APOD: The Magnetic Fields of Spiral Galaxy M77 (2019 Dec 16)

[APOD Robot]

The Magnetic Fields of Spiral Galaxy M77

Explanation: Can magnetic fields help tell us how spiral galaxies form and evolve? To find out, the HAWC+ instrument on NASA's airborne (747) SOFIA observatory observed nearby spiral galaxy M77. HAWC+ maps magnetism by observing polarized infrared light emitted by elongated dust grains rotating in alignment with the local magnetic field. The HAWC+ image shows that magnetic fields do appear to trace the spiral arms in the inner regions of M77, arms that likely highlight density waves in the inflowing gas, dust and stars caused by the gravity of the galaxy's oval shape. The featured picture superposes the HAWC+ image over diffuse X-ray emission mapped by NASA's NuSTAR satellite and visible light images taken by Hubble and the SDSS. M77 is located about 47 million light years away toward the constellation of the Sea Monster (Cetus).

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[alter-ego]

... HAWC+ maps magnetism by observing polarized infrared light emitted by elongated dust grains rotating in alignment with the local magnetic field.

For those interested, the question I had about grain orientation within our galaxy appears to be answered in today's linked paper:

Throughout the paper we assume that dust grain alignment is perpendicular to the
direction of the magnetic field.

Thus the simple interpretation that the grain rotation axis is parallel with the field lines is correct. The orientation of the grain is not stated nor implied. Took a little digging to ferret out what was actually assumed. I'm assuming the same is true for the dust grains at the center of our galaxy.

Of course, looking back after the fact, a linked NASA article in the earlier APOD says the same thing

[bystander]

viewtopic.php?t=40093

[Ann]

Click to view full size image

M77 in optical and near infrared light.
Photo: Hubble/Judy Schmidt (Geckzilla).

Click to view full size image

M77 in optical light and magnetic fields.
NASA, SOFIA, HAWC+; JPL-Caltech, Roma Tre. U.; ESA, Hubble, NuSTAR, SDSS

I find it most interesting how the magnetic lines enhance (and help create?) the inner elongated bar structure of M77.

I may be wrong, but I get the impression that the magnetic lines follow the major dust structures in M77.

Ann

[Ann]

... HAWC+ maps magnetism by observing polarized infrared light emitted by elongated dust grains rotating in alignment with the local magnetic field.

For those interested, the question I had about grain orientation within our galaxy appears to be answered in today's linked paper:

Throughout the paper we assume that dust grain alignment is perpendicular to the
direction of the magnetic field.

Thus the simple interpretation that the grain rotation axis is parallel with the field lines is correct. The orientation of the grain is not stated nor implied. Took a little digging to ferret out what was actually assumed. I'm assuming the same is true for the dust grains at the center of our galaxy.

Of course, looking back after the fact, a linked NASA article in the earlier APOD says the same thing

Thanks, alter-ego, that's really interesting!

Ann

[orin stepanek]

Me wondering if magnetic forces may be the future of space travel?

[BDanielMayfield]

... HAWC+ maps magnetism by observing polarized infrared light emitted by elongated dust grains rotating in alignment with the local magnetic field.

For those interested, the question I had about grain orientation within our galaxy appears to be answered in today's linked paper:

Throughout the paper we assume that dust grain alignment is perpendicular to the
direction of the magnetic field.

Thus the simple interpretation that the grain rotation axis is parallel with the field lines is correct. The orientation of the grain is not stated nor implied. Took a little digging to ferret out what was actually assumed. I'm assuming the same is true for the dust grains at the center of our galaxy.

Of course, looking back after the fact, a linked NASA article in the earlier APOD says the same thing

It makes sense that a free floating magnetic grain would align with the magnetic field that it's in, but must it also rotate? Is the rotation of such grains also helping to produce and/or sustain the field? In other words, what is the role of rotation?

[neufer]

Click to play embedded YouTube video.

Click to play embedded YouTube video.

[TheZuke!]

It appears that the X-rays show M77 infested with intestinal parasites.

"Paging Dr. McCoy! Paging Dr. McCoy!"

[Chris Peterson]

Me wondering if magnetic forces may be the future of space travel? :shock:

Well, since we're talking about field strengths that are orders of magnitude less than the Earth's own magnetic field, I don't see these fields being useful for moving anything.

[sillyworm 2]

Top video was interesting...bottom one was a little heady for only 2 cups of coffee....will give it a listen tonight.

[BDanielMayfield]

Top video was interesting...bottom one was a little heady for only 2 cups of coffee....will give it a listen tonight.

Careful though s2. Art has been known to artfully miss-lead from time to time. That first video he posted reminded me of an idea that has fallen way out of favor with consensus cosmology:

Plasma cosmology is a non-standard cosmology whose central postulate is that the dynamics of ionized gases and plasmas play important, if not dominant, roles in the physics of the universe beyond the Solar System.[2][3] In contrast, the current observations and models of cosmologists and astrophysicists explain the formation, development, and evolution of astronomical bodies and large-scale structures in the universe as influenced by gravity (including its formulation in Einstein's theory of general relativity) and baryonic physics.[4]

Some theoretical concepts about plasma cosmology originated with Hannes Alfvén, who tentatively[5] proposed the use of plasma scaling to extrapolate the results of laboratory experiments and plasma physics observations and scale them over many orders of magnitude up to the largest observable objects in the universe (see box[1]).

Cosmologists and astrophysicists who have evaluated plasma cosmology reject it because it does not match the observations of astrophysical phenomena as well as current cosmological theory. Very few papers supporting plasma cosmology have appeared in the literature since the mid-1990s.

The term plasma universe is sometimes used as a synonym for plasma cosmology,[2] as an alternative description of the plasma in the universe.[3]

The second video looks kosher (genuine and legitimate), but I haven't had the chance to watch the whole thing yet either. But the first vid had an "electric universe" look and feel to it. Those ideas have been pounded out of existence here on this forum. (And rightfully so.)

Bruce

[neufer]

Top video was interesting...bottom one was a little heady for only 2 cups of coffee....will give it a listen tonight.

Careful though s2. Art has been known to artfully miss-lead from time to time.

You've cut me to the quick

German philosopher Arthur Schopenhauer wrote in 1818: Der Wahrheit ist allerzeit nur ein kurzes Siegesfest beschieden, zwischen den beiden langen ZeitrÄaumen, wo sie als Paradox verdammt und als Trivial gering geschÄatzt wird.

which is translated as follows: To truth only a brief celebration of victory is allowed between the two long periods during which it is condemned as paradoxical, or disparaged as trivial.

But the fact that some geniuses were laughed at does not imply that all who are laughed at are geniuses. They laughed at Columbus, they laughed at Fulton, they laughed at the Wright Brothers. But they also laughed at Bozo the Clown. - Carl Sagan

That first video he posted reminded me of an idea that has fallen way out of favor with consensus cosmology:

Plasma cosmology is a non-standard cosmology whose central postulate is that the dynamics of ionized gases and plasmas play important, if not dominant, roles in the physics of the universe beyond the Solar System. In contrast, the current observations and models of cosmologists and astrophysicists explain the formation, development, and evolution of astronomical bodies and large-scale structures in the universe as influenced by gravity (including its formulation in Einstein's theory of general relativity) and baryonic physics.

Some theoretical concepts about plasma cosmology originated with Hannes Alfvén, who tentatively proposed the use of plasma scaling to extrapolate the results of laboratory experiments and plasma physics observations and scale them over many orders of magnitude up to the largest observable objects in the universe.

Cosmologists and astrophysicists who have evaluated plasma cosmology reject it because it does not match the observations of astrophysical phenomena as well as current cosmological theory. Very few papers supporting plasma cosmology have appeared in the literature since the mid-1990s.

The term plasma universe is sometimes used as a synonym for plasma cosmology, as an alternative description of the plasma in the universe.

The second video looks kosher (genuine and legitimate), but I haven't had the chance to watch the whole thing yet either. But the first vid had an "electric universe" look and feel to it. Those ideas have been pounded out of existence here on this forum. (And rightfully so.)

The University of Maryland where I studied astronomy in 1967/8
encouraged unorthodox ideas (...perhaps why they put up with me).

<<Sydney Chapman FRS (29 January 1888 – 16 June 1970) is recognised as one of the pioneers of solar-terrestrial physics. Chapman and his first graduate student, V. C. A. Ferraro, predicted the presence of the magnetosphere in the early 1930s. Chapman studied magnetic storms and aurorae, developing theories to explain their relation to the interaction of the Earth's magnetic field with the solar wind. Chapman disputed and ridiculed the work of Kristian Birkeland and Hannes Alfvén, later adopting Birkeland's theories as his own.>>

<<Kristian Olaf Bernhard Birkeland (13 December 1867 – 15 June 1917) was a Norwegian scientist. He is best remembered for his theories of atmospheric electric currents that elucidated the nature of the aurora borealis. Birkeland's theory of the aurora was eventually confirmed and accepted as correct.>>

Auroral-like Birkeland currents created by scientist Kristian Birkeland in his terrella, featuring a magnetised anode globe in an evacuated chamber.

---

<<After Kristian Birkeland first suggested in 1908 that "currents there [in the aurora] are imagined as having come into existence mainly as a secondary effect of the electric corpuscles from the sun drawn in out of space," the story appears to have become mired in politics. Birkeland's ideas were generally ignored in favor of an alternative theory from British mathematician Sydney Chapman.

Birkeland currents are one of a class of plasma phenomena called a z-pinch, so named because the azimuthal magnetic fields produced by the current pinches the current into a filamentary cable. This can also twist, producing a helical pinch that spirals like a twisted or braided rope, and this most closely corresponds to a Birkeland current. Pairs of parallel Birkeland currents will also interact due to Ampère's force law: parallel Birkeland currents moving in the same direction will attract each other with an electromagnetic force inversely proportional to their distance apart whilst parallel Birkeland currents moving in opposite directions will repel each other.

In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper published on the generation of the current from the Solar Wind. In 1964 one of Alfvén's colleagues, Rolf Boström, also used field-aligned currents in a new model of auroral electrojets.

Proof of Birkeland's theory of the aurora only came after a probe was sent into space. The crucial results were obtained from U.S. Navy satellite 1963-38C, launched in 1963 and carrying a magnetometer above the ionosphere. In 1966 Alfred Zmuda, J.H. Martin, and F.T.Heuring analysed the satellite magnetometer results and reported their findings of magnetic disturbance in the aurora. In 1967 Alex Dessler and graduate student David Cummings wrote an article arguing that Zmuda et al. had detected field-aligned currents. Alfvén subsequently acknowledged that Dessler had "discovered the currents that Birkeland had predicted" and they should be called Birkeland-Dessler currents. 1967 is therefore taken as the date when Birkeland's theory was finally acknowledged to have been vindicated. In 1969 Milo Schield, Alex Dessler and John Freeman used the name "Birkeland currents" for the first time.>>

[alter-ego]

...
It makes sense that a free floating magnetic grain would align with the magnetic field that it's in, but must it also rotate? Is the rotation of such grains also helping to produce and/or sustain the field? In other words, what is the role of rotation?

Grain rotation is required as it leads to a stable, non-random condition. FIR black-body emission from that grain will then have a constant preferential polarization direction. In short, all grains spin-up from incident light radiation but only weakly magnetic (paramagnetic) grains will develop a magnetic moment. These grains will precess in the external B field and eventually the rotation axis will align with the B field. The grain's long-axis ends up perpendicular to the external B field.
Again, it's the spin axis that aligns with the field, not the grain. If it weren't for the radiation-induced spin, grain orientations would be more chaotic due to collisions thus randomizing the FIR polarizations.

As noted above, whereas grains of any composition will (according to RAT [Radiative Alignment Torque] theory) be spun up by the radiation, only those made up of paramagnetic materials will align with the magnetic field. However, for strong, and directional enough, radiation fields, grains made up of other materials (such as diamagnetic carbon solids) might instead align with the direction of the light.
...
Once the grain acquires a magnetic moment, that magnetization will interact with an external magnetic field, and cause the grain’s spin axis to precess around the direction of the magnetic field (like a wobbly spinning top). As the radiation illuminates the different faces of the grain as it wobbles, the torques turn the spin axis to coincide with the magnetic field direction.

The external magnetic field is neither produced nor sustained by the rotating grains.
Extending the process for a single grains to large numbers, you can see how the B field lines can be traced in an image.

[BDanielMayfield]

...
It makes sense that a free floating magnetic grain would align with the magnetic field that it's in, but must it also rotate? Is the rotation of such grains also helping to produce and/or sustain the field? In other words, what is the role of rotation?

Grain rotation is required as it leads to a stable, non-random condition. FIR black-body emission from that grain will then have a constant preferential polarization direction. In short, all grains spin-up from incident light radiation but only weakly magnetic (paramagnetic) grains will develop a magnetic moment. These grains will precess in the external B field and eventually the rotation axis will align with the B field. The grain's long-axis ends up perpendicular to the external B field.
Again, it's the spin axis that aligns with the field, not the grain. If it weren't for the radiation-induced spin, grain orientations would be more chaotic due to collisions thus randomizing the FIR polarizations.

As noted above, whereas grains of any composition will (according to RAT [Radiative Alignment Torque] theory) be spun up by the radiation, only those made up of paramagnetic materials will align with the magnetic field. However, for strong, and directional enough, radiation fields, grains made up of other materials (such as diamagnetic carbon solids) might instead align with the direction of the light.
...
Once the grain acquires a magnetic moment, that magnetization will interact with an external magnetic field, and cause the grain’s spin axis to precess around the direction of the magnetic field (like a wobbly spinning top). As the radiation illuminates the different faces of the grain as it wobbles, the torques turn the spin axis to coincide with the magnetic field direction.

The external magnetic field is neither produced nor sustained by the rotating grains.
Extending the process for a single grains to large numbers, you can see how the B field lines can be traced in an image.

Thanks alter-ego for that fine explanation.