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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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 ...
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+50

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 ...
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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 $n_{HI} = \frac{M_{HI}}{m_H V}$...
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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 ...
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1 vote
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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 ...
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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 ...
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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) ...
134 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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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, ...
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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 ...
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1 vote
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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 ...
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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 ...
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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 ...
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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 ...
1 vote
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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 ...
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1 vote
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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 ...
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1 vote
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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 ...
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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 ...
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1 vote
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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 ...
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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 ...
1 vote
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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 ...
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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 $\sigma$ ...
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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 ...
1 vote
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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 ...
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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?
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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 ...
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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 ...
1 vote
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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 ...
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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 ...
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1 vote
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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 ...
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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 ...
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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?
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The centre of what we can now see in the universe

If the JWST has just discovered the furthest away galaxies in one place we can now triangulate to where the centre of what we can see This is observable and appears not symmetric with everything ...
1 vote
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Could the energy emitted by galaxies contribute to the dark matter phenomenon?

I'm pondering a concept regarding the energy-mass conversion in the context of cosmology, specifically related to the light emitted by galaxies over billions of years. Einstein's famous equation E=mc^...
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Gravitational field intensity in mass disk

To calculate the gravitational field intensity or acceleration in a mass disk (like a galaxy), should I do a(r)=G×Mt/r^2 or a(r)=G×M(r)/r^2 with Mt being the total mass of the disk/galaxy and M(r) the ...
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Age of the universe due to expansion

"If gravity working on matter is the only force at work on large scales, then the attactive force of gravity will act to slow the expansion. In this case, the universe was expanding more rapidly ...
1 vote
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How can I make the equation of warped disk?

My teacher gave us the assignment to find the moment of inertia of any shape you want. So I decided to find the moment of inertia of our milky way galaxy. I found out that our galaxy is shaped like a ...
47 views

How do we know if a distant galaxy's light we see today was also reaching our location before approximately 4.5 billion years (Earth formation)?

This has actually to do with the estimation of of how distant an observed galaxy is? How we can be sure if we do not know if we could see the light comming from the galaxy even before the Earth was ...
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1 vote
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Do all galaxies in our observable universe have more or less the same age?

What is the current consensus about the age variation of the existing galaxies in our observable universe? Not to be confused with the age of very distant galaxies as observed today by our telescopes ...
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How cosmologists know if the observed redshift of galaxies is due to expansion and not intrinsic to the galaxy?

Is there a way to experimentally test if the cosmological red shift observed is due to the expansion or intrinsic to the galaxy? If anyone knows how to do this or how to extract from the data please ...
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How do we know that there is matter-anti-matter asymmetry? [duplicate]

It is often said that there's more matter than anti-matter, How do we know that there is more matter, Can't there be a galaxy made up of antihydrogen? Will that galaxy be any different from ours? How ...
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At which rate does visible matter decrease from the galatic center?

At which rate does the visible matter density in galaxies decrease when moving away from the galactic center?
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How much greater is the average distance between stars in the spaces between spiral arms? How much lower is it in them?

I've read about the overall average distance between stars in the milky way, and I've read that one of the best explanations for spirals arms is that they are density waves - that is, they are indeed ...
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Is there dark matter in intergalactic space?

Reference: the rotation speed of galaxies in a galaxy cluster: Is the dark matter associated with each galaxy in a cluster, sufficient to explain the rotation speed of galaxies in the cluster, or ...
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Galaxy S4G database: convert pixel to kpc

I am referring to S4G database (https://irsa.ipac.caltech.edu/data/SPITZER/S4G/overview.html ). The information on the length of galaxies, and various length parameters are given in terms of pixels. ...
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