APOD: Arp 87: Merging Galaxies from Hubble (2019 Aug 11)

[APOD Robot]

Arp 87: Merging Galaxies from Hubble

Explanation: This dance is to the death. Along the way, as these two large galaxies duel, a cosmic bridge of stars, gas, and dust currently stretches over 75,000 light-years and joins them. The bridge itself is strong evidence that these two immense star systems have passed close to each other and experienced violent tides induced by mutual gravity. As further evidence, the face-on spiral galaxy on the right, also known as NGC 3808A, exhibits many young blue star clusters produced in a burst of star formation. The twisted edge-on spiral on the left (NGC 3808B) seems to be wrapped in the material bridging the galaxies and surrounded by a curious polar ring. Together, the system is known as Arp 87 and morphologically classified, technically, as peculiar. While such interactions are drawn out over billions of years, repeated close passages should ultimately result in the death of one galaxy in the sense that only one galaxy will eventually result. Although this scenario does look peculiar, galactic mergers are thought to be common, with Arp 87 representing a stage in this inevitable process. The Arp 87 pair are about 300 million light-years distant toward the constellation Leo. The prominent edge-on spiral galaxy at the far left appears to be a more distant background galaxy and not involved in the on-going merger.

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[BDanielMayfield]

This dance is to the death.

That's one way of looking at it. I can see the point being made; that ultimately there will come to be only one galaxy whereas now there are two, but that is a rather negative take IMO.

This is also a magnificently constructive, creative process. Look at all those bluish bright patches in the tidal streams around both galaxies. These are the areas of the most intense star formation. The light from the bright blue (especially) and bluish-white areas primarily come from the brightest stars, the ones that are said to "live fast and die young", but that is also describing an immensely creative process, for that is where the most heavy elements are being forged, later to be ejected out into the interstellar medium. The ever more enriched dust in these galaxies will continue to produce later generations of stars, and since planet formation is a by-product of star formation, literally billions of of planets might will likely result from this galactic merger.

Galactic "collisions" are way more constructive than destructive. They are hyper-productive, massively beneficial mergers. How's that for positive spin?

Bruce

[Ann]

This dance is to the death.

That's one way of looking at it. I can see the point being made; that ultimately there will come to be only one galaxy whereas now there are two, but that is a rather negative take IMO.

This is also a magnificently constructive, creative process. Look at all those bluish bright patches in the tidal streams around both galaxies. These are the areas of the most intense star formation. The light from the bright blue (especially) and bluish-white areas primarily come from the brightest stars, the ones that are said to "live fast and die young", but that is also describing an immensely creative process, for that is where the most heavy elements are being forged, later to be ejected out into the interstellar medium. The ever more enriched dust in these galaxies will continue to produce later generations of stars, and since planet formation is a by-product of star formation, literally billions of of planets might will likely result from this galactic merger.

Galactic "collisions" are way more constructive than destructive. They are hyper-productive, massively beneficial mergers. How's that for positive spin?

Bruce

Very good point, Bruce.

I read once that complexity happens on the way to uniformity and heat death. All right, so those two galaxies will merge and become one large elliptical galaxy where not much is going on. But before the galaxies reach that boring end stage, they face a future of amazing complexity. In a state of very low entropy not much happens, and when you have reached maximum entropy, nothing happens. Structure, complexity and even life happens between those two extremes.

Click to play embedded YouTube video.

[orin stepanek]

And the two shall become one!

[Leon1949Green]

Regarding the more distant galaxy not interacting with Arp 87. Now that we understand that the disc of our Milky Way is a bit distorted, what is being looked at to determine who we've encountered already to cause it to be that way? Thanks!

[BDanielMayfield]

I read once that complexity happens on the way to uniformity and heat death.

But, as we know, one shouldn't trust everything one reads.

I don't even let forecasts of ultimate heat death 'bring me down'.

Human reasoning on both the deep past and the deep future is handicapped by lack of information.

Bruce

[Ann]

I read once that complexity happens on the way to uniformity and heat death.

But, as we know, one shouldn't trust everything one reads.

I don't even let forecasts of ultimate heat death 'bring me down'.

Human reasoning on both the deep past and the deep future is handicapped by lack of information.

Bruce

To each his (or her) own, Bruce!

Ann

[BDanielMayfield]

I read once that complexity happens on the way to uniformity and heat death.

But, as we know, one shouldn't trust everything one reads.

I don't even let forecasts of ultimate heat death 'bring me down'.

Human reasoning on both the deep past and the deep future is handicapped by lack of information.

Bruce

To each his (or her) own, Bruce!

Ann

That is the nicest agree to disagree I've ever received. Thanks.

[BDanielMayfield]

Regarding the more distant galaxy not interacting with Arp 87. Now that we understand that the disc of our Milky Way is a bit distorted, what is being looked at to determine who we've encountered already to cause it to be that way? Thanks!

Are the Magellanic Clouds massive enough to have caused this warp in the MW?

[neufer]

Regarding the more distant galaxy not interacting with Arp 87. Now that we understand that the disc of our Milky Way is a bit distorted, what is being looked at to determine who we've encountered already to cause it to be that way? Thanks!

Are the Magellanic Clouds massive enough to have caused this warp in the MW?

The problem is their distance (and, to some extent, low angular motion).
Gravitational tidal forces drop off inversely with distance CUBED.

[Tekija]

Dance of death in the sky - La dance macabre by Leonie Justin Alexandre Petit (French, 1839–1884)

---

[neufer]

But, as we know, one shouldn't trust everything one reads.

I don't even let forecasts of ultimate heat death 'bring me down'.

Human reasoning on both the deep past and the deep future is handicapped by lack of information.

To each his (or her) own, Bruce!

That is the nicest agree to disagree I've ever received. Thanks.

---

<<"Jedem das Seine" is the literal German translation of Latin suum cuique, a fundamental juridical concept meaning "to each his own" or "to each what he deserves". Jedem das Seine has been an idiomatic German expression for several centuries. For example, it is found in the works of Martin Luther and contemporaries. It appears in the title of a cantata by Johann Sebastian Bach, Nur jedem das Seine (BWV 163), first performed at Weimar in 1715. Some nineteenth-century comedies bear the title Jedem das Seine, including works by Johann Friedrich Rochlitz and Caroline Bernstein. It is similar to "Whatever is, is right" employed by Leibniz, Hegel, and Scrope.

An ironic twist on the proverb, "jedem das Seine, mir das Meiste" ("to each his own, to me the most"), has been known in the reservoir of German idioms for a long time, including its inclusion in Carl Zuckmayer's 1931 play The Captain of Köpenick.

In 1937, the Nazis constructed the Buchenwald concentration camp, 7 km from Weimar, Germany. The motto Jedem das Seine was placed in the camp's main entrance gate. The gates were designed by Franz Ehrlich, a former student of the Bauhaus art school, who had been imprisoned in the camp because he was a communist.>>

[ems57fcva]

I am not seeing merging galaxies. NGC 3808B certainly came close to and has interacted with NGC 3808A. But I am not seeing a well-established bridge linking the galaxies. Instead I am seeing a lot of gas in front of NGC 3808A and then gas going around NCG 3808B. A connection may remain between the two, but it is tenuous at best. I read what I am seeing as indicating that NGC 3808B is coming most straight towards us, and is not gravitationally bound to NGC 3808A.

[Chris Peterson]

I am not seeing merging galaxies. NGC 3808B certainly came close to and has interacted with NGC 3808A. But I am not seeing a well-established bridge linking the galaxies. Instead I am seeing a lot of gas in front of NGC 3808A and then gas going around NCG 3808B. A connection may remain between the two, but it is tenuous at best. I read what I am seeing as indicating that NGC 3808B is coming most straight towards us, and is not gravitationally bound to NGC 3808A.

I don't think any of these inferences are reliable (or even possible) from examining an image like this. Certainly, the relative velocities of different parts of these galaxies will have been measured. The likelihood that a pair of galaxies would be coincidentally passing each other close enough to interact, with sufficient relative velocities that they are not in closed orbits around each other, seems vanishingly small. Indeed, I don't think any such galaxies have ever been observed. Galaxies form in clusters, with the members gravitationally bound.

[DL MARTIN]

Without context doubt remains that 2+2=4. In other words, until astronomers know what happens between then and now, claims of certainty are dubious.

[neufer]

Click to play embedded YouTube video.

The likelihood that a pair of galaxies would be coincidentally passing each other close enough to interact, with sufficient relative velocities that they are not in closed orbits around each other, seems vanishingly small. Indeed, I don't think any such galaxies have ever been observed. Galaxies form in clusters, with the members gravitationally bound.

[Chris Peterson]

Without context doubt remains that 2+2=4. In other words, until astronomers know what happens between then and now, claims of certainty are dubious.

Well, we can't see then, and we're unlikely to be around as a species long enough to see the future of this interaction, so all we have is now. Which is actually quite enough to draw some very solid conclusions.

[alter-ego]

Regarding the more distant galaxy not interacting with Arp 87. Now that we understand that the disc of our Milky Way is a bit distorted, what is being looked at to determine who we've encountered already to cause it to be that way? Thanks!

Are the Magellanic Clouds massive enough to have caused this warp in the MW?

The problem is their distance (and, to some extent, low angular motion).
Gravitational tidal forces drop off inversely with distance CUBED.

Solely talking about mass, apparently the MCs masses are enough to perturb the MW plane. However in a recent paper, ANTLIA2'S ROLE IN DRIVING THE RIPPLES IN THE OUTER GAS DISK OF THE GALAXY, the recently discovered dwarf galaxy, Antlia 2 is the best candidate collider to produce the observed MW distortions. GAIA measurements (again) have provided trajectory data to feed into to the collision calculations. It is expected the data releases will reduce uncertainties. The authors rule out the Sgr dwarf and both MCs as progenitors.

In summary, the orbital distributions for Antlia 2 have a signicant tail of low pericenters of 10 kpc for a
range of Milky Way masses commonly cited in the literature (from ~10¹²to 2x10¹²M☉).
A close interaction of this kind with a 1:100 mass ratio perturber is sufficient to explain the planar disturbances observed in the outer
HI disk of the Milky Way. Moreover, the phase of the disturbances has a flat radial variation for the HI data,
as do the Antlia 2 simulations with low pericenters, independently confirming that low pericenters are needed
to match the disturbances manifest in the outer gas disk of the Galaxy. We show that the tidal strength of the
Sgr dwarf is insucient to explain the disturbances in the outer gas of the Galaxy. Of the other tidal players of
the Milky Way, the Large and Small Magellanic Clouds are too distant and have not approached closer in the
recent past (Besla et al. 2007; Besla et al. 2012) to account for this level of Fourier power in the outer HI disk.
Thus, Antlia 2 is likely the driver of the observed large perturbations on the outskirts of our Galaxy.

With that said, the progenitor mass is in question:

But not everybody agrees with this conclusion. Vasily Belokurov (University of Cambridge, UK) cites a series of less than probable assumptions required for the Antlia 2 scenario. “Their simulations are rather simple and are produced for one realization of Antlia 2 only,” Belokurov explains. “There are already constraints on the mass of Antlia 2 and at the moment the mass appears lower than what they need.”

I suspect the uncertain contribution from Dark Matter in Antlia 2 is a big factor coupled with the uncertainty in total luminosity due to the poor visibility of this ultra-diffuse galaxy. Maybe improved GAIA data will at least narrow the MW collision zone.

[Ann]

Regarding the more distant galaxy not interacting with Arp 87. Now that we understand that the disc of our Milky Way is a bit distorted, what is being looked at to determine who we've encountered already to cause it to be that way? Thanks!

Click to view full size image

Warped disk galaxy ESO 510-G13. Photo: Hubble.

Click to view full size image

The Integral Sign galaxy, UGC 3697. NRAO/AUI/NSF

Good question, Leon. Really warped disk galaxies appear to be very unusual. I myself am an avid scrutinizer of galaxy images, and I can testify to the fact that I very rarely get to see strongly warped disk galaxies.

It appears that the Milky Way may be almost as warped as ESO 510-G13. I can't even begin to guess why our galaxy has such an unusual shape, although interactions with the Magellanic Clouds is really the only thing that comes to mind.

Ann

EDIT: Alter-ego, I didn't see your post before I posted mine. Thanks for pointing out the Antlia 2 dwarf galaxy as the likely culprit behind our galaxy's unusual shape!

[neufer]

Alter-ego, I didn't see your post before I posted mine.

Thanks for pointing out the Antlia 2 dwarf galaxy as the likely culprit behind our galaxy's unusual shape!

While it may well be true that "the tidal strength of the Sgr dwarf is insucient [sic] to explain the disturbances in the outer gas of the Galaxy" that doesn't make the recently discovered dim distant Antlia 2 dwarf galaxy the culprit based on one tentative paper.

https://en.wikipedia.org/wiki/Satellite ... _Milky_Way

insouciant (adj.) 1828, from French insouciant "careless, thoughtless, heedless,"

from in- "not" (see in- (1)) + souciant "caring," present participle of soucier "to care,"
from Latin sollicitare "to agitate" (see solicit).

[pferkul]

The word "violent" all depends on your point of view. On the one hand, altering the course of a hundred billion suns requires titanic forces, but on the other, the interaction of galaxies appears like puffs of smoke gently mingling over the course of a billion years.

[Chris Peterson]

The word "violent" all depends on your point of view. On the one hand, altering the course of a hundred billion suns requires titanic forces, but on the other, the interaction of galaxies appears like puffs of smoke gently mingling over the course of a billion years.

Except that puffs of smoke don't interact with each other to create temperatures high enough to initiate nuclear fusion.

[BDanielMayfield]

The word "violent" all depends on your point of view. On the one hand, altering the course of a hundred billion suns requires titanic forces, but on the other, the interaction of galaxies appears like puffs of smoke gently mingling over the course of a billion years.

Except that puffs of smoke don't interact with each other to create temperatures high enough to initiate nuclear fusion.

But pferkul has a good point too. When galaxies "collide" their stars, having such vast distances between them in the first place, will almost never collide. The "violence" of an event that takes a billion or more years to occur is overstated for dramatic effect.

Bruce

[Chris Peterson]

The word "violent" all depends on your point of view. On the one hand, altering the course of a hundred billion suns requires titanic forces, but on the other, the interaction of galaxies appears like puffs of smoke gently mingling over the course of a billion years.

Except that puffs of smoke don't interact with each other to create temperatures high enough to initiate nuclear fusion.

But pferkul has a good point too. When galaxies "collide" their stars, having such vast distances between them in the first place, will almost never collide. The "violence" of an event that takes a billion or more years to occur is overstated for dramatic effect.

Bruce

The "violence" I see describes the collisions of gas and dust, not stars. And the time scale seems to me irrelevant. I don't think it's unreasonable to describe as "violent" any physical process that releases a lot of energy over a relatively short period of time, regardless of the spatial scale, regardless of the temporal scale.

[neufer]

Click to play embedded YouTube video.

When galaxies "collide" their stars, having such vast distances between them in the first place, will almost never collide. The "violence" of an event that takes a billion or more years to occur is overstated for dramatic effect.

The "violence" I see describes the collisions of gas and dust, not stars. And the time scale seems to me irrelevant. I don't think it's unreasonable to describe as "violent" any physical process that releases a lot of energy over a relatively short period of time, regardless of the spatial scale, regardless of the temporal scale.