First image with the ATCA EW 352/367 configurations

The pairing of the newly commissioned EW 352 and EW 367 ATCA configurations offer almost complete coverage of all baselines from 31 to 367 m. Users of the ATCA's old 375 m configuration have long been plagued by the even multiples of the fundamental 15.3 m baseline increment, which resulted in more than a few images with a nasty grating lobe near strong sources. To overcome this problem observers needed an additional four arrays, the 750 m arrays, to fill in the inner u-v plane. The new EW 352 and EW 367 arrays provide, for the first time, an almost completely sampled inner u-v plane with only two arrays. The result is a synthesised beam with minimal sidelobes and excellent new images.

Figure 1: HI image at v=36.3 km/s of Galactic chimney GSH 277+00+36. This image was created from a 1065 pointing mosaic, mainly observed with the EW 352/367 arrays. The greyscale is linear from 5 to 90 K as shown in the wedge on the right.

In the 2000 September and 2001 January terms we used the new arrays for 21 cm continuum and HI spectral line observations of the Galactic chimney, GSH 277+00+36. Figure 1 is the HI image of the shell at v=36.3 km/s created from these observations. GSH 277+00+36 is a very large chimney approximately 6.5 kpc from the sun in the direction of the Sagittarius-Carina spiral arm (McClure-Griffiths et al. 2000). The chimney is believed to have been formed by hundreds of stellar winds and supernovae which pushed a shock wave through the Galactic HI to create a 600 pc diameter void with dense walls of piled-up HI, called an HI shell. As seen in Figure 1, the top and bottom of the shell are open. We believe that as the shell size exceeded the scale height of the HI disk, the top and bottom became dynamically unstable causing the shell to break and vent to the halo via the narrow vertical channels seen in the image. GSH 277+00+46 is one of only a handful of chimneys in the Milky Way and the only chimney known to have broken out of both sides of any galactic disk.

The approximately 100 square degree field observed with EW 352/367 contained 960 pointings. These data were combined with an additional 105 pointings from the Southern Galactic Plane Survey (SGPS). The 960 new pointings were observed in eight 60 second snapshots, four with the EW 352 and four with the EW 367. The deconvolved ATCA image was combined with an image from the Parkes multibeam for sensitivity to angular scales ranging from 3 arcminutes to 10 degrees. The improved resolution of new ATCA mosaic will allow us to explore the small scale structure of the shell and chimney walls. We hope the data will ultimately help us to understand the physics of chimney formation.

Naomi McClure-Griffiths and the SGPS team.
(Naomi.McClure-Griffiths@csiro.au)