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Galaxies


Black-Hole in M87, 2019 (2021, 2023, 2024 Updates)

The large elliptical galaxy M87 is the dominant member of the Virgo Cluster. The variation in brightness and a jet shooting out from its center indicate that there is a black-hole at the center (Figure 05-02r1). The table in Figure 05-02r2 lists some of the characteristics of
M87, Black-Hole M87 Parameters M87 and its black-hole (also see "Space Fact"). An image of this black-hole is presented to the world by the Event Horizon Telescope(s) (EHT) in April, 2019. However, close examination reveals that the image is a shadow (actually a silhouette) formed by dark object against radio frequency background, and the whole picture is in false colors. It is stitched together with missing data filled in by some sort of educated guessing, a.k.a. "Bayesian Inference".

Figure 05-02r1 M87, Black-Hole [view large image]

Figure 05-02r2 M87 BH Parms

The following shows the theoretical base and processing involved in the imaging.
See "SgrA* Radio Image" for comparison.

By combining past observational data and educated guessed-work, a series of M87 black hole images have been constructed from 2009 to 2017 (Figure 05-20w). It shows lopsided blob of light swirling around the supermassive black hole at the centre of M87*. The bright spot
M87 BH, 2009-2017 moves around because the environment around the black hole changes on a scale of several weeks. Strong magnetic fields stir the accretion disk and produce hotter spots that then orbit the black hole. See "The first-ever image of a black hole is now a movie".

Figure 05-02w M87 Black Hole,
2009-2017 [view large image]

In 2018, a separate team reported evidence of a blob of hot gas circling SgrA*, the Milky Way’s central black hole, over the course of around 1 hour. Because M87* is more than 1,000 times the size of SgrA*, the dynamics around M87* take longer to unfold.
[2021 Update] See "Milky Way Black Hole" published in May 2022.

[End of 2021 Update]

[2023 Update]

In year 2000, the Hubble Space Telescope produces a visible light image of M87 with a jet emanating from a bright source supposed to
M87 Jet Radiation Types be a black hole (see Figure 05-02z1). However, there is no jet in the 300 GHz radio image of the M87 black hole taken by EHT in 2019 and its supposedly refined image in 2021. The disappearance can be explained by weak synchrotron (non-thermal) emission at the higher frequency of 300 GHz (see Figure 05-02z2). The jet re-appears in the 2023 radio image at 100 GHz from the Global Millimetre VLBI Array (GMVA). It shows the twisted helix close to the black hole from the accretion disk. Thus, it requires lower frequency to see the non-thermal jet; while higher frequency shows the jet by thermal radiation.

Figure 05-02z1 M87 Jet
[view large image]

Figure 05-02z2 Radiation Types [view large image]

There is no jet ~ 300 GHz because both types of emission are weak near that point.
(see video in Figure 05-02u, and the "Formation of Jet").
In addition, there is an infrared image by the Spitzer Space Telescope. It shows the jet on the right, while there is also the shock created by the otherwise unseen receding jet on the left (see "The Galaxy, the Jet, and a Famous Black Hole").

[End of 2023 Update]

[2024 Update]

By 2024, it is clear that the thermal component of the jet is related to atoms or molecules in a state of ionization at temperature ~ 5000 oK (see thermal spectrum) as shown in Figure 05-02z3 taking from 2 research articles : "Outflows from the youngest stars are mostly molecular", and "A probable Keplerian disk feeding an optically revealed massive young star".

M87 Jet Jet and Magnetic Field Figure 05-02z4 shows the helically shaped magnetic field composing 2 components, e.g., Bz and B along the z and directions respectively. The thermal component moves along the z-axis collimated in the confined direction in a narrow beam. While only the lighter mass electrons can be bended over into the direction emitting the non-thermal radiation (see synchrotron in Figure 05-02z5).

Figure 05-02z3 Thermal Emission from New-Born Stars [view large image]

Figure 05-02z4 Magnetic Field

Synchrotron Radiation Synchrotron radiation is emitted by charged particles, usually electrons, moving at relativistic speeds in magnetic fields. In a magnetic field a charged particle is forced to circle around the field line in a helical path. An accelerating charged particle emits electromagnetic radiation that is radiated along the direction in which the particle is moving. A large population of relativistic particles moving in a magnetic field will radiate over a wide range of frequencies and has a high degree of polarization (Figure 05-02z5).

Figure 05-02z5 Synchrotron Radiation
[view large image]

Synchrotron Polarization Synchrotron Spectrum A moving charged particle in a magnetic field would be reflected by the Lorentz force F = q (vxB), where q is the charge, v is the velocity, and B is the magnetic induction. The magnetic field can in turn be generated by current loop. In a region with uniform magnetic field, the curvilinear path of the charged particle becomes a circle as shown partially in Figures 05-02z6,7 and if the velocity has a component in the direction of the magnetic filed then the trajectory will be a helix as in the jets of many astronomical objects such as quasars or black holes; we can further identify the accretion disk to be the current loop generating the magnetic field.

Figure 05-02z6 Synchrotron Polarization
[view large image]

Figure 05-02z7 Synchrotron Spectrum [view large image]

The cyclotron is a particle accelerator using the same principle to move the charged particles around. It is called synchrotron when the particle attains relativistic velocity. Radiation from such source is called synchrotron radiation as accelerating charge always produces electromagnetic wave. At relativistic speed the radiation pattern is collimated into a narrow beam.

Figure 05-02z6 shows an electron moves in an uniform magnetic field instantaneously. The orbital plane of the particle is in the x-y plane, the acceleration is in the y direction, the vector n points to the observer. The polarization vectors (in red) are on the orbital plane, and is orthogonal to both n and . For synchrotron emission, we only see the radiation when the particle is moving towards us, i.e., when ~ 0. Calculation shows that for a single electron, 7 times more power is radiated with the polarization than from . The spectrum of synchrotron radiation from a single electron is shown in the diagram on the upper right of Figure 05-02z7.

The combination of many individual emissions produces a power spectrum with the spectral index between -3 and +2.5. A positive value of indicates thermal emission in general. For :

Young Star, ~ -2 to -1,
Pulsar, ~ -3 to -2,
Milky Way Center, ~ -1.8 to +1,
AGN, ~ -1 to +1,
Radio Galaxy, ~ -0.7,
Black Hole, ~ -1.4 to -0.5.

[End of 2024 Update]

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