M 87 galaxy

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Insights into the M 87 Galaxy: A Comprehensive Overview

TeV Gamma-Ray Emission from M 87

The giant radio galaxy M 87 has been identified as a potential TeV gamma-ray emitter. Observations using the HEGRA stereoscopic system of imaging atmospheric Cherenkov telescopes have detected an excess of photons above an energy threshold of 730 GeV from M 87, with a significance level above 4σ. This corresponds to an integral flux of 3.3% of the Crab flux, making M 87 a promising candidate for TeV gamma-ray emission among giant radio galaxies. This detection, if confirmed, would establish a new class of extragalactic sources in this energy regime, as all other active galactic nuclei (AGN) detected at TeV energies are BL Lac type objects.

Complex Gamma-Ray Behavior

Recent studies have provided a detailed characterization of the high-energy (HE; >100 MeV) gamma-ray emission from M 87. Analysis of approximately eight years of Fermi-LAT data revealed evidence for HE gamma-ray flux variability and indications of a possible excess over the standard power-law model above 10 GeV. This suggests the presence of an additional component dominating the highest-energy part of the spectrum, which is likely jet-related and capable of month-type variability. The findings indicate that both the high-energy and very high energy (VHE; >100 GeV) emissions in M 87 may originate from the same physical component, with variability behavior providing further constraints on the location and nature of this component.

Accretion Events and Halo Formation

Kinematic and photometric evidence suggests that M 87 has experienced an accretion event in its halo within the last Gyr. Analysis of planetary nebulas (PNs) in the M 87 halo identified a chevron-like substructure, indicating the accretion of a progenitor galaxy. This event has significantly modified the outer halo of M 87, supporting hierarchical galaxy evolution models that predict the growth of cD halos through the accretion of smaller galaxies.

Radio Observations and Jet Dynamics

M 87's supermassive black hole, with a mass of 6.4×10^9 M☉, drives the activity responsible for the extended radio lobes surrounding the galaxy. Low-frequency observations using the Low-Frequency Array (LOFAR) have provided new insights into the past activity cycles of the active nucleus. The data suggest a continuous injection of relativistic electrons, with the lobes still being supplied by fresh particles from the active galactic nuclei. The halo's age is estimated to be around 40 Myr, with a jet kinetic power of 6-10 × 10^44 erg s^-1.

Jet Formation and Structure

M 87's jet, extending from sub-pc to kpc scales, is one of the best sources for studying jet formation due to its proximity and the large mass of its central black hole. High-resolution VLBI observations at 86 GHz have revealed a parabolically expanding limb-brightened jet emanating from a resolved VLBI core. The core's brightness temperature suggests magnetic energy dominance at the jet base, with the jet's sheath likely anchored in the inner portion of the accretion disk. Additionally, the jet exhibits episodic wiggling, possibly due to turbulent mass loading and local magnetic reconnection processes.

X-ray and Far-Infrared Observations

XMM-Newton observations have provided detailed X-ray spectra of M 87's nucleus and jet, revealing a radial temperature and abundance distribution of several elements. The data suggest a single-temperature structure for radii ≥2', with no indication of multi-temperature components. In the far-infrared, Herschel observations indicate that the emission from M 87 can be explained by synchrotron radiation, with no evidence of a diffuse dust component, likely due to the harsh X-ray environment at the core of the Virgo cluster.

Conclusion

M 87 is a complex and dynamic galaxy, exhibiting a range of high-energy phenomena from TeV gamma-ray emission to intricate jet dynamics and accretion events. Its proximity and the large mass of its central black hole make it an invaluable laboratory for studying the mechanisms of jet formation, high-energy radiation, and galaxy evolution. The ongoing and future observations will continue to unravel the mysteries of this fascinating galaxy.