(This post was co-written with Minnie Mao, an RGZ science team member and postdoc at the National Radio Astronomy Observatory in New Mexico.)

Thanks again for starting your work on the new images from the ATLAS survey! We wanted to talk more about how/why these images differ from the existing FIRST images, including details on the telescopes, survey data, and our science goals.

1. What kind of telescopes are used to take the new images?

The radio data in the new images is from the Australia Telescope Compact Array (ATCA), which is located in rural Australia outside the town of Narrabri. The ATCA has 6 separate radio dishes, each 22 meters in diameter. The Very Large Array (VLA), which took the FIRST images, has 27 dishes which are each 25 meters apiece; this means that ATCA has about 1/5th the collecting area of the VLA, and is less sensitive overall. The ATCA can still detect very faint radio objects, but they typically have to take longer exposures (integrate) than the VLA does.

The six radio dishes of the ATCA, located outside of Narrabri, NSW, Australia. Image courtesy CSIRO/Ettore Carretti.

The size of the arrays for the two telescopes is also different. The ATCA has a maximum baseline of 6km, which means that at 20cm (the wavelength used in RGZ images) you have a resolution of ~9 arcsec. This sets the smallest size of structures seen in the radio contours. The VLA has a longer maximum baseline of 36km, which means at 20cm you have a resolution of ~1.2arcsec. The configuration used for the FIRST images in RGZ has a resolution of about 5 arcsec, which is about twice as small as that in the new ATLAS data.

Finally, one of the biggest differences between the two telescopes comes from the arrangement of the dishes, not just their maximum size. The VLA is in a Y-shape which means imaging can be done in relatively short exposures, called ‘snapshots’. The ATCA is in a linear configuration running from east to west. Imaging with the ATCA requires observations over a large range of times so that observations are taken at a variety of earth rotation positions (filling the uv-plane). A full synthesis image with the ATCA requires 12 hours of observing.

The infrared data comes from the SWIRE survey carried out with the Spitzer Space Telescope. Spitzer is an infrared observatory launched by NASA in 2003 and is still operating today. One big difference between Spitzer and WISE is their relative sensitivities and field of view; Spitzer has a bigger mirror than WISE, but a much smaller field of view. Spitzer was designed mostly to study individual objects in detail and at very high sensitivity. WISE, on the other hand, was a survey telescope designed to sweep across the entire sky several times and detect all the infrared objects it could. So instead of mapping the whole sky, Spitzer carried out smaller observations of specific fields.

Spitzer had cameras that could image at a wide range of infrared wavelengths; the new images use Spitzer’s lowest-wavelength filter (3.6 microns) on the IRAC camera. This is almost exactly the same wavelength used for the WISE images (3.4 microns), so these are directly comparable. These near-infrared wavelengths are sensitive to emission from older/cooler stars, warm dust, and light from accretion disks that may surround black holes within galaxies.

2. Where in the sky were these new images taken?

The new images come from two fields in the Southern Hemisphere, called the Chandra Deep Field South (CDF-S) and the European Large Area ISO Survey South-1 (ELAIS-S1). If you know your constellations, these lie near Fornax and Phoenix, respectively.

These fields were chosen specifically so there weren’t bright radio sources in/near the fields. Moreover, these fields have tonnes of ancillary data! The CDF-S is one of the most intensely observed fields in the sky, with deep data from world-class telescopes from radio to gamma-ray! The CDFS (proper) is actually a MUCH smaller region than the ATLAS project observed… but the generally larger field-of-view from the radio telescope enabled a decent chunk of sky to be observed. This is critical to avoid problems such as cosmic variance.

A panoramic view of the near-infrared sky shows the distribution of galaxies beyond the Milky Way. SWIRE covers six small fields; two at the bottom right (Chandra-S and ELAIS-S1) are the ones now included in Radio Galaxy Zoo. Image courtesy NASA/T. Jarrett.

Deep fields like CDF-S and ELAIS-S1 enable statistical properties of galaxies to be determined over cosmic time, and of course understanding how galaxies have formed and evolved is probably the most important extragalactic astronomy question 🙂 These sorts of wide + deep observations also are great for discovering the ‘unknown’… 🙂

3. Why do these images look different than the ones already in RGZ?
This one is fun!! Mostly due to the VLA’s Y-shaped configuration, image artefacts tend to be hexagonally shaped (like a six-sided snowflake). Conversely, ATCA artefacts tend to look like radial spokes.

The ATLAS images also have ~10 arcsec resolution whereas the FIRST images have 5 arcsec resolution so the FIRST images might appear more ‘detailed’.

Both the ATLAS and SWIRE data are much more sensitive than the FIRST/WISE data because the telescopes integrated on this small part of the sky for much longer.

4. Why does the RGZ science team want to cover these fields?<

One reason is that ATLAS is what's called a "pathfinder" mission for an upcoming survey called EMU. EMU will use another telescope in Australia, named ASKAP, to do a deep survey over the entire sky. This is the best of both worlds, combining the sensitivity of ATLAS with the sky coverage of FIRST, and will provide ~70 million radio sources! A pathfinder mission like ATLAS is a smaller version which tests things like hardware, data reduction, and feasibility of larger surveys. We plan on asking citizen scientists to help with the EMU data as well, and so starting on the ATLAS images is a critical first step.

Since the area covered in these images is also much, much smaller than the FIRST survey, it was possible for small groups of astronomers to visually go through and cross-match the radio and IR emissions. Those results were published several years ago (led by RGZ science team member Ray Norris). Getting your results for the same set will help us to calibrate the new data from FIRST, which has many more galaxies and for which we don’t have the same information yet. We also want to see what new objects are left to be discovered in ATLAS (giant radio galaxies, HyMORS, WATs, etc.) that astronomers may have missed!