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Questions tagged [galactic-dynamics]

Questions regarding interactions between galaxies or between major parts of a galaxy.

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52 votes
2 answers
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Two species of dark matter?

At this point in time, evidence for the existence of dark matter has accumulated in many ways: it affects galactic rotation curves plays a major role in cosmology, and the evolution of structure in ...
Alexey Bobrick's user avatar
29 votes
1 answer
3k views

When will the number of stars be a maximum?

There are very roughly a "mole" of stars in the universe. Wikipedia quotes an estimate of $3 \times 10^{23}$ though the number is associated with some debate and uncertainly. I'd like to know if ...
uhoh's user avatar
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19 votes
2 answers
2k views

Questions about spiral galaxy arms

I have 2 questions related to spiral galaxies. Firstly, how did the arms form? Why would the stars accumulate into those specific areas? And secondly, why are they still intact? The inner parts of ...
stanley dodds's user avatar
17 votes
2 answers
4k views

Where was the solar system born? Alternatively, how far back in time can we retrodict the location of the solar system?

I have two related questions: Where in the Milky Way did the solar system form? Is there a particular nebula it can be traced to? How far back in time can we track the location of the solar system ...
Tim Campion's user avatar
13 votes
2 answers
3k views

Is Dark Matter possible if there is dynamical friction?

If dark matter existed: wouldn't it slow down the orbital velocity of stars in galactic disks by dynamical friction more than it would accelerate them through additional mass? The original orbital ...
Reggie Grünenberg's user avatar
12 votes
1 answer
453 views

Has the great Andromeda Galaxy ever collided with any galaxies?

We know that the Andromeda Galaxy is moving towards the Milky Way and vice versa, so there will be a collision. Because of the vast space between them, the stars as they are defined will not ...
Isopycnal Oscillation's user avatar
12 votes
5 answers
6k views

What mechanism causes oscillations of the solar system's orbit about the galactic plane?

In a recent paper (news release here) Lisa Randall and Matthew Reece propose that a dark matter disk coinciding with the galactic plane together with the solar system's oscillations through the ...
Chris Mueller's user avatar
11 votes
2 answers
549 views

Are there a lot of collisions between stars in the core of the galaxy?

I'm reading on Wikipedia about Halo Stars that orbit the center of the galaxy at a high inclination away from the plane of the Milky Way. It seems that at some point, these stars must dive back into ...
Kevin Holmes's user avatar
11 votes
1 answer
319 views

Dark Flow: statistical limits on existence

Dark Flow has been thrown around as a term a lot lately, especially with the latest Planck results. New Scientist reported last year that it was now statistically improbable, but the paper by Atrio ...
christopherlovell's user avatar
10 votes
3 answers
2k views

How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?

I just stumbled across the following answer "update" from 2013: This just in, recent observations have ruled out the possibility that SMBHs gain mass only through merging with other black ...
uhoh's user avatar
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10 votes
2 answers
2k views

What (actually) is the " deprojected half-light radius" of this almost-all-dark-matter Galaxy?

The recent news of the Ultra Diffuse Galaxy (UDG) Dragonfly 44 is an excellent example of what could be termed 'observe different' thinking. The dragonfly telescope is noted not for the size of its ...
uhoh's user avatar
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9 votes
3 answers
2k views

Are there some (simple) mathematical models which can simulate the cellular/web structure of the universe?

To visualize the structure of a solar system we have some simple mathematical model: Planets orbiting around a Star in circular motion. If we want to be more precise the circle becomes a ellipse, the ...
J. Doe's user avatar
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9 votes
1 answer
235 views

Half of the Milky Way's baryonic mass is from other galaxies - when did this happen?

The recent Phys.org article Milky Way's origins are not what they seem explains the recent MNRAS article Anglés-Alcázar et al. 2017; The cosmic baryon cycle and galaxy mass assembly in the FIRE ...
uhoh's user avatar
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9 votes
1 answer
136 views

Do we live in a galactic bubble?

I have heard a certain statement, which can be summarized succinctly into "we live in a local bubble", implying that solar system is located in an intestrellar dust underdensity region. This region ...
Alexey Bobrick's user avatar
8 votes
1 answer
298 views

Have we discovered a Galaxy lacking in Dark matter?

This question is relevant but it was speaking about a specific discovery reported by pieter van Dokkum, and the answer then mentioned it was disputed whether the paper's conclusion is even correct. I ...
Hisham's user avatar
  • 231
8 votes
1 answer
435 views

What is the characteristic time of the evaporation of the galaxies?

For a star, to leave its galaxy, it requires probably a 3-body near-collision at the edge of the galaxy. It is unlikely, but possible. And, if a star once somehow got the required velocity to escape ...
peterh's user avatar
  • 3,169
8 votes
2 answers
152 views

Do the orbits of satellite galaxies show the influence of dark matter?

I understand how galaxy rotation curves show the influence of dark matter, or something else that would produce similar behaviour, such as Modified Newtonian Dynamics (MOND). Do the orbits of ...
John Dallman's user avatar
8 votes
1 answer
693 views

Python 2D Jean Instability in Spiral Galaxy Simulation

I have ~1000 snapshots of a spiral galaxy simulation (from Molecular cloud perturbations t=0 to t>1Gyr. What I need to do is determine whether any position at any of the snapshot times meets criteria ...
voidedbowel's user avatar
7 votes
1 answer
818 views

Why wouldn't an elliptical galaxy collapse back into a spiral galaxy?

I should preface this post with two parts of my tentative (probably wrong) understanding first. First, it's been told to me that spiral galaxies typically form first, and then elliptical galaxies ...
Maximal Ideal's user avatar
7 votes
2 answers
2k views

Could dark energy be negative gravity?

Main question: Could dark energy (the mysterious accelerating expansion of the universe) be explained by "negative gravity"? "Spin off" questions: Does antimatter have negative gravity? If ...
Jonathan's user avatar
  • 4,381
7 votes
1 answer
710 views

Is our central black hole actually at the CG of the galaxy?

It seems fairly certain there is a large black hole, about 25,000 ly from Earth roughly in the center of our galaxy, in the direction of the Sag. A radio source. • In fact, is it thought to be ...
Fattie's user avatar
  • 946
7 votes
2 answers
112 views

Is the Astronomy community still concerned about the lumpyness of matter distribution in the universe?

A decade or so ago, when I was still a science undergrad, one of the open questions in astrophysics was to explain the uneven distribution of galaxies in the observable universe. That is, why did the ...
John F. Miller's user avatar
6 votes
2 answers
790 views

What are the arguments against the Feng and Gallo thin disk explanation of galactic rotation curves?

The well-known galaxy rotation problem is described here. Basically (as one moves outward from the galactic centre, $R$ increasing) the observed pattern of variation in orbital speed $V$ is very ...
steveOw's user avatar
  • 1,444
6 votes
1 answer
492 views

Does the Final-Parsec problem apply to stellar collisions?

The final parsec problem describes the difficulty of two blackholes merging with one another when their distances reach ~3.2 light years apart. Does this also then apply to stellar collisions, perhaps ...
Shawn Lim's user avatar
  • 385
6 votes
1 answer
806 views

Star density in- versus out- of the arms of the galactic pattern?

What is the density difference in our galaxy (or in a typical spiral), at A, B, C versus at a, b, c? Consider the yellow track. Do astronomers have a graph like this... ...which shows the shape of ...
Fattie's user avatar
  • 946
6 votes
0 answers
98 views

How many galaxies are currently merging?

On the homepage of the galaxies research group at CfA SAO, it states, "Astronomers discovered that as many as 25% of galaxies are currently merging with others." They do not provide a ...
Daddy Kropotkin's user avatar
6 votes
0 answers
45 views

Are metallicities of molecular clouds lower in the outskirts of the galaxy?

(this question was originally posted in an answer by user PSR-1937-21 to another post. I find it an interesting one, but since they don't seem to be active anymore, I'm posting it to see if somebody ...
SE - stop firing the good guys's user avatar
5 votes
3 answers
660 views

What happens to galaxies when they die?

Stars explode when they die and blast heavy elements into space. Do galaxies do the same thing?
Michael Lee's user avatar
5 votes
2 answers
3k views

What is the direction of the movement of the solar system in relation to the galaxy's plane

I got from this answer that the axis of the solar system is tilted of about 63 degrees in relation to that of our galaxy, so I can assume that we move through space (at least locally) roughly in a ...
TomCho's user avatar
  • 390
5 votes
2 answers
102 views

What is meant by " elliptical galaxies stabilize due to the chaotic motion of stars"?

I read that elliptical galaxies stabilize due to the chaotic motion of stars. As I understand that means that the stellar motions are more nearly random in direction. So they perform independent ...
trynerror's user avatar
  • 849
5 votes
2 answers
810 views

Why are stars more metallic closer as you move closer to the galactic bulge?

As I see it, most of the stars in the galactic bulge are Population I stars. However, as one moves farther from the galactic bulge, star metallicity decreases. In fact, halo stars are almost entirely ...
Sir Cumference's user avatar
5 votes
1 answer
676 views

Stellar systems: what is the difference between virial, dynamical and thermodynamic equilibrium?

I'm currently going through Binney & Tremaine (2008) on my own to learn about stellar dynamics. I also have been perusing additional online resources such as this scholarpedia wiki. Often when ...
quantumflash's user avatar
5 votes
1 answer
162 views

What parameters determine whether galaxies colliding will result in a merger or a hit and run?

Colliding galaxies sometimes merge and sometimes pass through each other. Either way, there are huge changes as a result. What are the parameters that matter in determining whether a collision will ...
Eubie Drew's user avatar
  • 1,080
5 votes
0 answers
85 views

Did the density wave theory for spiral arms take account of this phenomenon?

I came across this link. In it the author speculates that in the outer region of a galaxy, the spiral arms overtake the stars, and vice versa for the inner region. That means there must be a middle ...
longtry's user avatar
  • 403
4 votes
2 answers
2k views

Leaving the Milky Way

If we had a hypothetical spacecraft, would it be possible to exit our galaxy, wait for some time and enter back into another section? Will this method be more efficient that a more traditional flight ...
theVerma's user avatar
4 votes
1 answer
449 views

What does "unremarkable transverse peculiar velocity" mean exactly, and how is it calculated here?

The abstract of A magnetar parallax (also in MNRAS) contains the following: Combining our new observations with two archival observations from 2006, we have refined the proper motion and reference ...
uhoh's user avatar
  • 31.2k
4 votes
2 answers
332 views

Age of the universe

A Galactic year is estimated to be around 250 million "terrestrial" years. That means there will be about 4 rotations per billion years. And age of the universe is estimated to be near 13.8 billion "...
jorel's user avatar
  • 185
4 votes
2 answers
165 views

How is the potential-density pair relation derived?

How can it be shown that $$\nabla^2\phi = 4\pi G \rho(\vec{r})$$ ?
realanswers's user avatar
4 votes
1 answer
337 views

Distribution of Stars in Milky Way and globular cluster analogy

Just a quick (and hopefully simple) question on the distribution of stars in the Milky Way and in globular clusters. I was wondering why it IS NOT possible to describe the distributions of stars in ...
MichaelJRoberts's user avatar
4 votes
1 answer
316 views

What are typical peculiar velocities?

I'm wondering what sorts actual values we can expect to find for peculiar velocities of individual objects when observing the universe, both typically and on the high end. I'm interested in any ...
Mike's user avatar
  • 142
4 votes
1 answer
990 views

Direction of Earth’s poles relative to the Sun’s movement around the Galaxy

The plane of the solar system is at 63 degrees to the plane of the galaxy, then either the North or South Pole of Earth would be on the ‘leading edge,’ i.e. pointing roughly in the direction of ...
Rrobby's user avatar
  • 41
4 votes
2 answers
148 views

Can the collapse of a gas cloud lead to an elliptical structure?

Is it likely, unlikely, or impossible for an elliptical structure to form when a gas cloud collapses? Due to the conservation of angular momentum, one would expect that disk structures are much more ...
trynerror's user avatar
  • 849
4 votes
1 answer
270 views

How do two supermassive black holes reach "the last parsec" in the case of merging galaxies?

This answer to Why can't supermassive black holes merge? (or can they?) describes the barrier to merging that two supermassive black holes face when two galaxies are in the process of merging or have "...
uhoh's user avatar
  • 31.2k
4 votes
1 answer
182 views

Deriving the first moment of Collisionless Boltzmann Equation in Spherical Polar Coordinates

I am following these notes: Dynamics and Astrophysics of Galaxies. After equation 6.37, we have: \begin{equation*} p_r\,\frac{\partial f}{\partial r} + \frac{p_\theta}{r^2}\,\frac{\partial f}{\...
zabop's user avatar
  • 481
4 votes
0 answers
67 views

What's the angle between the sun's galactic speed and the ecliptic?

I know the sun wobbles up and down the galactic plane around 35myr. I don't mind about that component. I'd like the know the angle between the linear speed of sun around the Milky Way (just the ...
Yuta73's user avatar
  • 141
4 votes
0 answers
92 views

Why do the Centaurus A radio jets have two "arms" each? (Event Horizon Telescope image)

Fig. 2: Image analysis of the final model: a, Model image corresponding to the image from Fig. 1 with a pixel size of 2 μas. Here, the tentative position of the jet apex is indicated with a circle. ...
uhoh's user avatar
  • 31.2k
3 votes
2 answers
2k views

How can the (in my eyes quite ridiculous) conjecture of Sheldrake be disproven? [closed]

In this question on the physics site, it is asked if astronomical objects can be conscious. Now, in a sense, every object has to be "conscious" somehow, as every object is composed of the ...
Deschele Schilder's user avatar
3 votes
1 answer
321 views

Why don't globular clusters flatten with a galactic disc?

Globular clusters lie in the galactic halo, outside of the disc. However, galaxies are more or less a collection of material and objects — why is it, then, that most stars form a plane due to the ...
Sir Cumference's user avatar
3 votes
1 answer
196 views

Supergalactic and Equatorial coordinates

What's the difference between Supergalactic and Equatorial coordinates?
Max's user avatar
  • 31
3 votes
1 answer
82 views

Orphaned star systems and intergalactic travel (in fictional context)

Context: I'm looking for some authority on a particular idea that I brought up in regards to a new update brought out for a space travel sim game, Elite Dangerous The vessels in that game can travel ...
Andrew's user avatar
  • 33