Soft Gamma-ray selected radio galaxies: favouring giant size discovery

TL;DR

This study shows that soft gamma-ray surveys are particularly effective at detecting large and giant radio galaxies, revealing a higher fraction of such objects compared to traditional radio surveys, and suggesting a link to AGN properties.

Contribution

It demonstrates that soft gamma-ray selection favors the discovery of giant radio galaxies, highlighting a new method for identifying these rare and large-scale cosmic structures.

Findings

60% of gamma-ray selected radio galaxies are larger than 0.4 Mpc.

Approximately 22% are classified as Giant Radio Galaxies (GRGs).

The fraction of GRGs in gamma-ray surveys is higher than in radio surveys.

Abstract

Using the recent INTEGRAL/IBIS and Swift/BAT surveys we have extracted a sample of 64 confirmed plus 3 candidate radio galaxies selected in the soft gamma-ray band. The sample covers all optical classes and is dominated by objects showing a FR II radio morphology; a large fraction (70%) of the sample is made of radiative mode or High Excitation Radio Galaxies (HERG). We have measured the source size on NVSS, FIRST and SUMSS images and have compared our findings with data in the literature obtaining a good match. We surprisingly found that the soft gamma-ray selection favours the detection of large size radio galaxies: 60% of objects in the sample have size greater than 0.4 Mpc while around 22% reach dimension above 0.7 Mpc at which point they are classified as Giant Radio Galaxies or GRGs, the largest and most energetic single entities in the Universe. Their fraction among soft gamma ray selected radio galaxies is significantly larger than typically found in radio surveys, where only a few percent of objects (1-6%) are GRGs. This may partly be due to observational biases affecting radio surveys more than soft gamma ray surveys, thus disfavouring the detection of GRGs at lower frequencies. The main reasons and/or conditions leading to the formation of these large radio structures are still unclear with many parameters such as high jet power, long activity time and surrounding environment all playing a role; the first two may be linked to the type of AGN discussed in this work and partly explain the high fraction of GRGs found in the present sample. Our result suggests that high energy surveys may be a more efficient way than radio surveys to find these peculiar objects.