APOD: Arp 81: 100 Million Years Later (2014 Apr 23)

[APOD Robot]

Arp 81: 100 Million Years Later

Explanation: From planet Earth, we see this strongly distorted pair of galaxies, cataloged as Arp 81, as they were only about 100 million years after their close encounter. The havoc wreaked by their mutual gravitational interaction during the encounter is detailed in this color composite image showing twisted streams of gas and dust, a chaos of massive star formation, and a tidal tail stretching for 200 thousand light-years or so as it sweeps behind the cosmic wreckage. Also known as NGC 6622 (left) and NGC 6621, the galaxies are roughly equal in size but are destined to merge into one large galaxy in the distant future, making repeated approaches until they finally coalesce. Located in the constellation Draco, the galaxies are 280 million light-years away. Even more distant background galaxies can be spotted in this sharp, reprocessed, image from Hubble Legacy Archive data.

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[Ann]

Fascinating. I wonder what their color indexes are. Well, the galaxies are 280 million light-years distant, making it unlikely that my software will have color information about them.

Ann

[Nitpicker]

To put things in an Earthly timeframe, according to:
http://en.wikipedia.org/wiki/Timeline_o ... ry_of_life

... the galactic collision happened 380 million years ago, shortly after the first tetrapods on Earth were known to walk on land. And 100 million years ago (280 million years later), certain dinosaurs might have been able to witness the collision from Earth, had they been technologically minded. They might have had fun with grammatical tenses and metric tensors, too. They were certainly tense times.

[SouthEastAsia]

First off, I must say I do love fine cheeses with my fluffy cotton candy and popcorn...

That said, I must also say that whomever wrote this APOD's description is a born cosmic poet. Very fascinating and interesting APOD. Thanks...

[Boomer12k]

Truly Awesome encounter....Two Dragons twisting in Mortal Combat....only to merge and become stronger....."There can be only ONE".....

I always wonder if there was life over there, what could happen to them...get spun out of their system and freeze... or nothing???

:---[===] *

[Tszabeau]

The galaxy on the left appears to have revealed its' heart. What is the red streak on the right?

[starsurfer]

Two cosmic soulmates that belong in each others arms. This is the first APOD that has almost brought a tear to my eye.

[jambo]

"making repeated approaches"
Do I understand that they ricochet off each other like spinning tops, so in actuality they may be distinctly separate at this time, only to return possibly multiple times before completely merging?

[Psnarf]

Consider the center of mass of each galaxy connected via a rubber band. If you speed up the clock, you'll see the centers zip past each other until the rubber band is fully stretched, then back again. Off-center, they will not collide during this cosmic dance. Bad analogy, but it's a start. The individual stars do not come into contact with each other, the centers keep passing each other. Consider each galaxy center on the same elliptical track on opposite sides. The track rotates so as the two galaxies run around the track, each pass is from a different direction. Another stupid analogy.
What I don't understand is what happens when the two black holes get close enough to merge? Let's look again in a couple of million years and see what happens.

[geckzilla]

Ricochet is a bad word. It's best to just watch a simulation of merging galaxies to get a feel for what happens over millions of years.

Click to play embedded YouTube video.

Compare any simulations with real images of galaxies at various stages of merging:
http://www.spacetelescope.org/static/ar ... 0810ab.jpg

Frank Summers did exactly that with this video:

Click to play embedded YouTube video.

I can't think of a proper analogy. They are like giant, fluffy, crazy clouds which gravitationally coalesce, with spaces too far between individual stars for many actual collisions to occur.

[MarkBour]

geckzilla -- don't worry that you don't have a good analogy. The simulations you gave say it all quite eloquently.
I'm really impressed by the video that you credit to Frank Summers, that helps validate the simulation.

I wonder how many galaxy interactions astronomers have actually observed in space (total), and how they lay out in a 3-D map.
At first, I would have thought they were a rare phenomena, with only a few lucky examples ever noted.
If there are many examples, it would have significant cosmological implications.

[Anthony Barreiro]

The galaxy on the left appears to have revealed its' heart. What is the red streak on the right?

That red streak appears to be a background spiral galaxy seen edge-on. Maybe one of these, but I dunno.

[Chris Peterson]

I wonder how many galaxy interactions astronomers have actually observed in space (total), and how they lay out in a 3-D map. At first, I would have thought they were a rare phenomena, with only a few lucky examples ever noted.
If there are many examples, it would have significant cosmological implications.

Thousands. I don't think there's anything interesting about their distribution. Galaxies tend to have formed in clusters, and those galaxies always interact at some point. There are probably fewer galaxies that are pristine than ones that are interacting or have in the past.

[geckzilla]

geckzilla -- don't worry that you don't have a good analogy. The simulations you gave say it all quite eloquently.
I'm really impressed by the video that you credit to Frank Summers, that helps validate the simulation.

I wonder how many galaxy interactions astronomers have actually observed in space (total), and how they lay out in a 3-D map.
At first, I would have thought they were a rare phenomena, with only a few lucky examples ever noted.
If there are many examples, it would have significant cosmological implications.

They are relatively rare because they are transient but at the same time they are not difficult to find since all you really do is look for galaxies that look like they are colliding. It takes no special tool beyond a good telescope and no special skill except for the ability to recognize patterns which humans are innately proficient at. Projects like Galaxy Zoo utilize citizen scientists to classify galaxies in large numbers and there are plenty of videos generated by depth maps if you go looking.

[geckzilla]

There are probably fewer galaxies that are pristine than ones that are interacting or have in the past.

How could we recognize a pristine galaxy if we saw it?

[Chris Peterson]

There are probably fewer galaxies that are pristine than ones that are interacting or have in the past.

How could we recognize a pristine galaxy if we saw it?

Good question. It would probably be a spiral, since other forms are typically the result of mergers. But given the right sort of collision, spirals can "heal", and any evidence of a past collision would be subtle at best.

[Ron-Astro Pharmacist]

For anybody curious about the highly controversial hypothesis of entropic gravity you might want to investigate it's Wiki site and links.

http://en.wikipedia.org/wiki/Entropic_gravity

To me today's APOD provides some mental imagery for that process. The idea of two galaxies colliding initiates, in my mind, a loss of organized structure to a temporarily more random state that might spur interest for those motivated to examine such a concept. It's certainly not mainstream but when Benjamin Schumacher made it part of his course on gravity "Black Holes, Tides and Curved Spacetime: Understanding Gravity" I might think it warranted to bring up here in light of today's subject matter. (Noting Schumacher also stated it as an interesting notion with a dubious state of acceptance)

Hope it doesn't take 100 million years to get to the bottom of gravity.

[Anthony Barreiro]

There are probably fewer galaxies that are pristine than ones that are interacting or have in the past.

How could we recognize a pristine galaxy if we saw it?

Good question. It would probably be a spiral, since other forms are typically the result of mergers. But given the right sort of collision, spirals can "heal", and any evidence of a past collision would be subtle at best.

Would we have a better chance of seeing pristine galaxies at higher redshifts, since they would be younger and would have had less time to crash into one another? (Hmm, younger galaxies send us very old light. Thank God for verb tenses!)

[Chris Peterson]

Would we have a better chance of seeing pristine galaxies at higher redshifts, since they would be younger and would have had less time to crash into one another?

Seems reasonable to me.

[NGC3314]

Responding to a few questions that have come up -

When galaxies pass close enough to exchange enough energy, they become bound even if they weren't before (although in today's Universe, statistics indicate that most interacting and merging galaxies were already in weakly bound pairs). The exchange continues, so each approach makes the relative orbit tighter on a "death spiral". The energy lost from the galaxies' mutual orbit goes into a relatively small fraction of their particles (stars, gas clouds, and simulations say dark matter too) which are thrown out as tidal tails (star streams), some of which can become unbound and never return. So after a few passages the galaxies' main bodies interpenetrate and you get a complex thing like NGC 6240.

An interesting feature of this pair is the corkscrew shape of the dust lane passing in front of NGC 6622 - this and a couple of other examples (like NGC 1409/1410) suggest that this marks such a strong gravitational influence for the companion that gas may be transferred into the smaller galaxy and stay. Also, there is an enormous blue clump of star-forming regions right in between, just about where the combined gravity of the galaxies reduces the gradient in velocity so that really massive gas clouds could collect.

Ahh, let me see whether this will fit - here's an old attempt to fit a model to the structure of this pair, with a face-on time sequence of snapshots for comparison.

(I love what Martin Pugh has done with these data in bringing out all that detail at once).

[Chris Peterson]

When galaxies pass close enough to exchange enough energy, they become bound even if they weren't before (although in today's Universe, statistics indicate that most interacting and merging galaxies were already in weakly bound pairs).

Two bodies that are not bound (that is, in closed orbits about each other) can't become so without interacting with a third body. It doesn't matter how closely they pass. Of course, a galaxy isn't exactly a "body", but in a collision there will always be a substantial amount of material lost (ejected at greater than the escape velocity of the pair) if the two were not already in closed orbits before that collision.

[Beyond]

In geckzilla's 3-D video, I noticed that all the red-shifted galaxies disappear just after about halfway through the video. How come? Is that the limit of distance to be able to tell which direction the galaxies are going?

[SouthEastAsia]

Any educated guesses or assessment as to whether Massive Nearby Spiral Galaxy NGC 2841, e.g., (see April 21 2014 APOD) could be an exceptional massive 'Pristine' Galaxy? Or a colalesced end-product of at least one previous galactic collision??

[Ann]

Any educated guesses or assessment as to whether Massive Nearby Spiral Galaxy NGC 2841, e.g., (see April 21 2014 APOD) could be an exceptional massive 'Pristine' Galaxy? Or a colalesced end-product of at least one previous galactic collision??

Interesting, SoutheastAsia. When I checked NGC 2841 with my software, I could find no other galaxies that were obviously close to it.

NGC 2841 looks very undisturbed. From my complete amateur perspective, I'd say that this galaxy has remained undisturbed for a long time. Of course, I wouldn't know how to begin to guess if it has never had any encounters or mergers at all.

Ann

[Ann]

NGC 3314 wrote:
An interesting feature of this pair is the corkscrew shape of the dust lane passing in front of NGC 6622 - this and a couple of other examples (like NGC 1409/1410) suggest that this marks such a strong gravitational influence for the companion that gas may be transferred into the smaller galaxy and stay.

So which of these galaxies is the "main one" and which one is the companion?

NGC 6622 on the left looks smaller, but it has a large yellow bar. I wouldn't expect a "small" galaxy to have a large and bright yellow bar. Interestingly, too, NGC 6622 appears to lack star formation, since (almost) all the "blue clumps" here seem to be associated with NGC 6621, or with the "point of contact" between these two galaxies. If gas is being transferred from NGC 6621 into NGC 6622, doesn't that suggest that NGC 6622 is the more massive galaxy?

Also, there is an enormous blue clump of star-forming regions right in between, just about where the combined gravity of the galaxies reduces the gradient in velocity so that really massive gas clouds could collect.

Indeed, very fascinating. There is a smaller but also obvious blue clump seen in a dusty arm passing in front of the bulge of NGC 6621. The arms of NGC 6621 are generally rather blue, in stark contrast to the "arm" of NGC 6622. Or maybe that isn't an arm, but perhaps a central dust lane? Is it possible that NGC 6622 used to be an S0 galaxy with a dust lane, slightly like NGC 7814, before it got strongly disturbed by its interaction with NGC 6621? Perhaps, as is the case of the Antennae galaxies, this is a collision between one gas-rich and one gas-poor galaxy? There appears to be considerably less overall star formation in Arp 81 than in the Antennae galaxies, however.

Ann