Unlocking the Secrets of M87: What the Energy Densities Reveal About This Galactic Jet
"Scientists delve into the magnetic fields and relativistic electrons within M87's innermost jet to understand its formation and behavior."
The formation of relativistic jets in active galactic nuclei (AGNs) has been a long-standing puzzle in astrophysics. Estimating magnetic field energy density (UB) and electron energy density (Ue) is vital to understanding how these jets form, yet determining whether UB or Ue dominates at the base of a jet remains a challenge. Any deviation from equipartition (Ue/UB ≈ 1) is crucial for understanding relativistic jet formation, but obtaining a reliable estimate of Ue/UB at a jet base has proven difficult.
M87, a nearby giant radio galaxy, is the ideal subject for studying jet bases due to its proximity and supermassive black hole. M87 has been extensively studied across the electromagnetic spectrum, from radio waves to very high-energy (VHE) gamma rays. Causality arguments based on a VHE gamma-ray outburst in February 2008 suggest that the VHE emission region is less than 56 Rs, where Rs is the Schwarzschild radius. With VLBA resolution at 43 GHz reaching approximately 0.21×0.43 mas, corresponding to 5.3 × 10^16 × 1.1 × 10^17 cm, or 30 × 60 Rs, M87 offers an unparalleled opportunity for detailed investigation.
Recent advancements in VLBI observations have revealed the innermost structure of the M87 jet, including the frequency and core-size relation, and the distance and core-size relation, down to approximately 16 Schwarzschild radii (Rs). This makes the jet base of M87 the perfect environment for studying Ue/UB in close proximity to a central engine. Recent M87 observations provide two significant advancements, motivating this work. H11 successfully measured core-shift phenomena at the jet base at multiple frequencies. Also, core sizes were recently measured providing data for analysis.
Numbers Behind the First Submillimeter Polarization Census
A 2025 study published in Astronomy & Astrophysics (arXiv preprint 2505.10181, posted May 15, 2025) presented full-polarization observations at λ = 0.87 mm (345 GHz) conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) toward Messier 87 (M87) and seven other radio-loud active galactic nuclei (AGN). As reported by the arXiv abstract, linear polarization (LP) fractions across the targets spanned 1% to 17%, with Faraday rotation measures (RMs) exceeding the values captured in the study's central-region measurements. The aim, per the ADS abstract, was to characterize linear polarization fractions, measure Faraday rotation measures, and examine the magnetic field structures in the emission. These figures constitute one of the first systematic polarization and Faraday probes of extragalactic jets at submillimeter wavelengths.
GRMHD Simulations and the Limits of Jet Diagnostics
Within the central regions of active galaxies, accreting supermassive black holes launch strong jets, observed as narrow beams of relativistic plasma, yet the origin and early acceleration of these jets remain one of the most significant unresolved questions in contemporary astrophysics. One established approach takes published general relativistic magnetohydrodynamic (GRMHD) simulations, applies them to the M87 jet, and describes the evolution of physical quantities along the jet while quantifying the emission and absorption properties at each position. The ALMA polarization campaign illustrates a complementary observational method, using submillimeter polarimetry at 345 GHz and Faraday rotation measures to probe magnetic field structure directly. The coexistence of these techniques reflects an acknowledged limitation: model-based reconstructions alone have not yet uniquely resolved how and where jets are initially accelerated.
A Supergiant Laboratory: M87's Foundational Milestones
Messier 87 (also known as Virgo A or NGC 4486) is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars, making it one of the largest and most massive galaxies in the local universe. It hosts a large population of globular clusters — about 15,000 compared with the 150–200 orbiting the Milky Way — along with a jet of energetic plasma. That jet is central to the historical significance of M87: relativistic jets from active galactic nuclei efficiently transport energy from accreting supermassive black holes to galactic and intergalactic scales. Decades of such observations defined M87 as a target, setting the stage for the modern ALMA-era polarization studies of its inner jet.
What Can We Learn from the Radio Core at 43GHz?
This analysis focuses on the radio core at 43GHz. Select VLBA data observed after 2009 (H13) with good qualities to measure the core width and fit a single, full-width-half-maximum (FWHM) Gaussian to the observed core in the perpendicular direction to the jet axis to derive the core width (OFWHM). Using OFWHM at 43GHz, a model-independent value of Ue/UB in the 43GHz core of M87 can be estimated for the first time.
- Assume a uniform and isotropic distribution of relativistic electrons in the radio core at 43 GHz.
- Assume an isotropic tangled magnetic field. Hereafter, B is denoted as the magnetic field strength perpendicular to the direction of electron motion. Then, the total field strength is given by Btot = √3B.
- Assume the emission region is spherical with its radius R measured at the comoving frame. The radius is defined as 2R = θobs D where D is the distance to a source. A factor A defined as θobs = AθFWHM and 1 ≤ A ≤ 1.8 is assumed.
A Fast-Moving Research Front
Research on the M87 jet is advancing quickly, with new preprints and refereed papers appearing regularly on both observational polarization measurements and theoretical jet-structure modeling. The picture emerging from the recent literature is still incomplete, and specific quantitative claims should be treated as provisional snapshots of an evolving field. Readers should expect reported figures and model constraints to be refined as additional observations accumulate.
An Analytic Counterpoint to Simulation-Based Jet Models
An August 2025 study (arXiv 2508.06158) argues that certain jet properties can be constrained analytically, modeling stationary and axisymmetric active galactic nucleus jets while assuming energy conservation along each magnetic flux tube. Using very-long-baseline interferometry (VLBI) observations together with published general relativistic magnetohydrodynamic simulations, the work constrains the evolution of the bulk Lorentz factor, the magnetization parameter, and the magnetic field strength along the observed target. This represents a standing point of debate in the field between semi-analytic jet models and heavier GRMHD numerical treatments. Such analytic constraints offer an independent check on the magnetization and field-strength values inferred for jets like M87's.
More Checks on Simulation-Based Jet Models
Comparisons of different modeling strategies for the M87 jet remain an active area of ongoing work, but the comparative analysis is still incomplete in the refereed literature. Weakening the detailed tracing of jet structure is the more direct required way to test how assumptions differ between approaches, though this is still a developing effort. Rather than claiming firm agreement, current published results should be read as early, partial evidence that will require continued refinement.
Implications for Relativistic Jet Models
Based on VLBA observation data at 43 GHz, Ue/UB at the base of the M87 jet is explored. Using the basic theory of synchrotron radiation with the simplest geometry of one-zone sphere model for the radio core at 43 GHz. The upper limit of total jet power Ljet is imposed based on various previous works. The findings:
A Converging Picture, With Caveats
Across recent studies, the M87 system emerges as a place where sophisticated theoretical predictions meet high-resolution observations; however, the evidence is not yet mature enough to settle the central questions. Writers and reviewers increasingly stress that uncertainty remains high for quantities such as magnetization and early jet acceleration mechanisms. Synthesis of the current results is best understood as directional rather than conclusive.
Cross-Band Calibration as a Next Frontier
A key step for the next generation of M87 polarization work is verifying that submillimeter measurements are internally consistent across bands. In the ALMA study, the researchers report that the ALMA-VLBI measurements agree well with the AMAPOLA trends for most cases, helping to confirm the reliability of the polarization calibration across bands. This cross-checking matters because polarization systematics at submillimeter wavelengths can otherwise be mistaken for genuine astrophysical Faraday rotation. Extending such calibration checking to more targets and observing epochs should sharpen future constraints on magnetic field structure in extragalactic jets.
Measuring Magnetic Fields Beyond One Galaxy
The challenges illustrated by the M87 polarization campaign extend well beyond a single galaxy: magnetic field measurements of extragalactic jets remain technically demanding and difficult to standardize across facilities and epochs. Sparse observing campaigns and calibration limitations are systemic constraints that affect the whole field, not just one target. Progress will depend on coordinated multi-wavelength efforts rather than isolated measurements, though the required coordination is only beginning to take shape.
M87 as a Shared Cosmic Laboratory
M 87 has become an iconic target for the study of accreting supermassive black holes that drive active galactic nuclei, receiving special attention thanks to its proximity and the large mass of its central black hole. The nearby giant elliptical galaxy, situated at a distance of 16.7 Mpc, is described in the literature as an ideal laboratory for the study of extragalactic jets from low-accreting AGNs. Complementary work examines how a jet's transversal structure, captured through magnetohydrodynamic semi-analytical modeling, shapes the total intensity profiles of relativistic jets from active galactic nuclei. Together these efforts reflect a broad, ongoing community effort to turn one nearby object into the benchmark for testing black hole and jet physics.
The allowed range of B is 2 G < B < 13 G in the observed radio core at 43GHz with its diameter 0.11 mas ((16 R$)). This estimate of B is basically close to previous estimates in the literature, although fewer assumptions have been made in this work. This result excludes a strong magnetic field such as B ~ 103-4 G which is frequently assumed in previous works in order to activate Blandford-Znajek process. Although M87 has been a prime target for testing relativistic MHD jet simulation studies powered by black-hole spin energy, our result provides a very stringent limit on the maximum B,one of the critical parameters in relativistic MHD jets model.
The allowed region of Ue/UB is obtained in the allowed θobs and ye,min plane. The resultant Ue/UB contains both the region of Ue/UB > 1 and Ue/UB < 1. The deviation from Ue/UB ≈ 1 is not very large. It is found that the allowed range is 0.18 ≤ Ue/UB ≤ 66. This result gives a tight constraint against relativistic MHD models since they seem to postulate much larger UB/Ue at a jet-base than the maximum value obtained in this work. Therefore, the obtained Ue/Uß in this work gives a tight constraint on the initial conditions in relativistic MHD models.