APOD: Rotation of the Large Magellanic Cloud (2018 May 16)

[Ann]

So glad about gaia's data.

However, one of the sentences seems to be outdated information "The LMC [...] is a spiral galaxy that has been distorted by encounters with the greater Milky Way Galaxy and the lesser Small Magellanic Cloud."
Recently, I read some articles about the past encounters of Andromeda, in which they said that the LMC has been distorted then.
I may be wrong

This Wikipedia article about the Hubble sequence identifies the Large Magellanic Cloud as an irregular galaxy. However, I think it could be argued that the LMC might also be classified as an SBd galaxy:

Sd (SBd) – very loosely wound, fragmentary arms; most of the luminosity is in the arms and not the bulge.

If you take a look at this picture of the Large Magellanic cloud, you might perhaps agree that the scattered regions of star formation surrounding the bar of the LMC do show some slight simiarities to spiral arms.

Ann

Thank you for your answer, Ann. (I really appreciate your work on color, there are so many inconsistencies about it)

You're welcome!

English is not my native language,

It's not mine, either. I wonder how many gaffes I've made here over the years.

I probably did not express myself clearly. What I consider potentially outdated is that the LMC has been distorted by the Milky Way rather than by Andromeda.
I read a recent article about it, but I can not retrieve it.
Here are two
http://iopscience.iop.org/article/10.10 ... 2/924/meta
https://arxiv.org/abs/1010.2748

Well, personally I wouldn't think that M31 has affected the Magellanic Clouds all that much. The Astrophysical Journal doesn't seem to think so, either.

The Astrophysical Journal wrote:

For a range of LMC proper motions approximately 3σ higher than the mean and total Local Group mass >3.5 × 1012 M ☉, M31 can provide a significant torque to the LMC orbit. However, if the LMC is bound to the MW, then M31 is found to have negligible effect on its motion, and the origin of the high angular momentum of the system remains a puzzle.

Click to view full size image

The Local Group of galaxies. Illustration:
http://www.atlasoftheuniverse.com/localgr.html

So is the Large Magellanic Cloud bound to the Milky Way? Are both Magellanic Clouds bound to the Milky Way? I'd say that, yes, that seems highly probable.

Take a look at the map at left. It shows the Local Group of galaxies. The Milky Way is in the middle of the map. Can you see the two little elongated white things to the left of it? I'd say that those little objects are the Magellanic Clouds. Note how close they are to the Milky Way! Note that the Andromeda galaxy is much farther away. It, too, has two little objects right next to itself. Those two little satellite galaxies are M32 and NGC 205.

I believe that it is fairly generally accepted that the Magellanic Clouds have been interacting with each other for quite some time, perhaps for a billion years or so. Most likely, though, the Magellanic Clouds were not snared by the gravity of the Milky Way until more recently.

So I'd say that the distorted shape of the LMC is mostly due to interactions with the LMC, but it is also caused by the mighty pull of the Milky Way.

Ann

[neufer]

DR2 covers the first 22 months of data collection. DR1 was the first 14 months. It takes a lot of work before the data can be released in a usable form. The diagram is extrapolated from that. 22 months of proper motion would be sub pixel and invisible on a rendering like that.

I wouldn't be too concerned with "sub pixel."

Multiple pixels are involved with each spinning Gaia "snapshot" and

the center of each "snapshot" Airy disk can be determined to high "sub pixel" precision.

Yes. But that's not the point. At the scale of today's image, all the motion is subpixel, and since today's image contains no stellar images, there wouldn't even be centroids that could be calculated.

Today's image contains "exaggerated star trails for millions of faint LMC stars."

[Chris Peterson]

I wouldn't be too concerned with "sub pixel."

Multiple pixels are involved with each spinning Gaia "snapshot" and

the center of each "snapshot" Airy disk can be determined to high "sub pixel" precision.

Yes. But that's not the point. At the scale of today's image, all the motion is subpixel, and since today's image contains no stellar images, there wouldn't even be centroids that could be calculated.

Today's image contains "exaggerated star trails for millions of faint LMC stars."

Maybe, although I don't think that's quite accurate. The analysis technique provides flow patterns in structured data. I don't think the visual result is the same as you'd get projecting the measured proper motion of each star.

In any case, however, my point remains the same. The actual measured proper motion results in subpixel shifts at this image scale, and there are not stellar images in this product that could have their centroids determined.

[neufer]

Today's image contains "exaggerated star trails for millions of faint LMC stars."

Maybe, although I don't think that's quite accurate. The analysis technique provides flow patterns in structured data. I don't think the visual result is the same as you'd get projecting the measured proper motion of each star.

• 1) It a direct quote from the APOD itself.
  2) They are all thin straight lines of variable length.

In any case, however, my point remains the same. The actual measured proper motion results in subpixel shifts at this image scale, and there are not stellar images in this product that could have their centroids determined.

• 1) Which is why the actual measured proper motions were "exaggerated" to make them visible.
  2) The stellar images themselves are hidden at the center of each straight line.

[Chris Peterson]

Today's image contains "exaggerated star trails for millions of faint LMC stars."

Maybe, although I don't think that's quite accurate. The analysis technique provides flow patterns in structured data. I don't think the visual result is the same as you'd get projecting the measured proper motion of each star.

• 1) It a direct quote from the APOD itself.
  2) They are all thin straight lines of variable length.

Yes, but that's an oversimplification of the deconvolution technique used to extract this data.

In any case, however, my point remains the same. The actual measured proper motion results in subpixel shifts at this image scale, and there are not stellar images in this product that could have their centroids determined.

• 1) Which is why the actual measured proper motions were "exaggerated" to make them visible.
  2) The stellar images themselves are hidden at the center of each straight line.

I was addressing the original comment about subpixel shifts, which was accurate. There are NO stellar images to be found here, only synthesized data points extracted from numerical data.

[BillT]

I heard that Gaia's mission is to take up to 70 snapshots of its targets over 5 years [Wikipedia], and the elapsed time since Gaia's launch is 4 years, 4 months, and 27 days, I'd say the time period is around 4 years and 4 months. (Just my unofficial guess.)

DR2 covers the first 22 months of data collection. DR1 was the first 14 months. It takes a lot of work before the data can be released in a usable form. The diagram is extrapolated from that. 22 months of proper motion would be sub pixel and invisible on a rendering like that.

I wouldn't be too concerned with "sub pixel."

Multiple pixels are involved with each spinning Gaia "snapshot" and

the center of each "snapshot" Airy disk can be determined to high "sub pixel" precision.

I was referring to the rendering shown on APOD, not what is recorded on Gaia's cameras.

[AndyCanada]

The "Fingerprint" comment from several people is interesting, and the first thing I thought of (even though I new what it was) when I first saw it.

I suppose it isn't surprising that it looks like a fingerprint, since both are caused by chaotic systems. I imagine the same rule applies as well, there are no two galaxies with the same "Fingerprint".

Here's a bit on how fingerprints form:

Innumerable environmental factors are thought to influence the formation of fingerprints, including blood pressure, oxygen levels in the blood, nutrition of the mother, hormone levels, the exact position of the fetus in the womb at particular times, the exact composition and density of the amniotic fluid that’s swirling around the fingers of the fetus as they touch surrounding structures, and the pressure with which they touch their surroundings. These myriad variables decide how each individual ridge is formed.

The level of activity of a fetus and the general chaos of the conditions of the womb prevent fingerprints from developing exactly the same way in any two fetuses. The entire development process is so chaotic that, over the entire course of human history, there is virtually no chance that the exact same pattern formed twice. What this means, though, is that fingerprints are different on every finger of your hand, they’re different on the same fingers of opposite hands, and even the fingerprints of identical twins are different from each other.

[weird]

Why would it have that lobe above the center? Binary centers of gravity?

Gobbets of gravity all slogging around a common center. I like how the edges look all hairy. Highly perturbed areas form interaction w/ MW.

[JohnD]

Thnak you AndyCanada! Especially that you used the term 'Chaotic system', when that is usually the result of multiple influences, as you mention for the formation of fingerprints. The influences on the LMC are mentioned in the original caption, the Milky Way, SMC etc. showing the common underlying structure.

Are you quoting a list of fingerprint influences? Where from, please?
John

[neufer]

2 D phase space dynamics

---

Thank you AndyCanada! Especially that you used the term 'Chaotic system'...

Speaking of 'Chaotic systems' note that we have before us
a 4 D phase space diagram: 2 D of space & 2 D of velocity

If the LMC "BAR" we see is actually due to a quasi-standing wave convergence when the angular velocities within the "BAR" should be noticeably slower than the angular velocities (along the corresponding "finger print ridge") on either side of the "BAR." When will the first paper come out comparing the Gaia 4 D phase space diagram with actual galaxy models

Evolution of an ensemble of classical systems in phase space (top). The systems are a massive particle in a one-dimensional potential well (red curve, lower figure). The initially compact ensemble becomes swirled up over time.

In classical mechanics, any choice of generalized coordinates q_i for the position (i.e. coordinates on configuration space) defines conjugate generalized momenta p_i which together define co-ordinates on phase space. More abstractly, in classical mechanics phase space is the cotangent bundle of configuration space, and in this interpretation the procedure above expresses that a choice of local coordinates on configuration space induces a choice of natural local Darboux coordinates for the standard symplectic structure on a cotangent space.

[Chris Peterson]

The "Fingerprint" comment from several people is interesting, and the first thing I thought of (even though I new what it was) when I first saw it.

I suppose it isn't surprising that it looks like a fingerprint, since both are caused by chaotic systems. I imagine the same rule applies as well, there are no two galaxies with the same "Fingerprint".

Here's a bit on how fingerprints form:

Innumerable environmental factors are thought to influence the formation of fingerprints, including blood pressure, oxygen levels in the blood, nutrition of the mother, hormone levels, the exact position of the fetus in the womb at particular times, the exact composition and density of the amniotic fluid that’s swirling around the fingers of the fetus as they touch surrounding structures, and the pressure with which they touch their surroundings. These myriad variables decide how each individual ridge is formed.

The level of activity of a fetus and the general chaos of the conditions of the womb prevent fingerprints from developing exactly the same way in any two fetuses. The entire development process is so chaotic that, over the entire course of human history, there is virtually no chance that the exact same pattern formed twice. What this means, though, is that fingerprints are different on every finger of your hand, they’re different on the same fingers of opposite hands, and even the fingerprints of identical twins are different from each other.

I think it's a bit marginal to consider fingerprint formation a chaotic process. It's actually better seen as a slightly randomized process. N-body orbital systems are chaotic in the mathematical sense because of the way all the components interact with each other. Fingerprints are physical structures with only local interaction between regions (like most biological growth patterns). Sure, the environment that influences fingerprint formation is a complex system- chaotic. But that doesn't make fingerprint formation itself so.

[neufer]

---

.
.
"Fingerprint" diagram illustrating Lin & Shu's explanation
of spiral arms in terms of elliptical orbit confluences.

[MarkBour]

In this APOD, using the term "star trails" reminds of many nightly images taken from Earth, looking up. A favorite type of "star trails" image being an image from a camera aimed at Polaris. And this comparison begs a question. Is any of the motion indicated in the APOD the result of motion of GAIA? How is it possible to remove any such effects? I'm guessing that GAIA use a particular reference set for orientation(?) Perhaps some distant galaxies are treated as "fixed stars" for it?

A second question. If this is just numeric data turned into a visualization by drawing line segments, how is there color in the image? I think in addition to monotone line segments, there must be some other data imposed on the image. There was a discussion above of whether or not star airy disks were part of this image. Clearly there is something more than just rendering of line segments from ordered pairs of numbers.

[Chris Peterson]

In this APOD, using the term "star trails" reminds of many nightly images taken from Earth, looking up. A favorite type of "star trails" image being an image from a camera aimed at Polaris. And this comparison begs a question. Is any of the motion indicated in the APOD the result of motion of GAIA? How is it possible to remove any such effects? I'm guessing that GAIA use a particular reference set for orientation(?) Perhaps some distant galaxies are treated as "fixed stars" for it?

A second question. If this is just numeric data turned into a visualization by drawing line segments, how is there color in the image? I think in addition to monotone line segments, there must be some other data imposed on the image. There was a discussion above of whether or not star airy disks were part of this image. Clearly there is something more than just rendering of line segments from ordered pairs of numbers.

It's just numbers. The dataset is a catalog entry for each star which consists of the coordinates and a set of instrumental magnitudes as measured through different filters. That's where the color information comes from. And because the coordinates are remeasured regularly, there is also proper motion, which goes into the deconvolution process that produces the flow field (approximately, the trails, although if you look closely you'll see they aren't exactly trails).

Gaia's motion isn't a factor, other than in the sense that its own tiny vibrations and pointing errors contribute to the instrumental error in calculating the coordinates of the stars it measures.

[Bruce V. B.]

The image reminds me of a fingerprint. If each galaxy were to be photographed that way, I would guess that each galaxy would have its own "fingerprint", much as humans.