounter between the two Clouds 0.2 Gyr ago (Gardiner & Noguchi 1996). The HIPASS data reveal new structure along the Bridge, as well as an extension from the LMC which suggests that the Bridge is made of both SMC and LMC material. The Bridge is the only high-velocity cloud which is known to contain stars, and the age of many of the stellar concentrations (10 - 25 Myr) indicates that the Bridge is a star forming region (Grondin, Demers and Kunkel 1992).

Figure 1: Neutral hydrogen column density map of the Magellanic Bridge which joins the Large and Small Magellanic Clouds. The projection was chosen based on previous maps of the system and the main features are labelled. The contours are 1, 2, 4, 8, 16 and 32

x 1020 cm-2.

\begin{figure} \centerline{\psfig{,angle=-90,height=4in,width=4in,bbllx=5pt,bburx=560pt,bblly=80pt,bbury=715pt,clip=}}\end{figure}

Figure 2: Integrated intensity map of the Magellanic Stream (

vLSR = -400 to +400 km ${\rm s}^{-1}$), which starts at the SMC and Bridge and extends to declination $+02^{\circ }$. The location of the clumps MS I to MS IV, which were originally classified by Mathewson et al. (1979), are labelled for reference. The Stream passes through the velocity of Galactic emission at approximately declination $-35^{\circ }$, making it somewhat difficult to trace. At these velocities, Galactic emission has been blanked as much as possible.

\begin{figure} \centerline{\psfig{,height=6in,width=6in,bbllx=30pt,bburx= 500pt,bblly=2pt,bbury=500pt,clip=}} \end{figure}

The HIPASS map of the Magellanic Stream is shown in Figure 2. The Stream trails the Magellanic Clouds for over 100$^{\circ}$ and is not a confined filament as previous maps depict (Mathewson, Cleary and Murray 1974), but a complex network of filaments and clumps. The beginning of the Stream consists of multiple filaments and bow-shock structures as it spews from the northern side of the SMC and Bridge at

vlsr = 90 - 240 km ${\rm s}^{-1}$. Most of these filaments end at declination

$\sim 60^{\circ}$, but the main filament of the Stream continues to march northward, its bifurcated structure diminishing towards the northern tip1. Dense clumps follow the Stream in position and velocity, except for the region of the Sculptor Group which will be discussed in the next section. There are no stars in the Stream (e.g. Guhathakurta & Reitzel 1998), but H$_\alpha$ appears to be detectable at every location along the Stream with a column density greater than 1019 M$_{\odot}$ (Weiner & Williams 1996; Bland-Hawthorn & Maloney 1999a). These detections, together with the fact that no [OIII] has been detected from the Stream, indicates that the Stream is being ionized by photons escaping from the Galaxy (Bland-Hawthorn & Maloney 1999b).

The third Magellanic complex is a natural tidal counterpart to the Stream, and is called the Leading Arm (see Figure 3). This feature is more diffuse than the Stream and its leading position indicates that tidal forces are the dominant mechanism responsible for forming the Magellanic Stream. This complex was only recently confidently defined as Magellanic debris (Putman et al. 1998), as the course resolution and observing grid used in the previous surveys missed the filaments between the clumps and the connection to the Magellanic Clouds. Further evidence that this feature is of Magellanic origin is the metallicity determination of Lu et al. (1998), using the background galaxy NGC 3783.

Figure 3: A HIPASS peak intensity map which shows the full extent of the Leading Arm, as well as the Magellanic Clouds, the Bridge and the beginning of the Stream (as labelled). The position of the background galaxy, NGC 3783, is also noted (see text). To avoid the emission from the Galactic Plane (which extends out to 120 km ${\rm s}^{-1}$  in this direction), only velocities between 130 and 400 km ${\rm s}^{-1}$  were used. (Thus the truncated appearance of the SMC which begins at $\approx 80$ km ${\rm s}^{-1}$.)
\begin{figure} \centerline{\psfig{,height=4in,width=4in,bbllx=10pt,bburx= 550pt,bblly=5pt,bbury=550pt,clip=}}\end{figure}

Next Section: Is it all Magellanic
Title/Abstract Page: The Magellanic System's Interactive
Previous Section: Introduction
Contents Page: Volume 17, Number 1

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