APOD: Atoms for Peace Galaxy Collision (2010 Nov 16)

[APOD Robot]

Atoms for Peace Galaxy Collision

Explanation: Is this what will become of our Milky Way Galaxy? Perhaps if we collide with the Andromeda Galaxy in a few billion years, it might. Pictured above is NGC 7252, a jumble of stars created by a huge collision between two large galaxies. The collision will take hundreds of millions of years and so is effectively caught frozen in time in the above image. The resulting pandemonium has been dubbed the Atoms-for-Peace galaxy because of its similarity to a cartoon of a large atom. The above image was taken recently by the MPG/ESO 2.2 meter telescope in Chile. NGC 7252 spans about 600,000 light years and lies about 220 million light years away toward the constellation of the Water Bearer (Aquarius). Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown, no one really knows for sure if the Milky Way will ever collide with M31.

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[bystander]

ESO: Atoms-for-Peace: a galactic collision in action
http://asterisk.apod.com/vie ... 29&t=21973

[FrogSplash]

And people keep telling me that galaxies colliding is not catastrophic. This picture proves different.

FS

[Chris Peterson]

And people keep telling me that galaxies colliding is not catastrophic. This picture proves different.

It's catastrophic for the galaxies as a whole, in terms of their structure. But there are no star collisions, and for most of the stars it isn't a catastrophic thing at all.

[stowaway]

Amazing! How many galaxies in that picture?

[neufer]

And people keep telling me that galaxies colliding is not catastrophic. This picture proves different.

It's catastrophic for the galaxies as a whole, in terms of their structure. But there are no star collisions, and for most of the stars it isn't a catastrophic thing at all.

Streams of stars orbiting outside the plane of the Milky Way are thought to be the remains of small galaxies that were disrupted by tidal interactions and which are being assimilated into our own. Digital artwork © 2010 Don Dixon / cosmographica.com

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Sloan Digital Star Survey stellar map of the northern sky, showing trails and streams of stars torn from disrupted Milky Way satellites. Insets show new dwarf companions (credit: V. Belokurov).

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This one is not even all that catastrophic for the central galaxy.
NGC 7252 hardly seems annoyed at all by it's satellite galaxies:

• ____ King Lear Act 4, Scene 1
  
  GLOUCESTER: As flies to wanton boys, are we to the gods.
  ____ They kill us for their sport.

In NGC 7252, where we had discovered ~40 globular clusters
with the Hubble before its refurbishment, we have recently
discovered ~500 globulars with the refurbished telescope.
Each of bluish pointlike sources is a likely globular.
Credit: B. Whitmore (STScI)

http://antwrp.gsfc.nasa.gov/apod/ap100911.html
http://antwrp.gsfc.nasa.gov/apod/ap100415.html
http://en.wikipedia.org/wiki/NGC_5466

[orin stepanek]

The simulation link was very beautiful!

Click to play embedded YouTube video.

[orin stepanek]

This link is neat also.

Click to play embedded YouTube video.

[idahogie]

To me, this is the most interesting thing about today's post: "Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown,..."

Can anyone explain that to me in more detail? I assume that our relative motion on the axis between the Milky Way and Andromeda is known. But why is the "sideways velocity" unknown? Plus, it always seems to be presented as a given that the MW and Andromeda will one day collide. Now it's completely unknown whether that will happen? What changed?

[neufer]

To me, this is the most interesting thing about today's post: "Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown,..."

Can anyone explain that to me in more detail? I assume that our relative motion on the axis between the Milky Way and Andromeda is known. But why is the "sideways velocity" unknown? Plus, it always seems to be presented as a given that the MW and Andromeda will one day collide. Now it's completely unknown whether that will happen? What changed?

Andromeda's tangential velocity with respect to the Milky Way is known
but only to within about a factor of about two.

It's a 50/50 proposition so
it depends whether you are a glass half full or half empty type person.

[Chris Peterson]

To me, this is the most interesting thing about today's post: "Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown,..."

Can anyone explain that to me in more detail? I assume that our relative motion on the axis between the Milky Way and Andromeda is known. But why is the "sideways velocity" unknown? Plus, it always seems to be presented as a given that the MW and Andromeda will one day collide. Now it's completely unknown whether that will happen? What changed?

It is easy to measure radial velocity, by looking at the Doppler shift of light. But tangential velocity can only be inferred from indirect methods. We simply haven't observed long enough to see any actual tangential movement.

What this means is that we don't know with any certainty the orbital parameters of the galaxies in the Local Group. We do know that the galaxies are gravitationally bound, which means that all will eventually collide or be ejected from the cluster. But the Milky Way and Andromeda are far too massive to experience ejection from perturbations, so it is certain they will eventually collide. When that will happen is less clear, however.

[bystander]

it depends whether you are a glass half full or half empty type person.

I think the glass is just too big.

[neufer]

It is easy to measure radial velocity, by looking at the Doppler shift of light.
But tangential velocity can only be inferred from indirect methods.

• __ Hamlet, Prince of Denmark Act 2, Scene 1
  
  LORD POLONIUS: And thus do we of wisdom and of reach,
  __ With windlasses and with assays of bias,
  __ By indirections find directions out:
  __ So by my former lecture and advice,
  __ Shall you my son.

We simply haven't observed long enough to see any actual tangential movement.

I.e., Andromeda has made only the most im-proper motions thus far
(; a 100 km/s proper motion for M31 ~ 0.55 mas per 'score of years').

<<[Over five years of operation The High Precision Parallax Collecting Satellite (Hipparcos)] objective was to provide the positions, parallaxes, and annual proper motions for some 100,000 stars with an unprecedented accuracy of 2 mas (milliarcseconds), a target in practice eventually surpassed by a factor of two.

Since 1997, several thousand scientific papers have been published making use of the Hipparcos and Tycho Catalogues. A detailed review of the Hipparcos scientific literature between 1997–2007 was published in 2009. Some examples of notable results include (listed chronologically):

* studies of Galactic rotation from Cepheid variables[14]
* the nature of Delta Scuti variables[15]
* studies of local stellar kinematics[16]
* testing the white dwarf mass-radius relation[17]
* the structure and dynamics of the Hyades cluster[18]
* kinematics of Wolf-Rayet stars and O-type runaway stars[19]
* subdwarf parallaxes: metal-rich clusters and the thick disk[20]
* fine structure of the red giant clump and associated distance determinations[21]
* unexpected stellar velocity distribution in the warped Galactic disk[22]
* refining the Oort and Galactic constants[23]
* Galactic disk dark matter, terrestrial impact cratering and the law of large numbers[24]
* vertical motion and expansion of the Gould Belt[25]
* the use of gamma ray bursts as direction and time markers in SETI strategies[26]
* evidence of a galaxy merger in the early formation history of the Milky Way[27]
* study of nearby OB associations[28]
* close approaches of stars to the Solar System[29]
* studies of binary star orbits and masses[30]
* the HD 209458 planetary transits[31]
* formation of the stellar Galactic halo and thick disk[32]
* the local density of matter in the Galaxy and the Oort limit[33]
* ice age epochs and the Sun's path through the Galaxy[34]
* local kinematics of K and M giants and the concept of superclusters[35]
* an improved reference frame for long-term Earth rotation studies[36]
* the local stellar velocity field in the Galaxy[37]>>

What this means is that we don't know with any certainty the orbital parameters of the galaxies in the Local Group. We do know that the galaxies are gravitationally bound, which means that all will eventually collide or be ejected from the cluster. But the Milky Way and Andromeda are far too massive to experience ejection from perturbations, so it is certain they will eventually collide. When that will happen is less clear, however.

I think we know pretty well the orbital parameters of the galaxies in the Local Group within 1 million light years;
especially, galaxies like the LMC & SMC that leave star stream orbits around the Milky Way.

Streams of stars orbiting outside the plane of the Milky Way are thought to be the remains of small galaxies that were disrupted by tidal interactions and which are being assimilated into our own. Digital artwork © 2010 Don Dixon / cosmographica.com

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It is not at all clear to me that galaxies in the Local Group past 2 million light years
are gravitationally bound with the Milky Way.

Local Group

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[strangerbarry]

Maybe this is a really dumb idea. To estimate the sideways motion of the andromeda galaxy, would it be possible to measure the red shift of light reflected off distant intergalactic dust located at 90 degrees to our own line of sight to Andromeda ?

[neufer]

Maybe this is a really dumb idea. To estimate the sideways motion of the andromeda galaxy, would it be possible to measure the red shift of light reflected off distant intergalactic dust located at 90 degrees to our own line of sight to Andromeda ?

It would be way too dim.

But even if it wasn't ... how would you know what the dust velocity was?

[ZenGrouch]

This is probably a stupid rookie question, but I have to wonder how galaxies manage to collide, when the universe is expanding? Were they propelled in different directions, allowing this interaction, prior to the time they were formed, as the hydrogen atoms were drawn together?

[Chris Peterson]

This is probably a stupid rookie question, but I have to wonder how galaxies manage to collide, when the universe is expanding? Were they propelled in different directions, allowing this interaction, prior to the time they were formed, as the hydrogen atoms were drawn together?

It's a rookie question, but it isn't a stupid one <g>.

The force of gravity is stronger than the "force" of expanding space. Galaxies in clusters have enough gravity to hold space together locally, so the region doesn't expand. It is orbital dynamics, as governed by gravity, that hold galaxy clusters together. For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun.

Presumably, the member galaxies in a cluster all formed together, or nearly so, from material local to their region of the Universe, and have remained bound (sometimes loosely) since.

[strangerbarry]

I believe you're on the right track. There are "local" movements of nearby galaxies in different directions. As well gravitational attraction among local groups of galaxies can create localized areas where gravity acts to overcome cosmic expansion. The expansion applies on cosmic spatial scales with local areas of contraction. Think of a large chocolate chip cookie being baked in an oven where the cookie expands but the chips don't.

[owlice]

[snip] where the cookie expands but the chips don't.

Darn it!!

[ZenGrouch]

This is probably a stupid rookie question, but I have to wonder how galaxies manage to collide, when the universe is expanding? Were they propelled in different directions, allowing this interaction, prior to the time they were formed, as the hydrogen atoms were drawn together?

It's a rookie question, but it isn't a stupid one <g>.

The force of gravity is stronger than the "force" of expanding space. Galaxies in clusters have enough gravity to hold space together locally, so the region doesn't expand. It is orbital dynamics, as governed by gravity, that hold galaxy clusters together. For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun.

Presumably, the member galaxies in a cluster all formed together, or nearly so, from material local to their region of the Universe, and have remained bound (sometimes loosely) since.

Thanks for the explanation.

Unfortunately, your answer raised another 'rookie' question in my mind...

"For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun."

For argument's sake, if this weren't true, and space/time were expanding in our little piece of space, would we even be aware of it, or be able to measure it?

[neufer]

I believe you're on the right track. There are "local" movements of nearby galaxies in different directions. As well gravitational attraction among local groups of galaxies can create localized areas where gravity acts to overcome cosmic expansion. The expansion applies on cosmic spatial scales with local areas of contraction. Think of a large chocolate chip cookie being baked in an oven where the cookie expands but the chips don't.

Of course, unlike leavened bread, cookie dough expands in only two dimensions
while contracting in the third (much like Misner's Mixmaster Universe).

strangerbarr Universe

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WMAP Universe

---

<<The chocolate chip cookie was accidentally developed by Ruth Wakefield in 1930. She owned the Toll House Inn, in Whitman, Massachusetts, a very popular restaurant that featured home cooking in the 1930s. Wakefield is said to have been making chocolate cookies and on running out of regular baker's chocolate, substituted broken pieces of semi-sweet chocolate from Nestlé thinking that it would melt and mix into the batter. It clearly did not and the chocolate chip cookie was born. A different history of the cookie derives from George Boucher, who was at one time head chef at the Toll House Inn. Boucher said that the vibrations from a large Hobart electric mixer dislodged bars of Nestlé's chocolate stored on the shelf above the mixer so they fell into the sugar cookie dough it was mixing, then broke them up and mixed the pieces into it. Boucher claimed to have overcome Wakefield's impulse to discard the dough as too badly ruined to waste effort baking them, leading to the discovery of the popular combination.>>

<<Misner's Mixmaster Universe is a solution to Einstein's field equations of general relativity studied by Charles Misner in an effort to better understand the dynamics of the early universe. He hoped to solve the horizon problem in a natural way by showing that the early universe underwent an oscillatory, chaotic epoch.

The model is similar to the closed Friedmann-Lemaitre-Robertson-Walker universe, in that spatial slices are positively curved and are topologically three-spheres S³. However, in the FRW universe, the S³ can only expand or contract: the only dynamical parameter is overall size of the S³, parameterized by the scale factor a(t). In the Mixmaster universe, the S³ can expand or contract, but also distort anisotropically. Values of three shape parameters describe distortions of the S³ that preserve its volume and also maintain a constant Ricci curvature scalar. Therefore, as the three parameters assume different values, homogeneity but not isotropy is preserved.

Misner showed that the physical universe would expand in some directions and contract in others, with the directions of expansion and contraction changing repeatedly. Because the potential is roughly triangular, Misner suggested that the evolution is chaotic. Misner hoped that the chaos would churn up and smooth out the early universe. Since the directions of expansion and contraction varied, presumably given enough time the horizon problem would get solved in every direction.

While an interesting example of gravitational chaos, it is widely recognized that the cosmological problems the Mixmaster universe attempts to solve are more elegantly tackled by cosmic inflation. The metric Misner studied is also known as the Bianchi type IX metric.>>

[Chris Peterson]

For argument's sake, if this weren't true, and space/time were expanding in our little piece of space, would we even be aware of it, or be able to measure it?

Well, you could imagine a sort of change in the size of the Universe, as measured against some sort of hyper-universe, where the scale of everything changed. That sort of size would be meaningless from inside the Universe, though.

The expansion of the Universe as we recognize it is, in fact, an observable phenomenon, so I don't see why we wouldn't observe it locally if it were occurring. You don't need a ruler (which might also be expanding). We observe the expansion of the Universe by looking at redshifts and standard luminosities, observations that don't depend on a requirement that the local Universe around the observer be non-expanding. If we were getting farther from the Sun, we should observe that by redshift (assuming good enough instruments) even if our rulers were expanding along with everything else.

Indirectly, the local cohesion of space-time by gravity is predicted by General Relativity, and eliminating that component of the theory would alter or invalidate other parts- parts that are extremely well supported by varied observations. That's why most physicists have a high degree of confidence in anything predicted by GR, even where the specific element may have little observational evidence.

[neufer]

The force of gravity is stronger than the "force" of expanding space. Galaxies in clusters have enough gravity to hold space together locally, so the region doesn't expand. It is orbital dynamics, as governed by gravity, that hold galaxy clusters together. For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun.

Thanks for the explanation.

Unfortunately, your answer raised another 'rookie' question in my mind...

"For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun."

For argument's sake, if this weren't true, and space/time were expanding in our little piece of space, would we even be aware of it, or be able to measure it?

Well, for argument's sake, this isn't true,
and space/time is indeed expanding in our little piece of space,

and perhaps we are even able to measure it

[list]Hubble expansion H₀ = 70.6 (km/sec)/Mpc

H₀ = 2,228,000,000,000 (m/yr) / [206,265,000,000 AU]

H₀ = 10.8 (m/yr)/AU

H₀ = 27.6 (mm/yr)/(mean lunar distance)[/list][/color]Laser reflectors on the moon indicate that the Moon is spiraling away from Earth at a rate of 38 mm per year.
Perhaps 28 mm per year is due to Hubble expansion and only 10 mm per year is due to tidal forcing.

[Chris Peterson]

Laser reflectors on the moon indicate that the Moon is spiraling away from Earth at a rate of 38 mm per year.
Perhaps 28 mm per year is due to Hubble expansion and only 10 mm per year is due to tidal forcing.

I believe that the measured 38mm per year is very consistent with our understanding of momentum transfer and does not require any assumption about the expansion of space. I also believe that assuming the expansion of space to be happening between the Earth and Moon breaks GR, which seems to me a very unlikely thing.

[ZenGrouch]

The expansion of the Universe as we recognize it is, in fact, an observable phenomenon, so I don't see why we wouldn't observe it locally if it were occurring.

OK, makes sense, but in my naive thinking, I would think the small scale of our slice of the pie, would make the measurements impossible with our current technology. *I don't know... just thinking out loud, so to speak.*

One more question, regarding time/space and expansion. What are the popular beliefs about this? Is it believed that time/space expand, and if so, at what speed, relative to the matter of the universe. I'd think if they were expanding at the same speed, there would be no observable red shift. And if time/space are expanding at the same, or near, rates,<edit>sorry, meant to say differing rates<end edit> is it possible the expansion could be much greater or smaller than thought, since it would be a matter of relativity.

Sorry if these are thoughts are beyond ignorant...