Looking Deep into the Universe with ASKAP’s Powerful Eyes
We speak to the scientists involved in the recent cataloguing of data from the record-breaking Rapid ASKAP Continuum Survey.
Late last year, the ASKAP radio telescope broke new records when it surveyed the entire southern sky in the radio wavelengths in just over twelve days. The survey known as the Rapid ASKAP Continuum Survey (RACS) set new records in acquisition, data quality, and resolution, mapping the radio sky in unprecedented detail. The sheer amount of data collected by RACS is remarkable. The survey mapped approximately three million galaxies, a million of which were yet undiscovered until then. Other surveys would take sixteen weeks or more to achieve the level of depth and breadth achieved by RACS.
Unlike traditional radio telescopes, which present as single dish antennas, ASKAP (which is owned and operated by Australia’s national science agency, CSIRO), is an array made up of 36 12-metre antennas, spread over a large area where the longest baseline between any two individual antennas is six kilometres.
These dishes can work together to act as a single unit all scanning an area of the sky of about 30 square degrees in size (about the size of the Southern Cross constellation) by using technology developed by CSIRO, known as phased array feeds (PAF). The PAFs allow each antenna to have 36-individual beams, which then covers a larger portion of the sky - many times larger than other radio telescopes. With the ability to deeply observe such a large region of the sky, the array can achieve its observing goals in about fifteen minutes, which is why it only took about twelve days to survey the southern sky below 41-degrees north.
Now, a new paper (published in the journal Publications of the Astronomical Society of Australia - PASA) outlines the first catalogue of radio sources measured by RACS.
Unlike other emissions that generate the regular (optical, infrared, UV) light we see from stars, most of the radio emissions that are seen by RACS are not produced by thermal processes, but rather synchrotron radiation - when electrons are accelerated and spiral inside magnetic fields.
Radio wavelengths are much longer as well, ranging from millimetres to hundreds of kilometres in size (with ASKAP, these wavelengths are approximately 34-centimetres in length). Our atmosphere blocks out the biggest of these (usually above 20-metres in wavelength) but in the sense of traditional optical telescopes, radio telescopes need to be a lot bigger to detect them. Though, this comes with a catch - the bigger the radio telescope antenna, the more costly it is to build and maintain.
So scientists have devised a clever way to fix this - introducing more antennas.
By separating the antennas by some distance, then this simulates the ‘aperture’ of a much larger virtual antenna that looks out into the heavens. Some configurations and telescope collaborations can extend to many thousands of kilometres, with the recent first-ever image of the M87 supermassive black hole captured using telescopes all around the world - thus simulating a telescope eye as big as Earth itself.
This simulates a dish of significant size, without actually building a dish of significant size. In the case of ASKAP, the 36-array covers an area of six square kilometres; this can be approximated as a six square-kilometre dish that is made up of mostly holes with fragments of dish scattered amongst it. This makes the site easier to build and maintain while keeping pretty much the same functionality of a single dish of that size.
There’s another characteristic of the light (including radio light) received from space that is vitally important, and one that scientists study, called polarisation.
The Polarisation of Light
In a nutshell, it can be defined as the orientation that an electromagnetic wave travels through space. When talking about the polarisation of light there are three main flavours; Linear, Circular and Elliptical.
To visualise each of these, imagine you are holding a long length of rope. Now, say you start to move your hand up and down continuously. The result is a wave propagating down the length of the rope oscillating vertically with your hand. Say now you say moving your hand from side to side. Now the rope is oscillating horizontally. These waves are linearly polarised as they only oscillate in one direction - in fact, linear polarisation can be any single-plane oscillation and not just horizontal or vertical oscillations. That is to say, you can move your hand across your body in a diagonal motion, and the resulting wave would still be classified as linear polarisation.
Now imagine you start to move your hand in a circle, the rope now oscillates circularly. This is a circularly polarised wave. Depending on the direction you rotate your hand, the circular wave you produce has different classifications. If you rotate your hand clockwise, the wave is considered to be right-hand circularly polarised, and if you rotate your hand counter-clockwise then the resulting wave is left-hand Elliptical polarisation occurs when you favour a particular linear direction while making a circle. Elliptical polarisation can also have left- and right-handed variations.
In astronomy, the polarisation of light is further classified by what is known as Stokes Parameters. Stokes parameters are given by the observed light’s intensity and its degree of polarisation, which in turn provide a very useful way to describe how much the electromagnetic wave is polarised. This information plays a very important role, especially in radio astronomy.
There are four quantities in the Stokes Parameters (I, Q, U and V), which include the measurement of the intensity, and then the relationship between how the light is oriented in terms of the horizontal, vertical, elliptical and circular planes.
As part of this recent paper, the RACS project has generated one of the most valuable Stokes I (total intensity of radio sources) catalogues ever produced in radio astronomy, using 799 tiles that cover the entire sky between 80-degrees south and all the way up to 30-degrees north. It features 2.1 million sources, of which many are extra-galactic in nature - opening up a whole new way to look at the Universe.
We had a chat with three of the paper’s authors, Prof. Tara Murphy, Dr Minh Huynh, and Dr Dave McConnell about these exciting findings.
A Chat With ASKAP’s Radio Astronomers
Thank you so much for taking the time to speak with us. This really is an exciting time for radio astronomy - with results now pouring out of ASKAP as it ramps up, along with other telescopes, towards the SKA.
In particular, we’ve really been following the RACS project and are pleasantly in awe of how far things have come in the last few decades - where entire all-sky surveys can be completed in under two weeks. It is truly mind-blowing science when you consider the scale of the survey, the data output and processing, the number of people involved and so much more.
In terms of the RACS, what are the main populations of objects that are observed and why are these important?
“When we look at the sky with ASKAP, the main things we see are radio galaxies. Each of the bright blobs in a RACS image is radio emissions coming from around the supermassive black hole at the centre of a distant galaxy,” said Prof. Murphy. “The processes that happened at the centre of a galaxy are so powerful that jets of particles travelling at nearly the speed of light are emitted into the intergalactic medium. These are completely invisible in optical wavelengths and so radio telescopes provide a unique view of the Universe.”
“But not every blob is a galaxy: there are also nearby flaring stars and really rare events such as the afterglows from supernova and gamma-ray bursts that are caused when a massive star dies or when two neutron stars collide. RACS has already allowed us to discover a number of these more unusual objects,” she said.
I noted in the paper that you didn’t use all of the tiles from the initial survey. Was there a particular reason why some were excluded from the final results?
“Whenever you do a large observational project, you find that a small number of observations are not of the quality that you need to do good science,” said Prof. Murphy. “RACS was partly done as a test of ASKAP's capabilities and so we excluded a small number of the observations to make sure the final survey was as good as it could be.”
“The individual tiles had different resolution, or sharpness, but we needed to make tiles with a common resolution for this full-sky catalogue," said Dr Huynh, This is so we could measure the sources consistently across the whole sky. Some tiles were not able to be made with this common resolution, so they were excluded from this result” added Dr Huynh.
How does RACS and the Stokes I Catalogue compare with previous surveys? Has there been anything like it before?
“One of the exciting things about RACS is that we conducted the survey in four different polarisations. The regular radio image is called Stokes I and this shows you the total intensity of each object (how bright it is),” said Prof. Murphy. “However, we also measured linear polarisation and circular polarisation, which many previous surveys did not do. This has already led to some new discoveries, for example, a set of M-dwarf stars that had never been detected in radio before.”
What were some standout objects that you found in this survey, in relation to this paper?
"The primary purpose of a survey like this is to present a statistical description of the population of radio sources detectable to the telescope, and only secondarily to look for 'standout' objects. Now that the survey results are public, we expect other teams of astronomers to look for particular classes of object." said Dr McConnell, "Already there are some examples. A class of faint, cool stars called 'M-dwarfs' seen for the first time in radio by RACS. Another is the detection in RACS of the most distant known radio-loud quasar that has a red-shift of 6.44, meaning that the radiation left the object more than 10 billion years ago when the Universe was quite young."
What were some challenges that you encountered when putting together the results of this paper?
"RACS is the first all-sky survey conducted with ASKAP and so provided the very first opportunity to properly calibrate the telescope; that is to set the radio brightness scale so that the brightness of objects measured by ASKAP agrees with measurements made by other telescopes in their areas of overlap." said Dr McConnell, "It’s like making a new kind of thermometer, and ensuring that it reads 0 degrees for freezing water and 100 degrees for boiling water. Getting this right had to be done before publishing the new catalogue and involved getting an accurate understanding of ASKAP’s sensitivity pattern on the sky." added Dr McConnell.
How do you actually go about classifying and cataloguing so much data?
“The first step of creating a catalogue, is to make sure all the images are of high-quality and so we conduct a range of quality-control tests and exclude images that don't pass the bar,” said Prof. Murphy. “After that, the next step is to extract the astronomical objects from the image. To do this in a systematic and reliable way we use what is called source finding software. This uses an algorithm that runs through each image and finds objects that are significantly above the background noise level in the image. It records the position, brightness, and shape of each object and we use this information to form an initial catalogue.”
“We then do an enormous number of tests, comparing the RACS catalogue with previous catalogues of the same region to make sure all of the properties make sense. This is a job that requires a lot of care: when you are producing something that will be used as a reference for many different projects is important to make sure it is as accurate as possible,” she added.
Once you have classified and collated the results, how do you organise it so it can be easily accessed by scientists? Is this a particularly challenging part of the process?
"Astronomers maintain a number of internet-based archives of survey results including the catalogues (lists of sources and their characteristics) and collections of images. These provide easy access for all astronomers to the results from many different telescopes covering the electromagnetic spectrum." said Dr McConnell, "The primary store for ASKAP results, including RACS, is CASDA (CSIRO ASKAP Science Data Archive). The RACS images and catalogues have been loaded into that archive, a relatively straightforward process." added Dr McConnell.
"I’ll also note that we build Virtual Observatory protocols and services into our archive, CASDA," said Dr Huynh.
"This makes the data easier to access and use for scientists around the world. For example, we have the catalogue accessible by a Table Access Protocol and Simple Cone Search, so the 2 million sources can be easily queried by scientists, and they can find the radio sources of interest to them," she added.
What does this catalogue mean for future radio sky catalogues and why is this important?
“We set out to do the RACS survey because over the next five years ASKAP will conduct many large radio surveys,” said Prof. Murphy. “To get the most out of these it helps to have a really good model of the sky as seen by our telescope - this is called a sky model. RACS will be a basis for future surveys we carry out, as well as being scientifically interesting in its own right.”
What is next for all-sky surveys? Will you try to break the record made by RACS?
“Firstly, RACS isn't over yet! This paper present the results from one of ASKAP's observing bands but there are two other frequency bands that we are still exploring,” said Prof. Murphy. “We are currently extending RACS to these bands so that we can understand the sky model across all frequencies. When full ASKAP surveys are underway the EMU survey (which stands for the Evolutionary Map of the Universe) will be like RACS, but many times more sensitive.”
“We are also using RACS as a reference image for the ASKAP Variables and Slow Transients (VAST) survey which is looking for objects that change on very rapid timescales,” she added.
Building Towards the SKA
ASKAP is a precursor to the collaborative and Australia’s first mega-science project, known as the Square Kilometre Array (SKA). SKA will observe the sky across the mid and low-range radio frequencies across two sites, one situated in South Africa and the Murchison Radio-astronomy Observatory (MRO) in Western Australia - which is also the location of ASKAP.
Australia will host and operate the facility responsible for the low-range observation. This facility will be made up of over 130,000 antennae positioned across the MRO, with the greatest baseline equating to 65-kilometres. Essentially, ASKAP is just the first chapter in the story of large radio telescope arrays.
Video Credit: CSIRO RACS.
We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site.
The paper is now available in arXiv.org.