The countdown to survey science with ASKAP reaches its half-way point

Stage 5

Image of a complex field

Although 2020 has been a challenging year in many ways, the countdown to survey science with ASKAP continues. With additional precautions and procedures in place to protect our staff at the observatory and many of our scientists and developers working from home, data continues to flow from the telescope to supercomputers and astronomers around the world.

Today we showcase a new image and highlight GASKAP, a survey science team that uses ASKAP to study our nearest galactic neighbours.

Preparing ASKAP for surveys requires extensive testing of all the capabilities that will be required to achieve our science goals. The countdown milestones become successively broader and more ambitious as the cycle progresses and we begin to break new ground in terms of complexity and scale.

The half-way point is especially important because it marks the transition from self-contained tests on individual fields, to pilot surveys covering significant areas of sky, and even our first rapid all-sky survey. In fact, many of the later milestones are well underway and we look forward to sharing more results soon!

The 5th capability test fittingly involves the most complicated task for which ASKAP was intended – imaging of extremely large objects with extended radio emission that covers the telescope’s entire 30 square degree field of view. To test this capability, we re-visit one of our favourite dwarf galaxies, the Small Magellanic Cloud (SMC).

A New Approach to Imaging

The SMC is a well-studied but peculiar satellite galaxy of the Milky Way. Its relative proximity to our own Galaxy and complex structure makes it an ideal field to demonstrate ASKAP’s capability for cutting-edge astronomy research into the evolution of nearby galaxies.

The animation shown below is a data cube, which consists of two spatial dimensions (the 2D image in each frame) and one spectral dimension (each frame being a different radio frequency channel). The densest regions of neutral hydrogen correspond to the brightest yellow in each frame. By measuring the brightness and distribution of gas in each channel, we can build a 3D picture.

This particular cube is the result of about 12 hours of observing in a newly available Zoom mode, which allows us to step through the neutral hydrogen content of the SMC in smaller increments than ever before.

Additionally, since the observed neutral hydrogen extends over ASKAP’s entire field of view, we’ve had to apply a unique method of processing to these data. Typical ASKAP fields are produced by stitching together images from individual beams. In the case of the SMC, we risk losing important structure that extends over the boundaries of multiple beams during this stitching stage.

To ensure the most accurate possible image and circumvent computer memory limitations, Dr Nickolas Pingel wrote a custom imaging pipeline that utilizes the Common Astronomy Software Applications (CASA) package to stitch all of the ASKAP beams together simultaneously before progressing on to further processing stages. The GASKAP team then used Swinburne University’s OzStar supercomputer to process the full data set and construct the image cube. The result is a spectacular view of one of our nearest and most mysterious galactic neighbors.

Caption: Image cube of the neutral hydrogen in the Small Magellanic Cloud prepared by Dr Nickolas Pingel with data from ASKAP-36 and Parkes on the OzStar supercomputer.

Neutral Hydrogen in the SMC

Hydrogen is the most abundant element in the Universe, making up ~3/4s of all observable matter. Fortunately, due to the well-known interactions between a proton and electron, neutral hydrogen emits radio waves at a specific frequency (1.420 GHz) that is easily observable with ASKAP.

Through the Doppler effect, astronomers can measure the relative radial motions of the observed hydrogen gas. The small incremental steps shown in the animation provide novel insight into how the SMC rotates and interacts with our own Milky Way.

We also know that hydrogen is the raw fuel for star formation. The amazing abundance of small-scale structure seen in each step of the animation has important implications for understanding exactly how neutral hydrogen contributes to the conditions necessary for star formation.

On the other end of stellar evolution, the many observed holes in the neutral hydrogen distribution (e.g., check out the left side of frames 170-160 km/s) reveals the influence dying stars have on their surroundings when they go supernova. These novel ASKAP data provide astronomers an unprecedented view into the physics and life cycle of interstellar gas.

GASKAP

The Galactic ASKAP (GASKAP) Survey is one of several science surveys to be undertaken with ASKAP. GASKAP will provide unprecedented views of the distribution of neutral hydrogen and hydroxide within our own Milky Way and the nearby Magellanic System. GASKAP is an international collaboration that brings together world renowned expertise in the physics of interstellar gas, young and evolved stars, and galaxy evolution. Overcoming the immense technical challenge of imaging very extended emission —  as demonstrated by the presented data cube — ensures GASKAP will provide the gold-standard data set for the neutral hydrogen and OH associated with the Milky Way and Magellanic System for the next decade and beyond.

Over the coming months, we will work to improve the speed and efficiency of this new processing method in preparation for a hardware refresh at the Pawsey Supercomputing Centre, which should make it possible to scale up for the full GASKAP survey.