Sunday, March 26, 2017

One of the Largest Giant Radio Galaxies Discovered by Astronomers

Background SDSS image (composite from bands g, r and i) overlaid with white MSSS contours of the GRG at 2, 3, 4, 6 and 8 times the RMS noise (34 mJy/beam). Inset image:  SDSS image (composite 3 colour image from bands i, r and g on log stretch scale) with SDSS contours at 3 times the RMS noise after smoothing from bands z (white), i (cyan), r (red), g (green) and u (blue). Image credit: Clarke et al., 2017

An international team of astronomers led by Alex Clarke of the Jodrell Bank Centre for Astrophysics in Manchester, U.K., has detected a new giant radio galaxy (GRG). The new GRG, which has not received any official designation yet, has a projected linear size of 8.34 million light years - what makes it one of the largest galaxies in this class known so far.

The newly discovered GRG is associated with the galaxy triplet known as UGC 9555. Located some 820 million light years away from the Earth, UGC 9555 is a part of a larger group of galaxies designated MSPM 02158. This group of galaxies has been recently investigated by Clarke’s team analyzing the data provided by the Low Frequency Array (LOFAR). 

The researchers studied the data available in the LOFAR Multifrequency Snapshot Sky Survey (MSSS). It is the first northern-sky LOFAR imaging survey that covers the sky north of the celestial equator at frequencies from 119 to 158 MHz in eight separate 2.0 MHz bands. In result, LOFAR MSSS images allowed the scientists to distinguish a new giant radio galaxy.

“The discovery was initially made after looking through preliminary images from the survey. The survey has 3,616 pointings, and each pointing needs manual checking after the calibration and imaging, so the team had a lot of work to do to check all these images,” Clarke told Astrowatch.net.

LOFAR is crucial for such observations. It can produce arcsecond resolution maps, has a wide field of view, and has excellent ultraviolet coverage, which makes it sensitive to the large angular scales of nearby GRGs. Moreover, due to the nature of synchrotron emission, these objects are brighter at the low frequencies (150 MHz) where LOFAR operates.

GRGs are objects grown in low-density environments with an overall projected linear length exceeding one million parsecs (3.26 million light years). They are important for astronomers studying the formation and the evolution of radio sources. The largest GRG known to date is J1420-0545 - with a projected linear size of about 16 million light years.

“Giant radio galaxies have a loose definition that they must be over one million parsecs in total size, and these are not rare anymore as we have become much better at detecting them. They become rare once you get over two or three million parsecs when you are really only talking about a handful of objects,” Clarke said.

He added that this is where it gets more interesting as in this particular case of the new GRG, this galaxy is relatively close, what allows us to see the lobes on large angular scales. This means that astronomers can investigate the morphology of the lobes, their interaction with their environment and any link with the host galaxy. 

“We think that the low density environment is very important for such huge objects to grow, and this can only be investigated if it is near enough to us that we can resolve it on the sky,” Clarke noted.

According to the research, the newly detected GRG has integrated flux density at 142 MHz of 1.54 Jy over the whole dual-lobe emission, including underlying background point sources, which gives a total luminosity at 142 MHz of 11.6 septillion W/Hz. However, the available data are still insufficient to confirm the class of this GRG. Radio sources are divided into two classes: Fanaroff and Riley Class I (FRI), and Class II (FRII).

“This object jumped out really obviously, but you can imagine that once we get around to more detailed and automated checks, there will be much more to discover in this survey. Ultimately, we are working towards publicly releasing the survey as soon as we have the whole sky imaged and quality checked. This way, the whole community can benefit from the science, and discover new things,” Clarke said.

Clarke’s team assumes that the new GRG is a borderline case between FRI and FR-II, although the large size and therefore old age contributes to a decreased luminosity. The researchers hope that further data gathered from a deep LOFAR observation will clearly classify the properties of this GRG.

“We have been doing deeper observations and we intend to publish even more spectacular images later in the year,” Clarke revealed.

The paper detailing the discovery was published Feb. 6 and is available online on the arXiv pre-print repository.

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