Hidden Giants

University of the West Indies student Brianna Sampson presenting at AAS243 in New Orleans.

Giant Radio Galaxies (GRGs) are radio galaxies that have grown to megaparsec scales. The Milky Way galaxy is about 100,000 light-years across, but GRGs span millions of light-years. They are very rare, and their large apparent size can make them difficult to detect. But University of the West Indies student Brianna Sampson suspects there are lots of these giants hidden in the radio sky.

One way GRGs can form is when the powerful radio jets of a galaxy extend into a region of intergalactic space that is fairly empty. Thus, Double Radio sources associated with Active Galactic Nuclei (DRAGNs) are a good place to start when looking for these giant radio galaxies. To find GRGs, Brianna started with the TIFR GMRT Sky Survey (TGSS). The Giant Metrewave Radio Telescope (GMRT) is located in India with an excellent view of both the northern and southern sky, so TGSS covers almost the entire radio sky.

A confirmed GRG as seen in FIRST (left) and TGSS (right).

Using an algorithm known as DRAGNhunter, Brianna identified more than 1,400 potential GRGs. But here is where things get a bit complicated. Although TGSS covers almost the entire sky, its resolution is relatively low. Too low to confirm whether Brianna’s potential GRGs are actually giant radio galaxies. So Brianna looked at 30 of the potential GRGs also found in the Very Large Array’s Faint Images of the Radio Sky at Twenty Centimeters (FIRST). Completed in 2011, the FIRST survey only covered about 25% of the sky, but did so at a much higher resolution than TGSS.

Of the 30 potential GRGs found in both surveys, Brianna was able to confirm 27 of them as true Giant Radio Galaxies in the FIRST data. If this ratio holds in general, then there could be more than a thousand GRGs in the TGSS data. It will take future sky surveys at a higher resolution to confirm these GRGs, but it clear that the TIFR GMRT Sky Survey data contains hidden giants.