Study of H I and Star Formation Sites in the Magellanic Bridge

1. Study of HI and Star Formation Sites in the Magellanic Bridge Erik Muller: University of Wollongong, ATNF. Lister Staveley-Smith: ATNF. Bill Zealey: University of Wollongong. The expanding shell and stellar population. The overlaid crosses and circles in Figure 1a show the positions of OB associations (Bica & Schmitt, 1995) and apparently expanding HI regions (Muller et al. 2003). There is an apparent spatial correlation between the regions of bright HI, stellar associations and expanding shells in the Bridge, however a more qualitative analysis (Muller et al, 2003) has shown that the expanding shell and OB association population correlate quite poorly. However, the stellar associations appear to preferentially develop where the column density >7.5x1020 cm-2 (Muller et al, 2003) . Abstract: Tidal features represent an early stage in a process crucial for the recycling of matter across extremely large spatial scales. The Magellanic Bridge is the closest gaseous tidal feature to our own Galaxy, and offers an ideal opportunity to study the active processes in such phenomena at high spatial resolutions otherwise unavailable anywhere else. The neutral hydrogen of the western Magellanic Bridge, has been observed with both the Parkes 64m telescope and ATCA interferometer and the resulting dataset has shown that the Bridge exhibits extremely complex and chaotic morphology. We present these observations here, with reference to a variety of characteristics: The unusual dominant two-component velocity structure; the spatial correlation of apparently expanding HI shells with OB associations and the outcome of sensitive Ha and CO surveys. Introduction: The Magellanic Bridge is a filament primarily of neutral hydrogen (HI), extending approximately ~22 kpc between the Small and Large Magellanic clouds. The Bridge is clearly kinematically and morphologically contiguous with the SMC and is largely considered to be a tidal feature. This region has been observed the 21 cm wavelength using the 375m configuration of the ATCA and with the Parkes Telescopes. Position-position and position-velocity projections of the resulting dataset are shown as integrated intensity maps in Figure 1. b a Star formation sites: Detected CO regions. The estimated age of the observed OB stellar population in the Magellanic Bridge are relatively young: between 7 Myr 100 Myr (Hambly et al. 1994; Grondin et al. 1990) . In comparison with the estimated age of the Bridge itself of ~200 Myr (e.g. Fujimoto, 1976; Gardiner, Sawa & Fujimoto, 1994; Gardiner & Noguchi, 1996), this implies that stars are currently forming in situ, out of Bridge material. At this stage however, only one molecular regions in the Bridge has been detected (Lehner, 2002). Figure 2a shows a magnified part of the HI in the Magellanic Bridge, showing four regions studied with both the Mopra and SEST telescopes. This region was selected as a candidate CO emission site based on findings by Israel et al. (1993), where sites of CO emission were often found around regions which were showed 60 µm excess. Of the four tested locations shown in Figure 2a, pointings 1, 2 and 4 show emission at the 12CO(1-0) rotational transition (Muller, Staveley-Smith & Zealey, 2003). The four spectra shown in Figure 2b are those corresponding to each of the four pointings shown in Figure 2a. Empirical models relating the line width and radii of CO regions in the SMC (Rubio, Lequeux & Boulanger, 1993) applied here suggest that the CO emission region in the Bridge has a radius of <16 pc. This location is the first molecular region in the Bridge to be observed at the 12CO(1-0) rotational transition. Figure 1. (Top-Left, Panel a) The RA-Dec. Integrated intensity map of the observed HI in the direction of the Magellanic Bridge. The intensity wedge is shown with units in K.kms-1. Overlaid circles show the positions and sizes of the detected expanding shell population (Muller et al, 2003). Small crosses show the positions of known OB associations and clusters (Bica & Schmitt, 1995). Panels b (top) and c (left) show the RA-Vel and Vel-Dec projections respectively. Overlaid on panel c are lines of constant velocity relative to Galactic centre, of 38 and 8 kms-1. These lines highlight an apparent bifurcation in the velocity distribution of HI in the Magellanic Bridge (Muller et al, 2003). c b Figure 2. Panel a (left) shows a sub-section of HI map from Figure 1a. Overlaid are 60µm brightness contours of 0.1 to 1.20 MJy sr-1, in steps of 0.1 MJy sr-1. The Spectra in Panel b (right) shows results from four pointings as the numbered circles in Panel a. The numbering of the pointings is that of the order in which they were observed. a Ha emission objects. New, sensitive Ha plates have been searched for extended and discrete Ha emission regions has yielded approximately ten new obvious emission features (Muller & Staveley-Smith, in progress). Object catalogues have been consulted in an effort to locate possible sources of radiative ionisation. A study of continuum datasets is the subject of future work. Figure 3 shows an uncalibrated map of some new Ha objects (labelled as MBHa4 and MBHa3) found in the Magellanic Bridge at RA 01h 50m 05s, Dec. -74o 37’ 26”. The overlaid crosses and labels indicate the positions of catalogued objects within ~5 arcmin (as shown by the circle). These nearby objects are also listed in Table 1. Figure 3. (Right) New Ha emission feature found within the Magellanic Bridge, MBHa4 and MBHa3. MBHa4 is the crescent-like feature (image manipulation shows this to be a complete circular feature, Muller & Staveley-Smith, in progress), MBHa4 is the smaller diffuse patch to the south-west. This dataset is uncalibrated. Overlaid are the positions of relevant catalogued objects within ~5 acrminutes of the features. The extent of the object search is shown by the overlaid circle. Table 1. (Left) Positions of objects from Figure 3. References: Bica, E. L.D., Schmitt, H.R., 1995 ApJ Sup. Ser. 101, 41 Fujimoto, M., 1979, ‘Large scale characteristics of the Galaxy” conf. proc. 557 Gardiner, L.T., Sawa, T., Fujimoto, M., 1994, MNRAS, 266 567 Gardiner, L.T. Noguchi, M., 1996, MNRAS, 278, 191 Grondin, L., Demers, S., Kunkel, W.E, Irwin, M.J., 1990, AJ, 100, 663 Hambly, N.,Dufton, P.L., Keenan, F.P., Rolleston, W.R.J., Howarth, I.D., Irwin, M.J., 1994, A&A, 285, 716 Israel et al. 1993, A&A, 278, 25 Lehner, N., 2002, ApJ, 578, 126 Muller, E. 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