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17th of February 2016
21 cm HI-line spectra of southern ZOA galaxies
by Khaled Said (UCT)
Galaxies are distributed in a cosmic web consisting of high-density clusters and galaxy groups, which are embedded in walls, and filaments surrounding large low-density regions, the so-called voids. Tracing these Large-Scale Structures (LSS) of the cosmic web can be used to explain the formation and evolution of the universe. But many of these LSS are hidden behind the gas, dust and stellar disc our Milky Way, including such notably large structures as the Great Attractor, Perseus-Pisces Supercluster, Puppis Cluster and the Local Void. Extragalactic astronomers have traditionally focused their observations on the sky that is well apart from the Galactic Plane to avoid confusion and veiling effects. This results in the so-called Zone of Avoidance (ZoA), or a large region of the sky, 20 - 40%, that is sparsely studied. A full census of these important structures, both spatial and kinematic, is necessary in order to understand the evolving observable universe, and this requires the ZoA to be unveiled. Fortunately, the extent of the ZoA is wavelength-dependent, with infrared and radio techniques providing new windows to look through the Milky Way. Therefore we use the near-infrared bands, which is less affected by extinction than optical and the HI 21-cm, which is not affected by dust to measure peculiar velocities in the ZoA, engaged using the Tully-Fisher redshift-independent distance relation.

The attached image shows the distribution of the 394 inclined spiral galaxies selected from the Parkes HI Zone of Avoidance survey (HIZOA) color coded by their velocity. Follow-up narrow-band Parkes HI observations were obtained for 290 galaxies and additional 104 galaxies where derived directly from the HIZOA survey. The boundaries of the three HIZOA survey areas are plotted in blue (HIZOA-South), green (HIZOA-North) and red (Galactic Bulge).

Reference: Khaled Said, Renee C. Kraan-Korteweg, Lister Staveley-Smith, Wendy L. Williams, T. H. Jarrett, and Christopher M. Springob 2016, MNRAS, in press

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