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Questions tagged [galaxies]

Galaxies are gravitationally bound systems of stars, interstellar gas and dark matter, often hosting a central supermassive black hole. For questions about the structure, composition, dynamics, classification, etc. of galaxies. This includes small systems of interacting galaxies (i.e. merger, or galaxy + satellite system), but for galaxy groups and clusters use [galaxy-clusters], and for questions specific to our own Milky Way galaxy use [milky-way].

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Could dark matter be radiation pressure? [closed]

I watched Stephen Wolfram on a podcast explaining this idea last night. He said that dark matter could be explained as "spacetime heat". My contention is that stars and black holes trap a ...
Eschaton Magazine's user avatar
2 votes
1 answer
86 views

Are we certain of the mass we calculate for supermassive black holes?

If astronomers have concluded that the rotation speed of objects held firmly within a galaxy but far from a galactic center is too great to be explained by the visible matter of the galaxy alone, what ...
mdswartz's user avatar
0 votes
1 answer
25 views

What is emission line ratio?

As the header stated, what exactly is the 'emission line ratio'? Like, [O III]/Hβ or [Ne V]/[Ne II]. Recently I've been reading some research papers in astronomy and astrophysics pertaining to ...
ZenithalizeSquads's user avatar
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0 answers
25 views

What is the difference between the virial radius, the total mass-energy density radius and the critical density radius for a galaxy?

For a regular spiral galaxy like the Milky Way, how are these concepts different from each other?
Saminul Haque's user avatar
1 vote
0 answers
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Galaxy harassment; flyby encounter (tidal interaction)

This following image is from the paper https://adsabs.harvard.edu/pdf/1978AJ.....83..219R#page=4 It shows the path of a flyby encounter of NGC 3627 (M66) with the galaxy NGC 3628 (the Hamburger galaxy)...
Hey's user avatar
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4 votes
1 answer
384 views

What are the odds of a rogue planet that enters into a galaxy reaching the black hole at the center of the galaxy?

I am wondering if anybody has ever calculated the odds of a rogue planet, which has been traveling through interstellar space and then enters into a galaxy, being able to travel all the way to the ...
user57467's user avatar
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0 answers
27 views

What will happen to the Milky Ways nucleus when Andromeda collides with the Milky Way?

Andromeda will collide with the Milky Way in about 4.5 billion years. What I want to know is, will the Milky Ways nucleus reactivate when we collide with Andromeda, will it combine with Andromeda's ...
Kellan Heerdegen's user avatar
4 votes
1 answer
139 views

Quasars to a galaxy

Do quasars collapse and make a galaxy or do they eventually spew out enough matter to make one? Because I heard they are the center of galaxies, and I wonder if they can collapse from their own ...
Kellan Heerdegen's user avatar
0 votes
0 answers
28 views

New models for extreme scales? [duplicate]

Physicists struggling to explain too-fast spinning galaxies with standard models of gravity is weird to me. If we can accept that normal physics don't apply on a quantum level why shouldn't the same ...
Fayjakin's user avatar
1 vote
1 answer
125 views

What if gravitation was the only force? [closed]

This is a follow up of Interactions within constituents of dark matter . I wonder about dark matter, and, naturally, compare it with our observable world. If gravitation would be the only force acting ...
Gyro Gearloose's user avatar
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0 answers
99 views

Movement of galaxies vs expansion of space

I can readily accept the theory that the universe is expanding as a mathematical model to explain the fact that all galaxies are moving away from each other, but I have difficulty understanding ...
Ruye's user avatar
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0 answers
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Broad and narrow line regions of active galaxies: is the matter orbiting a massive black hole in the state of plasma?

The broad and narrow line regions (BLR and NLR) of an active galactic nucleus (AGN) are commonly described as regions of gas emitting atomic spectral lines whose width is broadened - via the Doppler ...
cosimoNigro's user avatar
6 votes
2 answers
2k views

Attraction between dark matter

Why does dark matter not attract other dark matter in space, and form a giant structure?
Super Vision's user avatar
2 votes
0 answers
30 views

Does the accelerated expansion of the universe have any effects in the orbital precession of galaxies? And in their eccentricity?

I found several papers (https://iopscience.iop.org/article/10.3847/2041-8213/ace90b; https://link.springer.com/article/10.1134/S1063772920100054; https://arxiv.org/abs/gr-qc/0301057; https://arxiv.org/...
vengaq's user avatar
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1 vote
1 answer
91 views

Dimensionless value to differentiate between concretated and dispersed mass systems

I want to find a dimensionless value that differentiates between concentrated mass systems such as the solar system and dispersed mass systems such as a galaxy. The only value I can think of is the ...
Manuel's user avatar
  • 476
-1 votes
3 answers
155 views

Mass multiplied by distance in exponential disk (galaxies)

I model an exponential disk resembling a galaxy with mass density: $\rho(r) = \rho_0 e^{(-r/h)}$ with $r$ the distance to the galatic center, $h$ the scale length of the galaxy and $\rho_0$ the ...
Manuel's user avatar
  • 476
0 votes
0 answers
43 views

How can I calculate the column density of Hydrogen of a galaxy?

I have the following data: Mass of HI of the galaxy and radius. I thought using the following equation: $N_{HI} = \int n_{HI} ds \quad$ where the number density would be $n_{HI} = \frac{M_{HI}}{m_H V}$...
MJ_'s user avatar
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1 vote
0 answers
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Dark energy, bound systems and orbits...?

As far as I understand it, dark energy can affect bound systems at cosmological scales (How does dark energy affect the dynamics of galaxy clusters?) effectively modifying their orbits. This ...
vengaq's user avatar
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1 vote
0 answers
35 views

Does Dark Energy contribute to increase the isothermal temperature of plasma in galaxy clusters?

I have a question about this work called "Dark energy and key physical parameters of clusters of galaxies" There, towards the end, the authors talk about the isothermal velocities and ...
vengaq's user avatar
  • 2,466
0 votes
2 answers
180 views

How much kinetic energy would a star in a galaxy have if it fell to the center?

I want to calculate the speed, or equivalently, the kinetic energy of a star, if it had no rotational speed and fell from a given radius to the center of the galaxy. I assume Newton's shell theorem ...
Manuel's user avatar
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0 answers
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Can dark matter be isolated from baryonic matter?

The above is an image to test Verlinde's emergent gravity theory (2016, https://arxiv.org/abs/1611.02269). The research team observered galaxies and masses beyond, used gravitational lensing (y-axis) ...
Koen de Jong's user avatar
-1 votes
1 answer
152 views

Which of Newton's shell theorems applies to a galaxy?

From this question I gathered that Newton came up with two sets of shell theorems, one for hollow spheres and one for solid. It was also said we should use the version inside a solid sphere to model ...
Livid's user avatar
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0 votes
1 answer
74 views

Size and boundary of the Milky Way Galactic disk

I have been pondering a question that arose while I was reading a research paper that mentions galactic disk stars have been found up to distances as far as 25 kpc from the galactic center and ...
HDhaliwal's user avatar
0 votes
0 answers
117 views

Is the rotational speed of stars at the outside of galaxies larger than their Newtonian escape velocity?

Stars at the outer edge of galaxies orbit faster than expected from Newtonian gravity, if no dark matter is assumed. Does this mean that the stars orbit so rapidly that their orbital speed is even ...
KlausK's user avatar
  • 727
1 vote
0 answers
44 views

Effects of dark energy in the kinetic energy of a body?

If I launch a ball into the sky it would reach a distance after which it would return into the ground transforming the potential energy into kinetic energy as it hits the ground This is similar to ...
vengaq's user avatar
  • 2,466
0 votes
0 answers
31 views

Expected Rotation Curves

As you know, it is expected that the velocity of stars and gas should slow down the further they are from a galaxy's center. However, in many cases, it does not slow down as expected, and this ...
mahsum's user avatar
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0 votes
0 answers
29 views

Difference between thermodynamical, statistical, and dynamical equilibrium

I have two related questions concerning the difference between thermodynamic, statistical mechanical, and dynamical equilibrium. In particular, I am thinking about the statistical physics of galaxies, ...
FriedBarking's user avatar
1 vote
1 answer
74 views

Tidal effects of galaxies orbiting one another in presence of dark energy?

I recently asked this question about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers ...
vengaq's user avatar
  • 2,466
1 vote
2 answers
171 views

How can I estimate the dark matter density for some specific galaxies by using the rotation curves?

I have rotation curve data (radius vs rotational velocity) for some specific galaxies. How can I estimate the dark matter density for those specific galaxies by using the rotation curves? Or is there ...
mahsum's user avatar
  • 19
0 votes
2 answers
115 views

Not an "intelligent design" question: How do galaxies collide given the Big Bang? [duplicate]

If all matter began from one infinitesimally small point, and flew outward from there. How can we have galaxies colliding? Did they make left hand turns or something? Or it is possible multiple ...
David Raymer's user avatar
2 votes
5 answers
1k views

Why is it that the further a galaxy is, the greater is its recessional velocity?

The exam question is: Explain how red-shift provides evidence for the Big Bang theory. One of the points in the answer is: the further away the galaxy is, the greater is their recessional speed ...
Radhi's user avatar
  • 21
7 votes
2 answers
2k views

Why are there no enormous galaxies near us?

Recently the James Webb space telescope detected six massive ancient galaxies. They are very old and very far away. But these galaxies must still exist today and be even heavier now. Why can't we see ...
Tony Häger's user avatar
1 vote
1 answer
54 views

What is the rough distribution for number of galaxies with a given mass in the observable universe?

Is there a rough formula for the fraction of galaxies in the observable universe with masses between M and M + dM? Or perhaps a graph that displays the same information? I've looked online but can't ...
Thanos's user avatar
  • 419
1 vote
2 answers
341 views

Gravitational potential energy of a galaxy

How can the total gravitational potential energy of a galaxy be calculated? Lets assume for simplicity that the entire galaxy follows an exponential mass density function for an infinitely small ...
Manuel's user avatar
  • 476
1 vote
0 answers
82 views

The 'core-cusp' problem for dark matter halos in larger galaxies

TLDR: Do observations of larger galaxies favour 'cuspy' dark matter halo distributions, as predicted by N-body simulations? I've been trying to understand the 'core-cusp' problem for dark matter halos ...
H-QM-W's user avatar
  • 11
0 votes
0 answers
31 views

Minimum angular size galaxy

I've heard a mention in a lecture that galaxies have a Minimum angular size. Naively, the angular size should drop as $1/d^2$. The effect is supposed to stem from the expansion of the universe. What ...
Rd Basha's user avatar
  • 2,141
1 vote
1 answer
38 views

Why do spiral arms occur at potential minima?

I've been learning about the density wave theory of spiral arms, and also how the gravitational potential of galaxies is non-axisymmetric, resulting in a sinusoidal spiral potential. I've then learnt ...
user374355's user avatar
0 votes
2 answers
394 views

How does the conservation of angular momentum contribute to the flattening of a galaxies shape?

I don't understand this but put down my best attempt at understanding why down below. After it I've included what chatgpt said about the matter. OK I understand why angular momentum is significant ...
Alright Then's user avatar
1 vote
1 answer
34 views

Seyfert Galaxies: How does this statistical deduction about the age of their nucleus make sense?

As per this book, An Introduction to Active Galactic Nuclei by Bradley Peterson: The nuclear emission must last more than $10^8$ years, because Seyfert galaxies constitute about 1 in 100 spiral ...
Arihant's user avatar
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0 votes
0 answers
40 views

Pressure and velocity dispersion

I’m reading this paper whose Eq(2) I’m a little concerned about. Sanders has tried to relate the pressure with the velocity dispersion of particles using the relation $P= \rho\sigma^2$ where $\sigma$ ...
Ambica Govind's user avatar
3 votes
1 answer
106 views

What happens to objects along spiral galaxy arms over long periods of time?

Observations of spiral galaxies reveal that objects within the same arm of a spiral galaxy move at around the same speeds, regardless of their distance from the center of the galaxy. Conversely, the ...
geoscience123's user avatar
1 vote
0 answers
64 views

Three-body problem with multiple systems?

The three-body problem has been known for a long time (https://www.spacedaily.com/m/reports/On_chaos_drunks_and_a_solution_to_the_chaotic_three_body_problem_999.html), in which two celestial bodies ...
vengaq's user avatar
  • 2,466
0 votes
1 answer
92 views

Can time in space be years faster than on Earth? [closed]

Is it possible that in some distant solar system from another galaxy, time will be dramatically different from our Earth time, with years passing far faster than on Earth time?
Omar Kashabash's user avatar
0 votes
1 answer
126 views

How would a Supermassive-black-hole-less galaxy behave?

The Supermassive black hole in the center of the galaxy only contains a tiny fraction of the mass of our Galaxy, so it has minimal direct effect on the orbits of most stars. However I have sort of one ...
blademan9999's user avatar
  • 2,908
0 votes
0 answers
44 views

If time stop at Sagittarius A event horizon, can time restart the other way inside? [duplicate]

From our point of view in our Galaxy, his center, Sagittarius A, is a super massive black hole. At event horizon of this singularity, gravity stop time. Is there any reason why time would not run in ...
Lunix Lunix's user avatar
1 vote
0 answers
48 views

How do we know that we will collide with Andromeda? [duplicate]

We know that Andromeda galaxy is heading towards the Milky Way. But how do we know that Andromeda doesn't have a large Transverse velocity? This would cause the Milky Way and Andromeda galaxies to ...
blademan9999's user avatar
  • 2,908
2 votes
1 answer
182 views

Stopping galaxies' rotation...?

I have been told that galaxies will never stop rotating because conservation of angular momentum But, there are planets inside of it can travel through dense nebulae and bodies of gas that would cause ...
vengaq's user avatar
  • 2,466
1 vote
0 answers
20 views

How long would take for other galaxies to be unobservable due to Cosmic Expansion?

I heard that if Universal Expansion continues at some point galaxies will be so separated that a future civilization would have no way to know there are other galaxies, for them the Galaxy they live ...
Daniel's user avatar
  • 111
0 votes
0 answers
23 views

How much gravitational lensing do we see from the Milky Way?

I assume that the Milky Way has a dark matter halo just like any other. If that is the case, if we look at a huge part of our own galaxy, do we actually see the gravitational lensing effect? How ...
Antoniou's user avatar
  • 495
0 votes
1 answer
37 views

Difference between star formation rate and star formation history

When we speak about galaxies evolution, what is the difference between the star formation rate and the star formation history?
Daniele Zambetti's user avatar

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