Â鶹ÒùÔº

Using the upgraded Giant Metrewave Radio Telescope (uGMRT), astronomers have performed radio observations of the Ophiuchus galaxy cluster. Results of the observations, August 26 in The Astrophysical Journal, deliver important insights into the nature of a fossil radio lobe in this cluster.

Fossil radio lobes are extended regions of radio emission that are remnants of past bursts of activity from active galactic nuclei (AGN) at the centers of galaxies. They are often found in the outskirts of galaxy clusters, far from their host galaxy. Observations show that although fossil radio lobes can be enormous in size, they are usually very faint and diffuse.

The Ophiuchus galaxy cluster is a massive, nearby cluster of galaxies located some 390 million light years away in the constellation Ophiuchus. Previous observations have found that it contains a giant radio bubble, which is considered to be a fossil radio lobe. However, the properties of this feature remain uncertain.

That is why a team of astronomers led by Simona Giacintucci of the Naval Research Laboratory (NRL) in Washington, DC, decided to take a closer look at the fossil radio lobe in this cluster. Using uGMRT in the 125–250 MHz and 300–500 MHz frequency bands, they investigated the structure of the lobe.

"This paper presents a deep radio follow-up of the fossil radio lobe in Ophiuchus using the upgraded GMRT (uGMRT) in Bands 2 (125–250 MHz) and 3 (300–500 MHz)," the researchers wrote.

These new observations allowed Giacintucci's team to trace the faint diffuse emission from the lobe to a distance of approximately 2,700 light years from the center of the Ophiuchus cluster.

The images acquired with uGMRT unveiled the presence of intricate spatial structures within the fossil lobe, including narrow (16,000–33,000 light years), elongated (230,000-330,000 light years) radio filaments embedded within the lower surface brightness emission of the lobe.

According to the paper, the detected filaments exhibit a very steep spectral index, significantly steeper than the underlying smoother radio emission from the lobe.

While their origin remains uncertain, the astronomers suppose that they may represent regions where the magnetic field has been stretched and amplified by magnetohydrodynamic turbulence within the rising bubble. This would result in accelerated aging of the relativistic electrons and steepening of their synchrotron spectrum.

The study also found that the spectrum of the brightest region of the fossil lobe, closest to the cluster core, displays a spectral break.

Based on this finding, the researchers estimated the radiative cooling age of the fossil lobe, which was found to be about 174 million years. They noted that such long radiative cooling raises the question of how the rapidly fading radio emission from such systems, like the Ophiuchus cluster, can still be visible in the radio band.

© 2025 Science X Network