Discovery of a giant radio fossil in the Ophiuchus galaxy cluster (2002.01291v1)

Abstract: The Ophiuchus galaxy cluster exhibits a curious concave gas density discontinuity at the edge of its cool core. It was discovered in the Chandra X-ray image by Werner and collaborators, who considered a possibility of it being a boundary of an AGN-inflated bubble located outside the core, but discounted this possibility because it required much too powerful an AGN outburst. Using low-frequency (72-240 MHz) radio data from MWA GLEAM and GMRT, we found that the X-ray structure is, in fact, a giant cavity in the X-ray gas filled with diffuse radio emission with an extraordinarily steep radio spectrum. It thus appears to be a very aged fossil of the most powerful AGN outburst seen in any galaxy cluster ($pV\sim 5\times 10^{61}$ erg for this cavity). There is no apparent diametrically opposite counterpart either in X-ray or in the radio. It may have aged out of the observable radio band because of the cluster asymmetry. At present, the central AGN exhibits only a weak radio source, so it should have been much more powerful in the past to have produced such a bubble. The AGN is currently starved of accreting cool gas because the gas density peak is displaced by core sloshing. The sloshing itself could have been set off by this extraordinary explosion if it had occurred in an asymmetric gas core. This dinosaur may be an early example of a new class of sources to be uncovered by low-frequency surveys of galaxy clusters.

• The paper reveals an ancient AGN outburst by detecting a giant radio fossil aligned with an X-ray cavity in the Ophiuchus Cluster.
• The study combined data from MWA, GMRT, Chandra, and XMM-Newton to analyze a steep radio spectrum indicative of advanced spectral aging.
• The findings highlight that aged AGN lobes can be identified even without a clear counterpart, underscoring the need for multi-band observational strategies.

Insightful Overview of "Discovery of a Giant Radio Fossil in the Ophiuchus Galaxy Cluster"

The paper "Discovery of a Giant Radio Fossil in the Ophiuchus Galaxy Cluster" by Giacintucci et al. provides a thorough investigation of an anomalous feature observed in the Ophiuchus galaxy cluster, uncovering evidence suggestive of a significant AGN outburst in the radio regime. This paper represents a detailed synthesis of radio observations from multiple surveys including those conducted by the Murchison Widefield Array (MWA) and the Giant Metrewave Radio Telescope (GMRT), in conjunction with X-ray data from the Chandra and XMM-Newton observatories.

Key Findings and Analysis

1. Identification of an X-ray Cavity: Through X-ray observations, the researchers noted a concave density discontinuity at the edge of the Ophiuchus cluster's cool core. Prior hypotheses wondered if this could represent the boundary of an AGN-inflated cavity; however, the energy requirements seemed prohibitive without corroborative evidence.
2. Radio Emission and Spectrum Analysis: Using low-frequency radio data, the paper identifies a diffuse radio emission aligned with the concave feature in the X-ray image. This emission has an unusually steep radio spectrum, hinting at advanced spectral aging of the ultrarelativistic electrons within. The calculated energy associated with this cavity is around $pV \sim 5 \times 10^{61}$ erg, indicative of an extraordinary AGN outburst without a clear counterpart, possibly due to asymmetric cluster characteristics or advanced aging beyond radio detectability.
3. Connecting Radio and X-ray Observations: The paper utilizes the alignment of radio emissions with X-ray cavities to posit a plausible connection between the observed radio fossil and an ancient AGN event. The spatial correlation suggests that the radio fossil fills the X-ray cavity, exhibiting characteristics similar to known AGN lobes in other clusters like Perseus and Hydra A.

Implications and Future Work

The findings underscore the potential of uncovering aged fossil AGN outbursts in clusters through combined radio and X-ray analyses. This work suggests a classification expansion within radio sources detected at low frequencies, particularly within the context of extragalactic radio surveys.

The absence of a corresponding opposite lobe, typically expected from AGN jets, raises questions about cluster asymmetry and dynamics potentially influencing detection feasibility. This raises the necessity for high-sensitivity observations across varied frequency bands to explore similar phenomena in other clusters.

Speculation on Future Directions

The pronounced spectral steepness and the scale of the detected radio feature suggest that further exploration into the nature of these radio fossils could enrich our understanding of AGN life cycles and their environmental impact on galaxy clusters. Upcoming high-resolution observational instruments could unveil additional cases, facilitating more comprehensive modeling and simulation efforts pertaining to AGN feedback cycles and cluster evolution.

In conclusion, this work offers an insightful addition to the paper of galaxy cluster environments and AGN dynamics, pushing the envelope in our interpretative methodologies of radioastrophysical phenomena. It paves the way for enhanced cross-disciplinary observational linkages which could reveal hitherto unrecognized population characteristics within cosmic structures.