European astronomers have conducted optical and radio observations of an enigmatic radio source designated GRS 0917+75. As a result, they found that GRS 0917+75 is a giant radio galaxy and determined its properties. The new findings were published July 3 on the preprint server arXiv.
Peculiar radio source in the spotlight
GRS 0917+75 was first identified in 1991 as an elongated, diffuse radio source. It appears to be associated with two galaxies at a redshift of approximately 0.125 and therefore lies at the same redshift as the adjacent Rood 27 supercluster. Some observations have suggested that GRS 0917+75 may be an inherent feature of the filamentary structure of Rood 27 or the remnant of a giant radio galaxy.
What makes GRS 0917+75 peculiar is that it is extended and diffuse, with a steep radio spectrum similar to radio halos or relics in clusters of galaxies. However, studies show that this source appears to be associated with a poor group without X-ray emission, while radio halos and relics occur in rich clusters with extended, diffuse X-ray emission.
To determine the true nature of GRS 0917+75, a team of astronomers led by Gabriele Giovannini of the University of Bologna in Italy decided to point the Galileo National Telescope and the LOw Frequency ARray (LOFAR) at this source. Their study was complemented by archival data from the Very Large Array (VLA).
“We conducted optical observations to obtain new spectroscopic data. We also acquired a LOFAR image at 144 MHz to derive information at low radio frequencies. Moreover, we performed a new analysis of archival VLA data in the L and C bands for a multifrequency study of the source properties in the radio band,” the researchers explain.
New giant radio galaxy
The study found that GRS 0917+75 is a giant radio galaxy (GRG) with a size of about 4.9 million light-years and an estimated age of 100 million years. In general, GRGs are radio galaxies with an overall projected linear length exceeding 2.3 million light-years. They are rare objects that grow in low-density environments and are important for astronomers studying the formation and evolution of radio sources.
The observations show that GRS 0917+75 is coincident with a poor galaxy group in a supercluster known as SCL245. The brightest galaxy member of this supercluster, designated BGG1, is a low-excitation galaxy (LEG) located at the center of GRS 0917+75 and coincident with a discrete radio source.
Furthermore, the observations detected an extended substructure at the center of GRS 0917+75 with a size of about 1.4 million light-years and a slightly higher surface brightness than the rest of the galaxy. The total source spectral index shows a clear steepening at high frequencies, and the spectral index distribution is relatively flat and uniform in the central region, steepening toward the outer eastern and western regions.
Unusual FR I-type GRG
The authors of the paper conclude that the morphology and brightness of GRS 0917+75 indicate that it is an FR I-type GRG. Generally, FR I sources are brightest toward the center, while FR II sources are brightest at the edges.
However, the scientists note that GRS 0917+75 shows a uniform radio brightness distribution and a large transverse size. This, together with the lack of a jetlike feature, is unusual for FR I sources. However, they conclude that similar peculiar FR I sources have been reported in previous scientific research.
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Publication details
G. Giovannini et al, The giant radio source 0917+75: Origin and properties, arXiv (2026). DOI: 10.48550/arxiv.2607.03368
Journal information:
arXiv
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GRS 0917+75 is a giant radio galaxy, observations find (2026, July 15)
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