EMU Project releases first results from Pilot Survey

A new, deep look into the radio sky using one of Australia’s most powerful radio telescopes has revealed a suite of interesting astronomical objects including what appears to be two dancing ghosts and a giant galaxy that no one has ever seen before.

These findings, and more, have been published as part of the first Evolutionary Map of the Universe (EMU) project’s pilot survey (PS), which utilised CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) telescope, located in central Western Australia.

The new paper, accepted in the Publications of the Astronomical Society of Australia (PASA), describes the first data release from the EMU-PS, which covers an area of the sky approximately the size of 1,350 full moons, down to a resolution of approximately 11 – 18 arcseconds.

“We are very excited to see these results. We’ve been working on this project for 11 years, and these first results are everything we dreamed of” said lead researcher Prof. Ray Norris from Western Sydney University and CSIRO.

“The EMU-PS covers only 1% of the area that will eventually be covered by the full EMU project, and so the new science from this 1% suggests that EMU is going to be very significant indeed for humankind’s view of the Universe.  It also places Australian scientists in a very strong position to take a leading role in the billion-dollar Square Kilometre Array project, which should start  probing even deeper into the sky about a decade from now.”

“EMU-PS has delivered a veritable zoo of radio sources, from Active Galactic Nuclei to Giant Radio Galaxies, starburst rings in nearby galaxies as well as halos and relics in galaxy clusters,” added Prof. Bȁrbel Koribalski from CSIRO and WSU, co-author and founder of one of ASKAP’s other surveys - the WALLABY project.

The international collaboration of scientists who worked on these first findings from the EMU-PS includes researchers from Western Sydney University, CSIRO, Monash University, Macquarie University, University of Tasmania, University of Technology Sydney, and the International Centre for Radio Astronomy Research.

The EMU survey began in 2009 and features more than 400 scientists working in institutions in 28 countries. The goals of the project are to make a deep continuum survey of the entire southern sky (up to 30-degrees north), expecting to build a catalogue of  40 million radio galaxies. Currently, approximately 2.5 million such galaxies are known. 

“It is incredibly gratifying to see the discoveries now coming from these early EMU observations,” said Prof. Andrew Hopkins from Australian Astronomical Optics, Macquarie University. 

“It is a credit to the EMU team, who have invested a vast amount of effort to develop and refine the scientific goals and design the survey, to now start seeing the exciting results that EMU has long promised.” 

“It is even more tantalising that what we are seeing just now is only the very tip of the iceberg! The main EMU survey will deliver so much more!” he said.

And it’s not just the scientists who work directly with the project that are involved in gathering data from the EMU survey or cross-referencing it against other observations made in other wavelengths. 

“The EMU collaboration works extremely well with dedicated students and postdocs in many countries around the world exploring the wealth of available data, developing new tools and injecting ideas. EMU also connects strongly to surveys at frequencies, e.g. the Dark Energy Survey in the optical and eROSITA in X-rays, enhancing the scientific understanding,” said Prof. Koribalski.

This information will help scientists build a better picture of how galaxies evolve over time, by finding a variety of evolutionary stages that galaxies are at throughout the Universe, then placing them into a sequence that helps establish how properties change over deep time. 

This also includes what happens when galaxies start to merge and collide, reviewing the historical star formation rate in galaxies, understanding the role that central supermassive black holes play in galactic evolution, as well highlighting the link between radio sources and dark matter haloes. 

And just like the uniqueness of many of the objects observed as part of the EMU-PS, each astronomer working on the project has their own particular findings in which they are hoping the survey will provide further insights. 

“There is so much that we don’t understand about galaxy evolution. For example, we know that most, and possibly all, galaxies have a supermassive black hole at the centre, but we have no idea how they got there or how they were formed,” said Prof. Norris. “By observing these supermassive black holes in the early Universe, I’m hoping EMU might give us some clues.”

“For me, EMU opens a new window on the processes of star formation in galaxies, and how it drives their evolution,” said Prof. Hopkins. “The joint impact of star formation together with the effect of a central supermassive black hole has long been expected from simulations and models to be a critical driver.” 

“Observationally, though, this is much harder to establish. EMU will provide the necessary measurements to help resolve just how the interplay between a supermassive black hole and star formation contributes to the overall evolution of a galaxy,” he said.

Whilst for Prof. Koribalski it’s the very faint and very big objects, detected through ASKAP’s gaze, that she is keen to see more from. 

“An area of great interest to me is the Low Surface Brightness (LSB) radio emission in the sky, which is one of ASKAP’s strengths This includes the faint outer lobes of (giant) radio galaxies and radio relics, the largest of which extend over half a degree, i.e. the size of the Moon projected onto the sky,” she said. 

What makes the EMU survey special is the ability to probe deeper into the Universe from other radio wavelength surveys of this nature and at higher resolution. To perform all these tasks, the EMU survey utilises ASKAP, which is developed and operated by Australia’s national science agency, CSIRO.

ASKAP is a radio interferometer made up of 36 antennas that are all spread across a region where the greatest baseline is six kilometres. This large baseline gives the telescope unprecedented resolution at radio wavelengths. Each ASKAP dish antenna is 12 metres in diameter and features a state-of-the-art phased array feed receiver unit, which forms 36 independent beams for each telescope, thus performing observation tasks in record time by scanning a larger portion of the sky. 

“ASKAP’s phased array feeds allow us to map 30 square degrees of the sky in each pointing while also delivering high angular resolution (~10 arcsec), making it a fast survey machine with enormous discovery potential,” said Prof. Koribalski.

To ensure even the most sensitive observations can occur, ASKAP is located at the Murchison Radio-astronomy Observatory (MRO), a government-designated radio quiet zone, where radio frequency interference from everyday objects like phones, TVs, and computers are completely limited. 

The EMU Pilot Survey is one of ASKAP’s pioneering surveys, aiming to test the strategies as well as the data processing pipeline, ironing out any issues before deeper surveys are conducted using the interferometer. 

To develop the most suitable conditions for EMU-PS, astronomers outlined some parameters that would help improve the data and results, such as avoiding observations near the Sun due to the radio interference it generates and utilising deep observations of the sky at roughly 10 hours of integration time for each of the 10 regional tiles selected. 

The telescope was also directed to observe at a particular bandwidth (800 - 1088 MHz, with a centre frequency of 944 MHz) avoiding any radio frequency interference, and the survey targeted parts of the sky that would complement existing surveys which have been observed in other wavelengths (such as optical). 

But such big projects don’t come without their many challenges - and for astronomers, engineers and computer scientists, these are often the first time anyone has experienced such issues - so there is no precedent established, and often the requiring to pave the road as you learn. For Prof. Norris, it was the development of the instrument which was one of the major hurdles. 

“The phased array feeds were completely innovative, and therefore a high-risk development. The CSIRO engineers who built the telescope had to face challenges that were completely new and untested,” he said.

“Some of our colleagues elsewhere in the world were sceptical that it would ever work, and so it is very gratifying, and a real testament to the skill and perseverance of those engineers,  to see it producing such world-leading science now.”

Observations across the 10 selected tiles were made during July - December 2019, and observed over 220,000 sources, of which roughly 180,000 are compact. The selected region of sky that was was a small section of field also observed with the Dark Energy Survey, which utilises a powerful optical camera to build images of an even larger portion of the sky. 

It’s hoped that by now having both complementary deep optical and radio surveys of these regions, a multi-wavelength approach (such as identification and redshift measurements) can be undertaken to analyse astrophysical data, helping develop a better understanding of the science objectives both surveys have outlined. Both the EMU and DES surveys have a memorandum of understanding to share data between the projects.

Even on its own, the EMU-PS paper has delivered a number of interesting findings, such as the peculiar radio source known as PKS 2130-538, or “the dancing ghosts’ - a set of two host radio galaxies located just over 1 billion light-years away. 

These findings are unlike anything astronomers have seen before. The anthropomorphised dancing shapes come from the supermassive black holes at the centre of these galaxies, thrusting out powerful streams of electrons which are then re-shaped by the intergalactic winds in the region.

Another peculiar item is the double-lobed active galactic nuclei, known as PMN J2041-5256, which resides a little closer, at 660 million light-years away. Astronomers working on the EMU project nicknamed this one the “Smoking Gun”.

A Giant Radio Galaxy (GRG J2051-5704) has also been found, featuring a huge north-south radio emission structure that can be traced across 22 arcminutes of the sky, giving it a linearly projected size of 4.9 million light-years in length. This is the first time the full extent of the giant structure has been observed. 

“We know that it’s started by jets of electrons streaming away from the supermassive black hole at the centre of the galaxy,” said Prof. Norris. 

“But why is it so much larger than other radio galaxies? We don’t know. I would have thought that the jet would be stopped by the intergalactic medium that we know surrounds it. But somehow it’s punching its way through, and we don’t know why.”

The EMU-PS has also found “odd radio circles” which are a new class of objects that are very large radio sources, with no optical or infrared counterpart detected. Astronomers are still trying to figure out what these objects can be but also have a few theories that they are currently working on. 

Additional to the complementary data sharing with the DES project, the EMU-PS also overlaps with another survey, the 6dF Galaxy Survey (6dFGS), which measured the spectra of 125,071 galaxies in the southern sky. From these, 2,506 were detected by the EMU-PS, allowing further multi-wavelength analysis of these objects in the radio bands. 

“The odd radio circles (ORCs) were definitely the most exciting thing to come out of the EMU-PS, so far,” said Prof. Norris. 

“They were completely unknown before we found them in EMU-PS, and for a while, we had no idea what they were. I’m now fairly certain they are a gigantic sphere of faint radio emission surrounding galaxies about a billion light-years from Earth, but we are still figuring out what generates them.”

Astronomers will now look at these latest findings in combination with data from another ASKAP survey known as WALLABY, then correlate both these surveys against other wavelength observations to create a very detailed overview of how star formation occurs in galactic evolution, where radio emissions come from in galaxies, and the important role supermassive black holes play in the evolution of these structures. 

“The two largest ASKAP survey science projects, WALLABY and EMU, complement each other in two ways,” said 

“While WALLABY is primarily an HI [atomic hydrogen] survey, it also delivers very deep radio continuum maps at ~1.3 GHz which, combined with EMU’s lower frequency maps, enhance the overall continuum survey sensitivity and allow us to derive better spectral indices.” 

“Secondly, most of the 500.=,000 HI-detected galaxies in WALLABY will also be detected by EMU in the radio continuum. This is because cold hydrogen is the fuel for star formation,” she added. 

“For each galaxy detected by WALLABY, we can easily measure its distance, gas mass, and total mass from the 21-cm HI spectrum, which also benefits EMU.”

The public is also encouraged to participate in helping find any new objects that might pop out of the survey results, through exploring the radio sky as captured from the EMU-PS. Eventually, the data from the survey will also be placed onto the CSIRO ASKAP Science Data Archive for all global astronomers to access as well. 

“As part of the EMU planning process, we started an experiment a few years ago called Radio Galaxy Zoo where we asked citizen scientists to help us identify the galaxies that were producing the jets of radio emission,” said Prof. Norris. 

“It was amazingly successful, with thousands of citizen scientists making over two million identifications, and it produced a pile of new science results and scientific papers.”

“That project is now finished, but its successor is now in development. We hope that by the time the main EMU starts, we will be asking citizen scientists for help in classifying the millions of radio sources in EMU,” he said.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site.