<p>Basically, since we know that the central region of these galaxies is occupied by a black hole, accretion by this black hole can power relativistic jets which result in radio emission. Radio emission from jets is most likely for high mass elliptical galaxies as they tend to host black holes with mass toward the high end of the supermassive black hole mass range. This seminal paper by <a href="https://arxiv.org/abs/astro-ph/0208001" rel="nofollow noreferrer">Merritt and Ekers (2002)</a> offers some context. They used this as a basis to hypothesize the double lobed radio galaxies (<a href="https://en.m.wikipedia.org/wiki/X-shaped_radio_galaxy" rel="nofollow noreferrer">X-shaped radio galaxies</a>) as originating from a spin flip due to a galactic merger.</p>
<blockquote>
<p>Although the reorientation of a SBH’s spin axis due to coalescence is not directly ob-
servable, any gaseous accretion onto the SBH is constrained by relativistic frame dragging
to be axisymmetric with respect to the black hole, and it is widely believed that the
jets emitted from the centers of active galaxies are launched perpendicularly to the inner
accretion disk; hence a jet should point in the same direction as the spin axis of the SBH
at its center. The extraordinary long-term stability of the jet direction in many radio
galaxies is strong evidence that jet orientations are regulated by black hole spins.
Because powerful radio galaxies comprise only a fraction, of order 1%, of all bright ellipti-
cal galaxies and because radio power is a rapidly increasing function of galaxy luminosity, we would expect only the more massive of the two merging galaxies to harbor a jet.
Hence a likely consequence of SBH coalescence in a radio galaxy is a sudden change in
the direction of the jet associated with the larger SBH, followed by the generation of a
new radio lobe at some (possibly large) angle with respect to the original lobe.</p>
</blockquote>
<p>They state further:</p>
<blockquote>
<p>In fact there is a class of radio sources which fit this description: the so-called
“winged” or “X-type” radio sources. X-shaped sources are characterized by two low-
surface-brightness radio lobes (the “wings”) oriented at an angle to the “active,” or
high-surface-brightness, lobes (Fig. 2); both sets of lobes pass symmetrically through
the center of the associated elliptical galaxy. The first winged source discovered, NGC
326, was initially interpreted by a model in which a single SBH undergoes slow geodetic
precession due to torques from an external mass, resulting in an S-shaped radio morphol-
ogy; later observations revealed the X shape of this source indicating a more
rapid change of jet direction. Other explanations for the origin of X-shaped sources have
been proposed but none has proved satisfactory; black holes are nearly perfect gyroscopes
and reorienting them via external forces is difficult. One proposed model is based on a
warping instability of accretion disks, but this model fails to explain why jet reorien-
tation occurs only once in the X-shaped sources and why most radio galaxies have stable
jet directions. Capture of a dwarf galaxy with mass comparable to M1 could reorient a
black hole but it is more likely that the infalling galaxy would be disrupted by tidal forces
before being accreted.</p>
</blockquote>
<p>So, as they state, the stability of the radio lobes is explained well by black holes since they &quot;are essentially perfect gyroscopes and reorienting them via external forces is difficult.&quot; They go on to explain how this is the case for several observed radio galaxies. They cite <a href="https://ui.adsabs.harvard.edu/abs/1984RvMP...56..255B/abstract" rel="nofollow noreferrer">this</a> seminal paper which explains that the observed radio emission generally requires relativistic flows, for which black hole accretion is a natural explanation. This continues to be a promising explanation for systems, see e.g.'s <a href="https://theconversation.com/experts-solve-the-mystery-of-a-giant-x-shaped-galaxy-with-a-monster-black-hole-as-its-engine-138205" rel="nofollow noreferrer">this</a>, <a href="https://astronomycommunity.nature.com/posts/new-insights-into-our-nearest-actively-feeding-supermassive-black-hole" rel="nofollow noreferrer">this</a>.</p>
<p>For an in depth explanation of the relevant physics and uncertainties, <a href="https://www.annualreviews.org/doi/abs/10.1146/annurev-astro-081913-035722" rel="nofollow noreferrer">this review by Heckman and Best (2014)</a> is very helpful. <a href="https://arxiv.org/abs/1403.4620" rel="nofollow noreferrer">Here</a> is the free version on arxiv. See for example Figure 3 which shows that both the main types of AGN, i.e., radiative and jet modes, involve a radio jet being sourced by a central supermassive black hole. The jet-mode AGN is more powerful and generally provides for stronger radio emission. Their Fig 4 summarizes this info nicely. See section 2.3.3 for how the mass of the black hole can be estimated.</p>

CC BY-SA 4.0
2022-10-22T20:37:44.063Z
From what I know the terms "SBH's accretion disk" and "SBH's photon ring" refer to the same phenomenon... - yes? There is one instance of the claim of direct observation of the the photon ring (yet it is disputed) https://www.sciencenews.org/article/disputed-photon-ring-supermassive-black-hole-image-eht                              If indeed those disks be observable - would it add to the credence of the radio lobes model?
Alex
CC BY-SA 4.0
2022-10-22T20:49:15.423Z
@Alex No, the photon sphere (or ring) is at the radius where light can orbit (although that orbit is unstable).  The accretion disk is an extended region, and its inner edge is further from the BH than the photon sphere. See https://en.wikipedia.org/wiki/Photon_sphere & https://en.wikipedia.org/wiki/Innermost_stable_circular_orbit
PM 2Ring
CC BY-SA 4.0
2022-10-22T21:17:40.827Z
@PM 2Ring - thanks. How those two parameters are used in calculating SBH's mass? Do they complement each other in SBH's mass calculations?
Alex
CC BY-SA 4.0
2022-10-22T21:29:17.000Z
@Alex For a Schwarzschild black hole (which has no spin), it's simple. The Schwarzschild radius is $r_s=2GM/c^2$. The photon sphere is at $1.5r_s$ and the ISCO is at $3r_s$. But most supermassive BHs have a *lot* of spin, see https://astronomy.stackexchange.com/a/20292/16685 and the equations  for the critical orbit radii are much more complicated. Also see https://www.nature.com/articles/s41598-021-93595-w
PM 2Ring
CC BY-SA 4.0
2022-10-22T17:26:35.120Z
2022-10-23T10:47:22.030Z
2022-10-23T10:47:22.030Z
Daddy Kropotkin
<p>According to Wikipedia:
<a href="https://en.m.wikipedia.org/wiki/Radio_galaxy" rel="nofollow noreferrer">https://en.m.wikipedia.org/wiki/Radio_galaxy</a>
&quot;A giant radio galaxy is a special class of object characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole.&quot;</p>
<p>How it was established that the giant radio galaxy radio lobes are indeed generated by the relativistic jets powered by the central galaxy's supermassive black hole? Could by any chance there be other (than the effect of the central galaxy's supermassive black hole) alternative possible sources and/or phenomena, which could cause the presence of radio lobes in such giant radio galaxy?</p>

<black-hole><radio>
How was it established that central galaxy's supermassive black holes are responsible for existence of radio lobes?
Alex
# How was it established that central galaxy's supermassive black holes are responsible for existence of radio lobes?

(Asked by: username_0 on 2022-10-22)

According to Wikipedia:
<https://en.m.wikipedia.org/wiki/Radio_galaxy>
"A giant radio galaxy is a special class of object characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole."

How it was established that the giant radio galaxy radio lobes are indeed generated by the relativistic jets powered by the central galaxy's supermassive black hole? Could by any chance there be other (than the effect of the central galaxy's supermassive black hole) alternative possible sources and/or phenomena, which could cause the presence of radio lobes in such giant radio galaxy?

**Tags:** black-hole, radio

---

## 1 answers

### Answer by username_1 on 2022-10-22. Score: 4

Basically, since we know that the central region of these galaxies is occupied by a black hole, accretion by this black hole can power relativistic jets which result in radio emission. Radio emission from jets is most likely for high mass elliptical galaxies as they tend to host black holes with mass toward the high end of the supermassive black hole mass range. This seminal paper by [Merritt and Ekers (2002)](https://arxiv.org/abs/astro-ph/0208001) offers some context. They used this as a basis to hypothesize the double lobed radio galaxies ([X-shaped radio galaxies](https://en.m.wikipedia.org/wiki/X-shaped_radio_galaxy)) as originating from a spin flip due to a galactic merger.

> Although the reorientation of a SBH’s spin axis due to coalescence is not directly ob-
> servable, any gaseous accretion onto the SBH is constrained by relativistic frame dragging
> to be axisymmetric with respect to the black hole, and it is widely believed that the
> jets emitted from the centers of active galaxies are launched perpendicularly to the inner
> accretion disk; hence a jet should point in the same direction as the spin axis of the SBH
> at its center. The extraordinary long-term stability of the jet direction in many radio
> galaxies is strong evidence that jet orientations are regulated by black hole spins.
> Because powerful radio galaxies comprise only a fraction, of order 1%, of all bright ellipti-
> cal galaxies and because radio power is a rapidly increasing function of galaxy luminosity, we would expect only the more massive of the two merging galaxies to harbor a jet.
> Hence a likely consequence of SBH coalescence in a radio galaxy is a sudden change in
> the direction of the jet associated with the larger SBH, followed by the generation of a
> new radio lobe at some (possibly large) angle with respect to the original lobe.

They state further:

> In fact there is a class of radio sources which fit this description: the so-called
> “winged” or “X-type” radio sources. X-shaped sources are characterized by two low-
> surface-brightness radio lobes (the “wings”) oriented at an angle to the “active,” or
> high-surface-brightness, lobes (Fig. 2); both sets of lobes pass symmetrically through
> the center of the associated elliptical galaxy. The first winged source discovered, NGC
> 326, was initially interpreted by a model in which a single SBH undergoes slow geodetic
> precession due to torques from an external mass, resulting in an S-shaped radio morphol-
> ogy; later observations revealed the X shape of this source indicating a more
> rapid change of jet direction. Other explanations for the origin of X-shaped sources have
> been proposed but none has proved satisfactory; black holes are nearly perfect gyroscopes
> and reorienting them via external forces is difficult. One proposed model is based on a
> warping instability of accretion disks, but this model fails to explain why jet reorien-
> tation occurs only once in the X-shaped sources and why most radio galaxies have stable
> jet directions. Capture of a dwarf galaxy with mass comparable to M1 could reorient a
> black hole but it is more likely that the infalling galaxy would be disrupted by tidal forces
> before being accreted.

So, as they state, the stability of the radio lobes is explained well by black holes since they "are essentially perfect gyroscopes and reorienting them via external forces is difficult." They go on to explain how this is the case for several observed radio galaxies. They cite [this](https://ui.adsabs.harvard.edu/abs/1984RvMP...56..255B/abstract) seminal paper which explains that the observed radio emission generally requires relativistic flows, for which black hole accretion is a natural explanation. This continues to be a promising explanation for systems, see e.g.'s [this](https://theconversation.com/experts-solve-the-mystery-of-a-giant-x-shaped-galaxy-with-a-monster-black-hole-as-its-engine-138205), [this](https://astronomycommunity.nature.com/posts/new-insights-into-our-nearest-actively-feeding-supermassive-black-hole).

For an in depth explanation of the relevant physics and uncertainties, [this review by Heckman and Best (2014)](https://www.annualreviews.org/doi/abs/10.1146/annurev-astro-081913-035722) is very helpful. [Here](https://arxiv.org/abs/1403.4620) is the free version on arxiv. See for example Figure 3 which shows that both the main types of AGN, i.e., radiative and jet modes, involve a radio jet being sourced by a central supermassive black hole. The jet-mode AGN is more powerful and generally provides for stronger radio emission. Their Fig 4 summarizes this info nicely. See section 2.3.3 for how the mass of the black hole can be estimated.

> **Comments:**
> **username_0**: From what I know the terms "SBH's accretion disk" and "SBH's photon ring" refer to the same phenomenon... - yes? There is one instance of the claim of direct observation of the the photon ring (yet it is disputed) https://www.sciencenews.org/article/disputed-photon-ring-supermassive-black-hole-image-eht                              If indeed those disks be observable - would it add to the credence of the radio lobes model?  
> **username_2**: @Alex No, the photon sphere (or ring) is at the radius where light can orbit (although that orbit is unstable).  The accretion disk is an extended region, and its inner edge is further from the BH than the photon sphere. See https://en.wikipedia.org/wiki/Photon_sphere & https://en.wikipedia.org/wiki/Innermost_stable_circular_orbit  
> **username_0**: @PM 2Ring - thanks. How those two parameters are used in calculating SBH's mass? Do they complement each other in SBH's mass calculations?  
> **username_2**: @Alex For a Schwarzschild black hole (which has no spin), it's simple. The Schwarzschild radius is $r_s=2GM/c^2$. The photon sphere is at $1.5r_s$ and the ISCO is at $3r_s$. But most supermassive BHs have a *lot* of spin, see https://astronomy.stackexchange.com/a/20292/16685 and the equations  for the critical orbit radii are much more complicated. Also see https://www.nature.com/articles/s41598-021-93595-w  

---

