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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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'...
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25 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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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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1answer
147 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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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. ...
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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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29 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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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 ...
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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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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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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. ...
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1answer
118 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 ...
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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 ...
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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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45 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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2answers
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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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29 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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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 ...
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3answers
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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 ...
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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 ...
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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?
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1answer
66 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 ...
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1answer
101 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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36 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
25 views

Relative size of galaxy calculation

So this is a past exam problem, however I am confused to a question which is related to this graph and the questions states: Question: On the graph, one galaxy is labelled A. Determine the size of ...
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1answer
47 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 ...
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2answers
317 views

Could the cosmic fluid be better described as a kind of van der Waals gas, or a kind of foam, instead of a dust gas?

In cosmology, it is usually assumed that the cosmological fluid made of galaxies could be described as a gas of "particles" without any pressure (the dust gas), of density $\rho_{\text{matter}} \...
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0answers
95 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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2answers
46 views

How can a galaxy not be disk shaped [closed]

If a galaxy is the accretion disk for a supermassive black hole, how can it be a shape other than disk?
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101 views

Why aren't shape of galaxies depicted as a funnel?

Let's say if a galaxy is moving, shouldn't it be shaped like a funnel? I am saying this because it would take considerable time for the outermost star to know that the center of gravity has shifted. ...
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0answers
42 views

Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
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1answer
64 views

What's the eccentricity of our sun's orbit around the center of the galaxy? [duplicate]

Just thinking about "galactic habitability zones." Is this even a valid question?
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1answer
58 views

A question about horizontal doppler effect on galaxy hydrogen radiation observation

The figure shows the positions of the solar and neutral hydrogen regions A to D rotating in a circular orbit relative to the center of the galaxy. The sun and A to D assume a Kepler rotation. ...
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2answers
237 views

Dark matter density calculation

I find that the general dark matter density of the Milky Way is $$6.87 \times 10^9 \: \rm GeV/m^3$$ or $$1.225 \times 10^{-17} \: \rm kg/m^3$$ (by taking the size of the Milky Way and dividing it to ...
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1answer
100 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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1answer
69 views

Do attraction between two galaxies count for dark matter?

We know that the uniform rotation curve of individual spiral galaxies does require presence of some form of dark entity (matter). Does the attraction between two (or more) spiral galaxies also require ...
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1answer
98 views

What does the gravitational potential of the Milky Way do to the CMB?

We sit in a gravitational potential, so there should be a blue shift on the CMB light from the potential of the Milky Way. Is this blue shift dependent on direction? Is it being subtracted from the ...
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4answers
3k views

Could dark matter consist of the supermassive black holes at the centers of galaxies?

Inspired by this question about whether dark matter is matter, noting that dark matter tends to be clumped in galaxies near the center and less so on the edges, accepting that many (most?) galaxies ...
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1answer
52 views

If a galaxy forms from a spherical stationary cloud, how much of the gas will escape?

Let's ignore the dark matter legend and stay with Keplerian physics. Assuming that there is a cloud with $N$ stationary particles with the same size uniformly distributed in a sphere and they ...
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1answer
59 views

Orbits within orbits

Do galaxy clusters have structured orbits (e.g. ellipticals)? If not, why not? Moons orbit planets, planets orbit stars, and stars orbit galactic centres. Is there something different about the ...
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40 views

Does a disk galaxy wobble (galactic wobble)?

The center of mass of the solar system occasionally wanders up to 1/2 the solar diameter outside the sun. The sun rotates around this CM. Does something similar occur in spiral galaxies? If so, ...
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2answers
198 views

What's wrong with the Big Spin Model?

A Dr. Serkan Zorba has a paper on arxiv in which he considers, what if the universe is actually slowly rotating? This gives rise to centrifugal and Coriolis forces on a galactic scale that seem to ...
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1answer
89 views

Is the dark matter present only around the galaxies?

Dark matter is believed to give galaxies its shape and prevent them from flying apart. Is dark matter present only on outer edges of the galaxy or is it present throughout the galaxy?
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3answers
59 views

What prevents galaxies from collapsing towards the galactic center? [duplicate]

Galaxies have supermassive black holes at their cores exerting huge gravitational field. What is the reason that galaxies don't contract towards the center? What prevents this from happening.
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1answer
273 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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0answers
19 views

Population histograms and relationship between size of galactic disk and amount of dark matter

I am interested in seeing a plot of dark to baryonic matter ratios in terms of the galactic disk size (when it applies, as not all galaxies fit into disk shapes, and probably analyzing disk galaxies ...
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1answer
191 views

Why does dark-matter distribute in spherical shape around the galaxy?

For visible matter, due to spin dominance over gravity while formation of galaxy, planar symmetry of a galaxy is achieved and the galaxy will be planar (spiral). But dark matter's distribution is ...
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1answer
138 views

Cosmological redshift : “time of emission” vs “during travel”

Let’s consider a very distant galaxy, suppose it has no peculiar velocity, and let’s assume space expansion; If I understand correctly there would be a redshift due to the value of space expansion at ...
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0answers
67 views

Barkana & Loeb's Virial Radius Plot [closed]

Recently I have been trying to teach myself the basics of modern cosmology. To do so I have been using Barkana & Loeb. I've been working my way through and come to virial radius. They give the ...