Observing gas, mass and motions in the “Zone of Avoidance”

We are not living in a fixed rest frame on Earth: the Earth is orbiting around the Sun, the Sun is moving around the centre of our galaxy, and the galaxy itself is moving towards the nearest mass over-density around us, which is the nearest galaxy cluster in Virgo. Similar motions arise from even larger scales and larger mass densities, such as the gravitational anomaly “Great Attractor”. To get a complete overview of these motions, we need a whole-sky map of the galaxies, together with these gravitationally induced motions.

A major stumbling block in these efforts is our own galaxy, which blocks the sky behind it from our view. Many of the large-scale structures are hidden behind the dust and stellar density of our Milky Way, notably the Great Attractor, Perseus-Pisces Supercluster, Puppis Cluster and the Local Void. This results in the so-called “Zone of Avoidance” (ZoA).

Contrarily to optical light though, radio waves travel unhindered through the galaxy. Researchers are therefore using the Parkes radio telescope to measure the gas in galaxies hidden behind the Milky Way. Combining these observations with near infrared (NIR) imaging allows measuring the distance and gravitationally induced peculiar velocity via a distance indicator called the Tully-Fisher (TF) relation.

CAASTRO PhD student Khaled Said, who is jointly supervised by researchers at the Astrophysics, Cosmology and Gravity Centre at the University of Cape Town, South Africa, and CAASTRO Deputy Director Prof Lister Staveley-Smith (ICRAR-UWA), recently published his analysis of 290 new galaxy observations with the Parkes Multibeam Receiver. The team also used 104 additional galaxies from the existing HIZoA survey. The final sample contained 342 inclined spiral galaxies in the southern ZoA with adequate signal-to-noise and HI profiles suitable for their TF analysis.

The aim of this analysis was the development of a systematic processing pipeline for HI line spectra for future applications of the TF relation in the ZoA. Their current sample, in conjunction with their previously published newly calibrated TF relation and current NIR data, will be the first accurate determination of flow fields in the southern ZoA. Their results will be an important contribution to the forthcoming WALLABY survey (Widefield ASKAP L-band Legacy All-sky Blind surveY) and its northern hemisphere counterpart, the Westerbork Northern Sky HI Survey (WNSHS).

Publication details:

Khaled Said et al. in MNRAS (2016): “NIR Tully-Fisher in the Zone of Avoidance. – II. 21 cm HI-line spectra of southern ZOA galaxies”