APOD: Galaxies in the River (2018 May 10)

[APOD Robot]

Galaxies in the River

Explanation: Large galaxies grow by eating small ones. Even our own galaxy practices galactic cannibalism, absorbing small galaxies that get too close and are captured by the Milky Way's gravity. In fact, the practice is common in the universe and illustrated by this striking pair of interacting galaxies from the banks of the southern constellation Eridanus, The River. Located over 50 million light years away, the large, distorted spiral NGC 1532 is seen locked in a gravitational struggle with dwarf galaxy NGC 1531 (right of center), a struggle the smaller galaxy will eventually lose. Seen edge-on, spiral NGC 1532 spans about 100,000 light-years. Nicely detailed in this sharp image, the NGC 1532/1531 pair is thought to be similar to the well-studied system of face-on spiral and small companion known as M51.

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[Ann]

I'm always glad to see a galaxy picture, and NGC 1532 is an interesting and rarely photographed "island universe" in the process of digesting a really large snack!

I will hopefully comment more later, when I have more time.

Ann

[robotwisdom]

There was a recent result claiming all galaxies rotate approximately once per billion years. So no galaxy has rotatedmore than 15 times. So if they're busy colliding and devouring, those timescales must be much quicker than the spiraling... which seems intuitively wrong to me. How do the spirals stay so organized when most of their velocity is in another direction?

[Boomer12k]

There was a recent result claiming all galaxies rotate approximately once per billion years. So no galaxy has rotatedmore than 15 times. So if they're busy colliding and devouring, those timescales must be much quicker than the spiraling... which seems intuitively wrong to me. How do the spirals stay so organized when most of their velocity is in another direction?

I believe that "result" to be incorrect as the Milky Way completes a rotation once every 200-250 million years, or so, depending on source... thus 4-5 in a billion...depending on the distance from the center...
https://en.wikipedia.org/wiki/Milky_Way

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[Boomer12k]

Interesting side on view of a Merger...maybe it is not "Cannibalism"...maybe it is "Oneness"...Cosmic Zen...

:---[===] *

[smitty]

Question: if two galaxies -- each of which has a black hole at its center -- merge, what is current thinking about the fate of the black hole at the center of the smaller of the two galaxies? Where does it end up? And what havoc does it cause over the course of the merger?

[Chris Peterson]

Question: if two galaxies -- each of which has a black hole at its center -- merge, what is current thinking about the fate of the black hole at the center of the smaller of the two galaxies? Where does it end up? And what havoc does it cause over the course of the merger?

The central black holes orbit one another for a long time, but slowly come together (through the dynamical friction process under discussion a day or two ago in another thread here) and merge into a single, more massive black hole. Large black holes' regions of significant gravitational influence (where they might cause "havoc") are pretty small compared to the size of a galaxy, so most of the merging galaxies is probably not severely impacted. Of course, when you mix things up you increase the possibility of one or both black holes becoming active, and they can produce huge jets of charged particles. If those are tipped near the plane of either galaxy, there could be large regions which would become hostile to life.

[Chris Peterson]

There was a recent result claiming all galaxies rotate approximately once per billion years. So no galaxy has rotatedmore than 15 times. So if they're busy colliding and devouring, those timescales must be much quicker than the spiraling... which seems intuitively wrong to me. How do the spirals stay so organized when most of their velocity is in another direction?

The spirals of spiral galaxies are not directly produced by rotation. The spiral hasn't been produced by "winding up" the entire structure as it rotates. Spiral galaxies are apparently quite stable, and maintain their structure, until such time as they encounter another galaxy close enough to be tidally disrupted, after which they lose much of their spiral structure, often becoming an irregular or elliptical galaxy. Galaxy collisions tend to occur inside fairly dense galaxy clusters, but a great many galaxies are either isolated or in loose clusters, and the Universe isn't old enough for such galaxies to have come close to other galaxies yet.

(As Boomer notes, a rotation period on the order of 100 million years is probably a better estimate for spirals than a billion years... but that isn't relevant to the issue of spiral structure or collision frequency.)

[heehaw]

Notice how blind we are to the dark matter, and how dazzled we are by the bright matter. What happens with galaxy mergers is largely dependent on the masses that we cannot see!

[Chris Peterson]

Notice how blind we are to the dark matter, and how dazzled we are by the bright matter. What happens with galaxy mergers is largely dependent on the masses that we cannot see!

Imagine what the Universe would look like if we could see gravity. Talk about dazzling! (Of course, we are getting good at using gravity to make dark matter visible in images, just like we can make x-rays visible.)

[neufer]

Notice how blind we are to the dark matter, and how dazzled we are by the bright matter.
What happens with galaxy mergers is largely dependent on the masses that we cannot see!

Imagine what the Universe would look like if we could see gravity.

God made iron cannonballs to see gravity & iron filings to see magnetism:

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Newton's 'Treatise of the System of the World'

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[MarkBour]

Newton's 'Treatise of the System of the World'

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I've always liked that diagram. As a person who has learned just enough physics to be a danger to myself and others ... it occurred to me it would be fun to calculate just what speed it would take to get exactly once around the Earth and to land at your feet, say, just 1 foot behind you. (We must be precise ... we don't want the cannonball to hit us in the back of the head, or even in the derriere.)

So, if you fired an 8 kg cannonball at 30 degrees up from horizontal (let's just fire it from the ground, not a tower) and you imparted to it a velocity of 500 m/s, it would arc up and come back down, landing about 10,000 m away. (That's from real experimental data http://www.desertrats.org.uk/equipartillery.htm.)

Starting with a "no-air-resistance" approach, there is a nice calculator for this at: http://www.convertalot.com/ballistic_tr ... lator.html. Playing around with this, I get to 21.306 km/s muzzle velocity, at 30 deg elevation, would land a distance of 40,075 km away, which is the circumference of the Earth.

Fun enough to that point. What will be the effect if I could make a calculator that included air resistance? Obviously, a cannonball shot at 21 km/s would not make it around the Earth once. Air resistance would slow it down. But by how much? And if we were to try to overcome that, by putting larger initial velocities into this hypothetical calculator, how high would we have to go to overcome air resistance and then get a range of 40,075 km? It is possible that no velocity will work no matter how high, because air resistance apparently grows as the square of a projectile's velocity. And actually, there appears to only be a certain range of velocities within which this quadratic relation holds, so I don't know what goes on at super-high velocities (other than immense heating).

A nice calculator that includes air resistance and gives its result with graphical output is at: http://dynref.engr.illinois.edu/afp.html.

None of the above discussion actually takes into account Newton's point in his cannonball thought experiment, that the Earth is curved, not flat. This of course helps, and as Newton pointed out from the diagram, it can produce infinite-length flight paths. However, if a projectile does not get out of the atmosphere, the drag from the air will defeat this. In his thought experiment, the projectile keeps its speed, whereas within the atmosphere, this would not occur.

[Chris Peterson]

Fun enough to that point. What will be the effect if I could make a calculator that included air resistance? Obviously, a cannonball shot at 21 km/s would not make it around the Earth once. Air resistance would slow it down. But by how much? And if we were to try to overcome that, by putting larger initial velocities into this hypothetical calculator, how high would we have to go to overcome air resistance and then get a range of 40,075 km? It is possible that no velocity will work no matter how high, because air resistance apparently grows as the square of a projectile's velocity. And actually, there appears to only be a certain range of velocities within which this quadratic relation holds, so I don't know what goes on at super-high velocities (other than immense heating).

The immense heating is your fundamental problem if you want to make the problem realistic. Closely related is the material strength of the cannonball. 8 km/s is your minimum speed without air. With it, you'd need hugely higher, and that would translate to high enough ram pressure to fragment any practical material (indeed, maybe even any theoretically possible material) and enough heat to ablate any material away in probably no more than a few hundred kilometers of travel. You're basically making a meteor. It would probably break up or ablate before it even finished accelerating to the necessary speed to overcome air resistance.

[neufer]

As a person who has learned just enough physics to be a danger to myself and others ... it occurred to me it would be fun to calculate just what speed it would take to get exactly once around the Earth and to land at your feet, say, just 1 foot behind you. (We must be precise ... we don't want the cannonball to hit us in the back of the head, or even in the derriere.)

So, if you fired an 8 kg cannonball at 30 degrees up from horizontal (let's just fire it from the ground, not a tower) and you imparted to it a velocity of 500 m/s, it would arc up and come back down, landing about 10,000 m away. (That's from real experimental data http://www.desertrats.org.uk/equipartillery.htm.)

Starting with a "no-air-resistance" approach, there is a nice calculator for this at: http://www.convertalot.com/ballistic_tr ... lator.html. Playing around with this, I get to 21.306 km/s muzzle velocity, at 30 deg elevation, would land a distance of 40,075 km away, which is the circumference of the Earth.

Your nice calculator at: http://www.convertalot.com/ballistic_tr ... lator.html is also a "Flat Earth" approach.
(The Earth's escape velocity of 11.186 km/s would make your 21.306 km/s cannonball disappear for good.)

A realistic "no-air-resistance" approach would produce Keplerian ellipses that may or may not intersect the Earth:

No Keplerian ellipses would land at your feet just 1 foot behind you (for that would require a spiral).

All Keplerian ellipses which do not hit the Earth first
(requiring 8 to 11 km/s muzzle velocities, at 0 deg elevation fired from the N or S pole)
would hit you, more or less, in the back of the head.

[Mr Wiffle Ball]

... it occurred to me it would be fun to calculate just what speed it would take to get exactly once around the Earth and to land at your feet, say, just 1 foot behind you. (We must be precise ... we don't want the cannonball to hit us in the back of the head, or even in the derriere.)

So if you're worried about getting clobbered by your cannonball then it would have to land well short of you to come to a rolling stop 1 foot away.

[MarkBour]

Chris, Art, and "Wiffle" --

Thanks for indulging in my sub-problem of Newton's thought experiment, however naively flawed it was, and for the interesting points you added to consider. I also forgot to consider the rotation of the Earth during the flight.

I am tempted to continue ... For example, one could instead consider a rocket launch, but still have the flight become "external ballistic", after a brief launch phase. Another possible modification would be to try it on our Moon, to avoid an atmosphere.