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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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17 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
35 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
295 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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20 views

Are galaxies rotating faster or slower over time?

Independent of the discussion around flat rotation curves and dark matter, is there something known about the change of the rotation curves over time?
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0answers
62 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
45 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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2answers
90 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
39 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
51 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
50 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
96 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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0answers
40 views

Convergence of Milky Way and Andromeda

It is definitely a certainty that Andromeda will collide directly with the Milky Way in ~4 billion years. Has it been estimated how long the two galaxies have been attracted to each other to the ...
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1answer
50 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
52 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
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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
44 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
55 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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0answers
34 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
173 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
55 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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0answers
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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
149 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
84 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
49 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 ...
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1answer
48 views

Could space repel mass?

I'm not an advanced physics person, so any explanations need to be reasonably low-level... I was looking at oil/water bubbles (hydrophobic vs hydrophylic molecules) and it got me thinking about the ...
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3answers
135 views

Are galaxies really structured the way they look in pictures?

Are real galaxies really structured the way they are in pictures online? I'm wondering this because if the speed limit of the universe is light speed, which means stuff we see on the sky or detected ...
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0answers
13 views

Morphology-density relation and Morphology-radius relation

I have a sample of 500 galaxies with classification as ellipticals (E) or spirals (S). To each one of these galaxies I have the value of the radius R/R200. I want to check in this sample the ...
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1answer
115 views

Galaxy Rotation Speeds and General Relativity

Is it true that the predictions from the General Theory of Relativity don’t match the observations of galaxy rotation speeds, and that this then started the search for dark matter? Is it only the ...
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3answers
418 views

Astronomical sources of muons

I am looking for some sources of muon particles (μ) in astronomy. By this, I mean I am interested in any astrophysical event where theoretical presence of muons is suggested. For instance, does the ...
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1answer
68 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?
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1answer
86 views

Why can't the Higgs vacuum energy clump to galaxies and act as dark matter (instead of dark energy)? [closed]

The Higgs field has a nonzero vacuum expectation value which contributes to the energy density of the Universe. Energies contribute to the curvature of spacetime and affected by gravitational ...
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1answer
58 views

What causes structure formation: Baryon acoustic oscillations or inflation?

Formation of structures such as galaxies and galaxy clusters require anisotropies in the cosmic plasma which (as I understand) were due to the anisotropies or density inhomogeneities created by Baryon ...
3
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1answer
121 views

Why are there no more galaxies moving as relativistic speeds?

Introductory note: I am not discussing galaxies that are going away from us or that are at the border of the universe. If there are no preferred frames of reference, no galaxy or matter ensemble can ...
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1answer
65 views

Article presenting first evidence for dark matter?

What are the papers where the first evidence for dark matter related with the stability of galactic discs, and the orbits of galaxy satellites?
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2answers
102 views

How do we know that galaxies are not in the process of flying apart?

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
88 views

What is Cosmic Downsizing?

I've had a quick look at a few lecture pdfs and papers as supplementals to my own given lecture notes, but I can't seem to get a proper explanation for what cosmic downsizing is. The closest ...
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1answer
33 views

In theory, can we count every galaxy in the observable universe one-by-one?

Suppose I have a very advanced telescope that can detect long wavelengths of light, so I can deal with extreme redshift. I am also a very quick counter, so I can get through all of the galaxies before ...
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1answer
66 views

Intuitive understanding of the virial radius/mass

I'm currently trying with no avail to understand the intricacies that define the virial radius or mass in a way that is different from the actual galactic radius/mass. I understand it is derived in ...
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0answers
45 views

Understanding the tidal force tensor

I'm just looking to gain a basic intuitive understanding of the forces at work in tidal stripping and their direction/motivation etc. I'm struggling to grasp the underlying principles and it is making ...
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6answers
7k 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 ...
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3answers
5k views

How did we 'discover' dark matter? [closed]

I'm an astrophysics student and I've been researching this topic and there is one point that keeps eluding me. How did the scientific community realize that there had to be dark matter in the ...
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1answer
236 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
41 views

Estimating total mass inside the Sun's orbit

I'm trying to estimate the local density of dark matter at a given radius, $r=R_0=8\text{kpc}$. I also have values for the core radius, $a=5\text{kpc}$, and circular velocity of the Sun, $v_{\text{...
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1answer
79 views

Given a fixed spectral resolution, what is the minimum detectable redshift?

Given an spectral resolution $R$ (for example $R=1000$), what is the minimum redshift which I can measure? How is this related to the minimum velocity among galaxies? If this velocity is related with ...
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0answers
24 views

Estimating the reddening of a star? [closed]

I'm stuck on a question concerning how to estimate the reddening of a star. The question is: "A star near the Galactic plane has observed apparent magnitudes in the blue and visual bands of $B = 15....
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2answers
172 views

The Sun's Orbit - Is it What We Think?

I was thinking that the sun must orbit something within our spiral arm in the Milky Way, or be affected by other astronomical bodies - surely not just the supermassive black hole centre. I have ...
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0answers
24 views

Estimating the age of the Fermi Bubbles

Assuming the Milky Way’s Fermi Bubbles were caused by a single event near the galactic center, what are the limits to when that event could have happened? The bubbles reach 25,000 light years high, ...
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2answers
62 views

Understanding a plot obtained from the 2dF redshift survey

The diagram below (taken from here) represents the distribution of Galaxies obtained from 2dF redshift survey. I have a few very naive questions about this. Which point does the centre correspond ...
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1answer
215 views

What is the current shape of the Milky Way?

Things in the Universe which are thousands of light years away from Earth can be seen, or pictures can be taken by Hubble Telescope, because those stars, galaxies, etc. emitted light thousands, ...