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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What are other potentials like the Henon-Heiles Potential? [closed]

The Henon-Heiles Potential, as I have learned, is a two-dimensional nonlinear axi-symmetric potential of galactic systems. I wanted to know if there are any other potentials that also describe ...
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How did astronomers at Edwin Hubble time decide the size of the Milky Way Galaxy and settled the Great Debate?

I have been reading some books about Great Debate that whether the Milky Way Galaxy was the entire universe and Edwin Hubble settled the debate by identifying some Cepheid variable star (V1) in ...
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How to interpret line of sight velocity distributions and dispersions of a galaxy?

What I did: I am analysing some simulation snapshots of the Milky Way and it is modelled as a DM halo – bulge – disc system. I produced some mean line of sight velocity (LOSV) and dispersion of mean ...
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Does barred in spirals galaxies like ours have the same speed (spin) as the center black hole?

In the case of old galaxies (elliptical) the black hole, like one in M87 have 2 jets. This look like polarisation pushing from central BH. In barred spiral galaxies, the bar have to cross the central ...
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Rotation of galaxies and gravitomagnetism [duplicate]

Can we explain the bigger speed of rotation of the galaxies from gravitomagnetic field acting from the "inner layers" of the galaxy to the "outer layers" of the galaxy?
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Effect of Newton's law to the orbits inside a galaxy like Milky Way

Does Newton's law describe the orbital motion of stars in a galaxy? Is it true that the closer a star is to the center, the lower is the gravity, so the lower the speed of the star?
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4answers
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What would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? [closed]

So, what would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? What was the configuration of the gas, the history of the primordial stars, for this ...
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Why do they say universe will become cold with expansion?

The Universe is expanding and they say it will eventually become cold, and new star formation will stop. But galaxies are only moving away from each other, and each galaxy itself stays intact and the ...
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1answer
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Finding the dominant frequency of a stellar orbit

I was reading this paper on astro-ph (Sanders et al. 2020, "Models of Distorted and Evolving Dark Matter Haloes"). It's about modelling the gravitational potential in a cosmological ...
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4answers
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Would we even notice the merger with the Andromeda Galaxy?

I have read this question: When galaxies collide it is not that their stars crash into each other, because their individual cross-sections are extremely small when compared to the space between them. ...
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Can you explain what is Heliocentric velocity

In this photo, What is the meaning of heliocentric velocity ? Is the velocity at which the Sun's motion was corrected or before the Sun's motion was corrected?
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If gravity is attractive, why doesn't the Milky Way contract?

Einstein introduced the cosmological constant because without it, one cannot get a static universe - gravity would cause the universe to contract. Given that, why is the Milky Way not contracting? For ...
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1answer
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Age determination of stars

In the report 'New observations of the most distant galaxies close in on cosmic dawn', an astronomer states: 'Using a well-understood age indicator based on the amount of hydrogen absorption seen in ...
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Galaxy statistics calculation in Saslaw's book

I am trying to follow a calculation from the book of William C. Saslaw, The Distribution of the Galaxies: Gravitational Clustering in Cosmology. The calculation is shown on the pages following page ...
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1answer
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Galaxies: Negative pressure, positive feedback

As part of research trying to develop a cosmological model that can be static (scale-factor), but in dynamic equilibrium - a way is being considered whereby dense collapsing regions of matter 'bounce' ...
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Compute the variance of spectroscopic Shot Noise in cosmology context

I would like to know the right expression for the expression of variance of Shot noise in spectroscopic probe. Sometimes, I saw $$\sigma_{SN,sp}^{2} = 1/n_{sp}$$ with $n_{sp}$ the average density of ...
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Velocity change of stars in subject galaxy by passage of perturbator cancels for all stars

Consider a subject galaxy where a star $\alpha$ is moving with velocity $\mathbf{v}_\alpha$. The galaxy is then perturbed by the high-speed passage of a massive system. The velocity of the star $\...
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2answers
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Forces besides gravity acting at astronomical distances?

It seems like all one reads about is gravity affecting planets and stars whereas at the atomic level there are other forces and gravity is unimportant. How do we know there are not other forces ...
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Is there any $N$-Body software available that allows you to change the code to try different laws of gravity?

I am looking for some software (free or for a charge) that allows the user to modify the code to try different laws of gravity. This will be used to model galaxies, dark matter, galaxy clusters, etc.
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2answers
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How much faster are the stars on the outskirts of galaxies going than they are 'meant' to as predicted by general relativity?

this is in reference to dark matter. I've look on the internet for this but I can't find precisely what I am looking for.
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How does Collisional Ionization cause cooling?

I am trying to understand the different forms of radiative cooling such as recombination, collisional excitation, collisional ionization etc. In recombination and collisional excitation, the cooling ...
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1answer
285 views

Does General Relativity allow a reduction in the strength of gravity?

Observations of galaxies commonly show ‘jets’ and other ejection phenomenon like these ‘Fermi bubbles’ The Fermi Bubbles are two enormous orbs of gas and cosmic rays that tower over the Milky Way, ...
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Calculating redshift from distant galaxies using spacetime metric

In Sean Carroll's GR book, pg. 104, it was said that at a rigorous level, the notion that galaxies are "receding away from us" at a speed defined by their redshift is nonsense, since ...
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2answers
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Gravitational Assists: star-hopping - what's the limit?

By dipping deep into a star's gravitational well, one can take advantage of the Oberth effect and of gravity assists. From the Oberth effect, one can control large changes in direction from relatively ...
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Keplerian galactic rotation curve

I am trying to plot my obtained galactic rotation curve from Hydrogen line measurements with Keplerian galactic rotation model. However what values should I use for Keplerian galactic rotation model ...
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What reduces the efficiency of galaxy formation at lower and higher masses?

What reduces the efficiency of galaxy formation at lower and higher masses? (Feedback mechanisms) I understand its something to do with supernova feedback and AGN feedback but I can't seem to ...
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1answer
67 views

Why is the distribution of Dark Matter in the Universe inhomogeneous?

As far as I know, there exist galaxies containing lots of DM and other galaxies with almost no DM at all. This implies that the distribution of DM in the Universe is inhomogeneous. Do we have any idea ...
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2answers
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Are galaxies moving away from each other at a constant speed?

Im trying to understand what Hubble's law $$v = H_0 d$$ means. The thing is I keep getting two different explanations. Galaxies are moving at a constant speed, the ones further away are so because ...
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1answer
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Is galactic gravitational lensing self-magnification big enough to contribute noticeably to the galaxy rotation curve problem?

Gravitational lenses magnifies the appearance of themselves, so in the case of a galaxy it looks bigger than it is. Thus the outer stars in a galaxy seems to have a higher tangential velocity. For ...
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Minimum mass of galaxies and dark matter

I have read somewhere that a relativistic dark matter particle can “free-stream” out of a primordial overdensity, and erase such densities within the comoving horizon distance, and that this can be ...
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1answer
71 views

What happens when a galactic body gets bigger?

In my simple non-quantum non-nuclear but enthusiasm-filled mind, I fascinate that as a galactic body become much more massive, it can reduce matter first down to its building blocks, then eventually ...
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1answer
58 views

Mass of Dark Matter Particle

I have heard that dwarf galaxy masses can be used to determine the mass of a dark matter particle. I have tried looking online for methods cosmologists use (https://cds.cern.ch/record/2702846, De ...
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1answer
75 views

The average velocity of galaxies in a galaxy cluster [duplicate]

On slide 24 of these lecture slides (Its title is 'worked example, what Zwicky did'), the author says that $\langle\langle v^{2} \rangle\rangle = 3 \langle\langle{v^{2}}_{s}\rangle\rangle$. Can ...
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Vertex deviation of the velocity ellipsoid

Context I will be working here in galactic coordinates. This system uses distance from the Sun $(r)$, longitude $(l)$ and latitude $(b)$. The latter two are angles defined as shown in the following ...
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Do gravitational lensing experience Doppler effect?

I watched dark matter explained by Perimeter Institute on YouTube and they used it as gravitational lenses, they also explained Doppler effect so we know how fast things are relative to us. I wonder ...
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1answer
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Why is speed measured along the line of sight (LOS) a factor $\sqrt{3} $ smaller for randomly distributed velocities?

I encountered this problem from Introduction to Modern Cosmology by Andrew Liddle. I had the exact same doubt as the OP of that question had, and the first answer makes sense but I don't understand ...
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2answers
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Does it make sense to discuss the "pressure" of the universe?

I've heard the galaxies of the universe analogized as particles in a gas. If we consider this analogy, and understand that the universe is expanding while the temperature at this point in time is ...
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1answer
38 views

Do irregular galaxies host a supermassive black hole?

Do irregular galaxies (eg the LMC) host a supermassive black hole?
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1answer
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How could I calculate the circumferential velocity of stars in an idealized spherical galaxy from the distribution of mass through that galaxy? [closed]

We will assume that the galaxy is a sphere where stars orbit around the center on different radii. Since there are so many stars, we will not deal with individual stars, but we will assume that the ...
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1answer
49 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}{\...
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0answers
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Star formation rate in different galaxies

I was looking for some scale to compare my result of star formation rate and see if it is moderate or high or low. But I couldn't find any scale. Is there any paper related to such scale where Star ...
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4answers
105 views

Are orbiting masses in a uniform disc affected by masses outside its orbit?

For a sphere of uniform density, we know an orbiting mass experiences a net attraction only by the mass inside its orbit, and that the mass outside its orbit exerts a net zero force (using Newton's ...
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1answer
79 views

How did the Holm 15A Galaxy BH reach 40 Billion Solar Masses?

The most massive supermassive black hole discovered to date lies within the Abell 85 galaxy cluster. At the heart of this cluster is the galaxy Holm 15A, containing an estimated 2 trillion solar masse ...
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1answer
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Can the gravity of one entire galaxy slow down an astrophysical jet emitted from its central black hole?

Let say we are talking about our Milky way and Sagittarius A* emitted a astrophysical jet. If the galaxy was 150 000 ly wide would its gravity with dark matter included eventually slow down that jet ...
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1answer
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Will spiral galaxies collapse when expansion of the universe isolates them from the rest of the universe? [closed]

If a galaxy gets isolated when the speed of the expansion of the universe exceeds the speed of light, will it collapse? The rotation of a spiral galaxy prevents it from collapsing due to gravity. So ...
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1answer
31 views

Galaxy Superclusters

Okay quick question... why is it that when galaxies group together in superclusters they form a sort of branch-like shape? I personally would think they’d all just group together in some sphere or ...
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What are Luminous Red Galaxies (LRG's) and why are they useful?

That's the question. I've been encountering LRG's in papers such as https://arxiv.org/abs/astro-ph/0608632 and I don't really find enough liteterature to understand what are the main characteristics ...
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2answers
67 views

How can we know the velocity curve past the cut-off radius of visible matter in a galaxy?

I am trying to understand a typical explanation for the need of a dark matter halo in galaxies. The rotational velocity of stars around the centre of a galaxy seems to be constant past a certain ...
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2answers
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Single galactic proof of Dark matter?

I've heard tell that many galaxies have visible mass profiles which do not match up to a predicted Gravitationally Newtonian velocity distribution. Can anyone here find me an accessible paper of an ...
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3answers
67 views

What's stopping stars in galaxy from traveling at infinite speed?

The dark matter in galaxies causes the starts to orbit with a higher orbital speed. Now, assuming that there is a constant force from the dark matter on the starts, there should be constant ...

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