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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4
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1answer
184 views

Tidal tails of galaxies after collision

When there is a collision of 2 disc shaped galaxies, there is a tail formation created from both the galaxies. I read here that this was due to tidal forces, but I couldn't figure out how this happens....
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1answer
497 views

What is the meaning of 'systemic velocity' of galaxies?

I came across the term systemic velocities of galaxies. Can you please explain what it means?
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1answer
162 views

Formula for the bias of galaxies

From the article "Large-Scale Galaxy Bias", I try to deduce the equation that my teacher told me which links 2 quantities: the global number density of galaxies the local number density of galaxies ...
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3answers
118 views

Schwarzschild Radius of a Galaxy

If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius, how can it be observed from the outside of the Galaxy?
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3answers
3k views

Would a galaxy be visible from outside, but nearby?

We all know the typical sci-fi image of a guy standing on the ship deck and able to see a full galaxy. If you somehow were able to stand a few lightyears away from a galaxy would you be able to see it ...
2
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1answer
104 views

What is the future of the CMBR?

Galaxies farther away than let's say z=2 are beyond our reach. If we today receive a message from such a galaxy, our response message will never reach back to them. So goes the current knowledge. This ...
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2answers
63 views

Can an observer perceive a single source of light as multiple sources because of curved spacetime?

Is it possible that some of the stars and galaxies we observe here on Earth are actually replicates of other stars and galaxies we are observing from different angles because of curved spacetime? Let'...
1
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1answer
84 views

Amount of Information of a Galaxy? [closed]

Just like the title suggests, how much data/information does a galaxy have in (bits)? Like the Milky Way galaxy for example? For example, this is based on the Bekenstein Limit formula where mass can ...
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0answers
26 views

Generally how much faster are stars on the outskirts of galaxies orbiting than they should be?

This assumes there is no mysterious matter causing the extra speed. Are they orbiting say 10% faster than expected etc?
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2answers
82 views

How will Andromeda collide with Milky Way in spite of Hubble's law? [duplicate]

According to Edwin Hubble our Universe is expanding because he noticed that other galaxies are moving away from us. But then how Andromeda galaxy will collide with Milky Way in ~4.5 billion years, as ...
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2answers
162 views

Is the Maffei 1 galaxy gravitationally bound to the Milkway Galaxy?

I watched a Kurzgesagt video explaining how the limit human exploration is the local group, because outside galaxies are accelerating away from us faster than the speed of light, (or faster than we ...
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1answer
37 views

Database of Galactic Mass?

I'm currently working on a paper investigating any relationship that might exist between the mass of the AGN, or more specifically the relativistic mass at its center, and the mass of the host galaxy. ...
2
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0answers
92 views

Spiral galaxy stability

Is there a limit of baryonic masses $M_{B}$ ($M_{B}=M_{\star}+M_{g}$), beyond which a spiral galaxy is no longer rotationally supported? Like for example: Could spiral galaxies of baryonic masses $M_{...
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1answer
278 views

Is the Andromeda Milky Way collision the most likely form of intergalactic space travel?

Assuming that some form of sentient organization gradually spreads within the Milky Way through technology predicated on currently known physics, is the Andromeda-Milky Way collision in 4 billion ...
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1answer
38 views

Are wet mergers good for star formation?

In my astrophysics lecture my professor said wet mergers are bad for star formation since it often results in gas being fed to the black hole. In the wiki however it says the opposite. What is the ...
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1answer
3k views

Difference between Gunn Peterson trough and the Lyman Alpha Forest? Cosmological implications?

I'm having difficulty understanding the full implications of the Lyman alpha forest and its use in cosmology. My understanding is this: we detect features in the Intergalactic Medium (IGM) by very ...
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0answers
30 views

Texts on Hoag's Object

Are there any good texts online that someone can recommend on Hoag's Object? It is a very weird galaxy and I would like to learn more.
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1answer
53 views

Why are galalxies spiral? [duplicate]

In every picture we see that galaxies are spiral, why so? are there any other shapes possible?
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2answers
1k views

Why is the distribution of dark matter in a Galaxy different from the distribution of normal matter?

The dynamics of a galaxy are driven by gravity. And dark matter experiences the same gravitational forces as normal matter. The effects of other forces are very small in comparison. So why is the ...
5
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0answers
153 views

Spatial dimension of galaxy filaments

Galaxy filaments are "amongst the largest known cosmic structures in the Universe. They [...] form the boundaries between large voids in the Universe." As their name suggests, filaments are ...
16
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1answer
1k views

When galaxies collide, what happens to dark vs. light matter?

I have read that galactic collisions have been used as evidence demonstrating the effects of dark matter. In particular, that luminous matter slows down and interacts while dark matter shoots through, ...
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0answers
15 views

How well supported by observations is the hypothesis that concentrations of dark matter are essential to galaxy formation?

Much work regarding the formation and early evolution of galaxies seems to assume that galaxies form based on clumps dark matter. The following observations indicate that some galaxies started with ...
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6answers
18k views

Why the center of our galaxy doesn't absorb us?

Depending on the theories, the center of our galaxy is a super massive black hole, this is easy to accept as a truth, but what I couldn't simply devour is how the solar system is orbiting around it ...
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0answers
37 views

Gravitational redshift and mass decrease

If there are two distant gravitationally unbound galaxies and the observer galaxy loses its mass twice faster than the distant galaxy. So gravitational blueshift decreases in the observer galaxy. ...
14
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3answers
8k views

How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?

The actual number: How far apart are galaxies on average? An attempt to visualize such a thing: If galaxies were the size of peas, how many would be in a cubic meter?
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1answer
119 views

Is a galaxy made up of dark matter really a galaxy?

So I am watching a video discuss Dragonfly 44. The galaxy is made up almost entirely of dark matter and is 100x dimmer than the Milky Way galaxy. Wouldn't this mean it is just a large clump of ...
5
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3answers
173 views

Distribution of dark matter in galactic halos

Often dark matter around galaxies is referred to as a 'halo'. I've seen the galactic rotation curves, but I'm having trouble visualizing how the dark matter is distributed for a typical rotating ...
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1answer
64 views

Could six copies of the universe exist only interacting by gravity? [closed]

The ratio of dark matter to matter is roughly in proportions $1:5.4$ This suggests that if there were about 6 Universes placed in the same space that were completely invisible to each other, except ...
11
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2answers
1k views

$N$-body simulation in General Relativity

How would one perform an $N$-body simulation in General Relativity (GR) for something like galaxy formation or galactic dynamics? Suppose one wants to simulate the rotation curve $v(r)$ for galaxies ...
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0answers
12 views

How do Oort's constants relate to shear and vorticity?

I am told to show that Oort's constants represent shear and vorticity in the velocity field of an orbiting star. I know that Oort's constants are defined as: $A = \frac{1}{2}(\frac{v_{\bot,0}}{R_{0}}...
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0answers
48 views

Gravitational potential energy of an isothermal sphere

I am trying to calculate the gravitational potential energy, W, defined as: $W = -\frac{1}{2}\int\rho(r)\Phi(r)d^{3}r$ for an isothermal sphere. I am given that the density profile varies with r as:...
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0answers
31 views

Jeans equation for a fully isotropic velocity dispersion tensor

Given a known spherically symmetric gravitational potential, $\Phi(r)$, I need to calculate the value of the velocity dispersion tensor $\sigma_{rr}$. To calculate $\sigma_{rr}$ I am using Jean's ...
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2answers
170 views

Why can't I see whole galaxies with the naked eye?

I have read these questions: At what distance could you see andromeda with the naked eye? Do all the individual stars that we can see in the night sky belong to Milky Way? I look at the night sky ...
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2answers
127 views

Galaxy Cluster Data

I have a modification to ${\rm MOND}$ that appears to work much better than 'Standard' ${\rm MOND}$ at the galactic scale. I want to test this model now on larger structures. Does anyone know where ...
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0answers
68 views

How to derive the gravitational potential on an infinitely thin disc of finite radius?

I am stuck on a problem that requires me to calculate the gravitational potential on an infinitely thin disc of finite radius $R$ with mass $M$. This infinitely thin disc models a galaxy. Once I have ...
6
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1answer
186 views

How Much Do Spiral Arms Affect Their Galaxies' Rotation Curves?

According to Jerry Sellwood of Rutgers University, in the article What process creates and maintains the beautiful spiral arms around spiral galaxies?: Fortunately, nearly every one agrees that ...
39
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6answers
8k views

What is the evidence for a supermassive black hole at the center of Milky Way?

Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific ...
10
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2answers
3k views

What is the frequency of occurrence of stellar classifications off the HR main-sequence?

An alternative version of this question would be: "if was to pick a star from the $10^{11}$ or so in our galaxy at random, what are the probabilities of it being various kinds of star?" (and I do mean ...
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4answers
2k views

Shapes of galaxies

I've heard most of galaxies are spiral or ellipsoid shaped. Is it true? If true, then why they form in such shapes? How did arms of the spiral galaxies form?
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3answers
205 views

How is a star's parent galaxy recognized?

A star is probably visible/detected by it's radiation. But that star may or may not belong to our own galaxy ... yet news reports speak of detecting a star/nova in a distant galaxy. How does one ...
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1answer
50 views

Could dark matter be normal matter that is beyond the observational horizon?

As I understand it, matter that is too distant for its light to reach us would not be observable, and the CMB is the limit of what is observable from our location within the universe. Could what is ...
55
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3answers
31k views

Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?

Question: As we know, (1) the macroscopic spatial dimension of our universe is 3 dimension, and (2) gravity attracts massive objects together and the gravitational force is isotropic without ...
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3answers
3k views

Expansion of the Universe: is new space(time?) being created or does it just get stretched?

Is new space(time?) being created as the Universe expands, or does the existing spacetime just get stretched? If it just gets stretched, why do galaxies move along with the expansion instead of just ...
46
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2answers
8k views

Does a star need to be inside a galaxy?

Must a star belong to a galaxy, or could it be completely isolated? In case it can be isolated (not belong to a galaxy), could it have a planet orbiting around it?
2
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2answers
131 views

How do we know that galaxies are not losing significant numbers of stars from their outer regions?

A major reason for believing in the existence of dark matter is that the stars in the outer regions of galaxies are moving too fast to remain bound to their galaxies, given only the estimated mass of ...
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1answer
67 views

Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
2
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1answer
106 views

Effects of dark matter

I was wondering, when sims of galaxies show that we need dark matter to account for the movement of outer stars, are models taking into account relativistic effects on gravity? The tips of spiral ...
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0answers
37 views

Does dark matter experience drag?

Is dark matter dragged around the galaxy by gravity as stars and groups of stars orbit the galaxy, and, if so, is its shape/distribution deformed? Similarly, would the Oort cloud be deformed by being ...
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1answer
48 views

Doubts about the formation of the bullet cluster

The bullet cluster is formed by the collision of two clusters of galaxies. After the collision, the stars and galaxies in those two clusters passed through each other. But the intergalactic gas clouds ...
5
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1answer
2k views

Why are there not many detectable supernovae?

Astronomers estimate that there are between 200 billion to 400 billion stars contained within the Milky Way, and that the Andromeda galaxy probably has 1 trillion stars. There may be around 500 ...