LOFAR radio image of GRG-J2233+1315 at 144 MHz. Credit: Dabhade et al,
An international team of astronomers has made radio and optical observations of a peculiar giant radio galaxy known as GRG-J223301+131502. The results of the observing campaign shed more light on the properties of this galaxy and have revealed its unique jet structure. The findings are reported in an article published September 26 on arXiv.org.
Giant radio galaxies (GRGs) are radio galaxies with a total projected linear length of at least 2.3 million light-years. They are rare objects that grow in low-density environments. In general, GRGs are important for astronomers to study the formation and evolution of radio sources.
So far, around a thousand GRGs have been detected, of which only ten exceed 10 million light-years in size. The largest is J1420-0545, with a projected proper longitude of approximately 16 million light-years. Therefore, GRGs and also the rest of the megaparsec-scale radio galaxies are assumed to be the largest single-galaxy-induced phenomena in the universe.
At a redshift of 0.093, GRG-J223301+131502 (GRG-J2233+1315 for short) is a giant radio galaxy first identified in 2017 as part of the Search and Analysis of GRGs with Associated Nuclei (SAGAN) project. It has a total projected linear size of almost 5.57 million light-years and is housed in SDSSJ223301.30+131502.5, a galaxy of type S0-a that exhibits a large diffuse stellar halo.
Previous studies of GRG-J2233+1315 have found that this galaxy resides in a dense cluster environment, which is in contrast to current theories that GRGs generally reside in a sparse environment. So, to verify this, a group of astronomers led by Pratik Dabhade of Sorbonne University in Paris, France, made deep multi-frequency radio observations of GRG-J2233+1315 with the Giant VHF Radio Telescope (GMRT) and low frequency. Array (LOFAR), as well as spectroscopic observations using the William Herschel Telescope (WHT).
Deep, high-resolution radio images revealed a huge jet, about 772,000 light-years in size, emanating from the radio core of GRG-J2233+1315 and leading to a “kink” that extends about 326,000 light-years. . The images also show that the galaxy exhibits lobes without prominent hotspots and closely resembles a bar, which is why the researchers named GRG-J2233+1315 Barbell GRG. These detected lobes turned out to have a magnetic field strength of around 5 µG and spectral ages between 110 and 200 million years.
The observations found that the Barbell GRG has a redshift of about 0.99 and that its linear size is larger than previously thought: nearly 6 million light-years. The results indicate that the host galaxy has a relatively low rate of star formation, at a level of about 0.001 solar masses per year.