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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Would time dilation make it impossible to accurately convert between calendars across the galaxy?

A hypothetical signal is emitted from a known exoplanet thousands of light years away. The signal marks the occurrence of an epoch-worthy event on that planet, and we (the receivers) want to know ...
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Cross correlations between 2 sensors - Approximation of a 2D + 3D synthesis

I am interested, in the context of my work, in the cross correlations between a spectroscopic probe (which gives a 3D distribution of galaxies with redshifts, which is also called spectroscopic Galaxy ...
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Formula of black hole gravitational sphere of influence

Well, I want to derive the formula $$ r = \frac{GM}{\sigma^{2}} $$ which happens to be the radius of the gravitational sphere of influence of a supermassive black hole inside a galaxy. How can I do ...
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Will Milky way and Andromeda collide for sure?

Is it sure that Milky way and Andromeda will collide or might they just start a rotation around their center of masses?
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Theories of galaxy structure

If you look up galaxies on Wikipedia (at least as of August 14, 2020) you will find a list of classes of galaxies that progresses roughly as follows: Elliptical galaxies Shell galaxies Spiral ...
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Barred galaxies and the velocity at the tips of the bar

Ofcourse there must be something with the arrangement of masses, more precisely dark masses in barred galaxies but do scientists figure out are the velocities of stars along the whole circumference ...
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1answer
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Gravitational bonding energy of a galaxy

If most mass from a proton comes from the nuclear force's bonding energy between quarks that make-up this proton, why wouldn't most mass from a galaxy come from the gravitational force's bonding ...
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At which point in the universe history will inflation prevent galaxies feeding from intergalactic matter?

It's my understanding that galaxies formed from accretion of intergalactic matter around supermassive black holes. As the universe expands the amount of matter entering a galaxy decreases, until at ...
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Why is dark energy dominant between galaxies but not inside galaxies?

The ideas of dark matter and dark energy are mind blowing. Why is it said that dark matter overcomes dark energy in galaxies but it loses the battle in intergalactic space? In other words, why is dark ...
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What causes peculiar velocities of galaxies?

The Wikipedia page on the subject states: Galaxies are not distributed evenly throughout observable space, but are typically found in groups or clusters, where they have a significant gravitational ...
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Dynamical Mass of the Galaxy

What does the dynamical mass of a galaxy represent? Is it the Mass of the gas in the galaxy or the total mass of the galaxy ? What can we infer from the rotation curves, is it dynamical mass or mass ...
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What experiment can I do at home to show different formation of spiral galaxy and elliptical galaxy?

I am wondering if there is a simple experiment that can be performed at home to demonstrate the formation of galaxy with arms and no arms(density is smoothly spread out)? The experiment must help ...
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Which parameters can be used to differentiate between bulge and bulgeless galaxies?

I started doing a project which needs a set of bulgeless galaxies, but I don't how to differentiate between bulge and bulgeless galaxy. It would be useful if you could specify which parameters can be ...
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Galactic structure vs distance from earth (ie age)

In both theories of galactic formation, top-down and bottom-up, we understand that galaxies become increasingly structured as they age. By increasingly structured, I mean they are more spiral or more ...
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1answer
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How astronomers estimate the redshift of galaxies clusters?

I don't understand how astronomers estimate the redsfhit of a cluster. As far as I understand a cluster of galaxies is something really "big", so I expect that different galaxies in the ...
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1answer
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Why Andromeda is approaching the Milky Way in an expanding universe? [duplicate]

We know from Hubble law that the galaxy is receding from each other. Also, the general theory of relativity predicts the metric expansion of space ( voids are increasing). Then why Andromeda is ...
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I cant understand here why is receding acceleration is taken equal to gravitational acceleration?

Here is the full document link https://www.lapasserelle.com/cosmology/ Click on lecture 1 notes for the document Now my question is why the acceleration with galaxies is receding is taken equal to ...
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Are X galaxies (PKS 2014-55) collisions?

Recent radio imaging by Meerkat of PKS2014-55 show an xshaped galaxy with a distinctive core and lobes. https://scholar.google.co.uk/scholar?hl=en&as_sdt=0%2C5&q=Pks+2014+meerkat&btnG=#d=...
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1answer
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Does the mass of a galaxy change over its lifecycle?

I expect there is change as energy converts into matter and matter converts into energy. Is there an overall curve we have seen through observation that a galaxy goes through in its lifecycle or does ...
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Galactic bars and star formation

I have recently been reading on barred galaxies and am confused as to whether they aid or obstruct star formation as a general rule. Some papers state that they aid star formation, whilst others ...
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Why are the most distant galaxies “only” around 13 billion light years away?

According to Wikipedia's List of the most distant astronomical objects, the most distant galaxy (GN-z11) is estimated to be around 13.39 billion light years away from earth. However, the observable ...
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What is the gas distribution in galaxies?

I'm fitting a number of galaxy rotation curves, and I need the density distributions of stars, gas, and DM. I know that the baryonic disk densities fall off exponentially (the characteristic radii can ...
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1answer
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Does the $M/L$ ratio have an implied band?

When I look at the NED database, I see the Luminosity quoted for six different bands (X-Ray, UV, Visible, Near IR, Far IR, Radio). My issue is trying to interpret the $M/L$ ratios I've seen ...
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What is the luminosity of the Milky Way?

This question may appear to be the same as What is the luminosity of the Milky Way galaxy?, but that question references a value of $5\times 10^{36}\space W$, which comes from a Wiki page which ...
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What is a good database of galaxy luminosities?

I need to do some mass/luminosity relationship calculations. Where can I find good, referenceable absolute luminosity data?
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Galactic Dynamics Binney and Tremaine

I was reading up on galactic dynamics, especially rotation curves and came across this equation in Galactic Dynamics by Binney and Tremaine(2.208 pg 111): $$ \psi(m)= \rho_{b0}\int^m_0 dm^2 \left(\...
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Is this a valid way to estimate the solar system's orbital velocity around the galactic center?

I know there are better ways to derive this, such as finding orbital velocities of other stars at approximately the same radius from the GC as ourselves and finding our own velocity $v_0$ as within ...
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3answers
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Why is the Andromeda galaxy coming closer to Milky way?

If both these galaxies are on collision path because of their gravity but how could that be? They are 2.5 million light-years apart now but I imagine they are still n collision path for a long time ...
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Galaxy is 8 times closer 13 billion years ago? [duplicate]

Based on below, How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ? I ...
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Calculate distance of galaxy 13 billion years ago [duplicate]

How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ?
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What would happen if our galaxy and an identical anti-matter galaxy annihilated each other?

What would happen if every bit of matter in our galaxy annihilated with an anti-matter equivalent at the same time? Such as that stars completely annihilated with identical antimatter stars, planets ...
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3answers
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If the distribution of dark matter is not uniform, why it is not attracted towards center of galaxies?

The dark matter hypothesis was formed due to difference in theoretical and observed speed of the outer parts of galaxies. Therefore, there is more dark matter in the outer parts of the galaxy than in ...
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2answers
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Why are galaxies so far apart?

With the Big Bang, one would expect the distribution of mass to be somewhat uniform, but we have galaxies that have billions of stars and there are huge distances between galaxies. Why is there is so ...
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1answer
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How do I decompose my overall rotation curve for M31 into a rotation curve for the disk and halo?

How do I decompose my grand rotation curve into rotation curves for the disk and halo, like the image below? The picture is from this link pages.pomona.edu/~tmoore/RotCurve The data points are ...
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Questions about Dark Matter and Galactic Rotation

First, does the dark matter halo and galactic rotation of a galaxy depend on it's size? So bigger galaxies, bigger dark matter halo. Secondly, is the amount of dark matter for a galaxy universal? So ...
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Required Preconditions for Galactic Formation

In recent weeks, I have been working on building simulations to look at the formation of galaxies from dust clouds. It's a relatively crude simulation at this point as I needed a working model to ...
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4answers
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A star or a galaxy?

When we look into the beautiful sky in the night, exclaiming how beautiful these shining stars are. My question is how could we tell, whether any of these shining "point" is a star or a galaxy? If ...
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1answer
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Why don't the galaxies collide due to gravity?

Why doesn't the galaxies collide due to gravitational attraction and why doesn't all the matter in the universe stick together due to gravitational attraction?
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1answer
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If Earth is mostly fluid, why does it not stretch as it rotates? [duplicate]

Discs are naturally occurring shapes in the world, because any soft object experiencing rotation on a single axis will stretch itself into a disc shape such as pizzas and galaxies due to centrifugal ...
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2answers
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Can one galaxy have a higher dark matter ratio than another?

Dark matter do not interact with ordinary matter and light, it has mass and is affected by gravity. If the distribution of dark matter and ordinary matter is evenly spread at the early universe then I ...
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Galactic Birkeland currents

Does Milky Way or any other galaxy produce Birkeland currents such as seen on Earth in auroras? If yes, how strong would they be? According to this questions answer, galaxies do have magnetic fields. ...
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Is it possible that galaxies formed from incredibly fast neutron star collisions?

In the recent Kurzgesagt youtube video there is mention of how neutron star collisions are the likely candidates for creating the heavier elements in the universe. It is also possible that it created ...
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1answer
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View from inside clashing galaxies

The below image from Hubble show two clashing galaxies. Is there a simulation (or whatever) as how the sky would look withing those galaxies? For example, would the night sky be noticeably brighter ...
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1answer
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How scientists look for anti-matter?

If matter and anti-matter look exactly the same, then how can scientists look for anti matter from a distance (other galaxies)?
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Is dark matter inside galaxies different from dark matter in intergalactic space?

I just read a text about astronomy and when talking about dark matter the author says: [...], the dark matter responsible for the orbits of the stars in the Milky Way is probably different from the ...
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Where can I find the star distribution data of our galaxy?

I am doing some research on dark matter and for that I need data of star distribution in our Galaxy. Is there any mathematical law that predicts the distribution pattern of stars in our Galaxy?
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3answers
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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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2answers
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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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2answers
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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
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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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