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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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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245 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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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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62 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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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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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
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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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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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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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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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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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Do irregular galaxies host a supermassive black hole?

Do irregular galaxies (eg the LMC) host a supermassive black hole?
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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
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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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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
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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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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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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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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
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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
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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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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
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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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How can we detect antihydrogen?

From a mathematical standpoint (CPT symmetry) it is most probable that antihydrogen has the same spectra (absorption and emission) as hydrogen. The CERN confirmed this hypothesis to a high accuracy ...
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1answer
69 views

Why does the jet come out perpendicular to the accretion disc?

Why does the jet always come out perpendicular to the plane of the accretion disc around a black hole?
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1answer
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How to calculate the strength (“space force”) of dark matter within galaxies?

Let's first have a look at the observed speed within galaxies (source): On the left side we can see the speed of celestial bodies within galaxies derived by gravity only (gravity is usually created ...
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2answers
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Andromeda & Milky Way Merger: Gravitational Waves

When the Andromeda galaxy and Milky Way merge in the future, the super-massive black holes at their respective galactic centers will likely eventually merge. Similarly to the gravitational waves ...
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Highly inclined galaxies

I found in the literature that galaxies with an inclination angle relative to the line of sight greater than 60 degrees are considered highly inclined. Does anyone know why this particular angle is ...
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Are there any databases for astronomical measurements (specifically amount of dark matter per galaxy)?

I was looking online for amount of dark matter in certain galaxies. I couldn't find any data on google. Does any one here know a trusted database which contains amount of dark matter in galaxies? ...
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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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1answer
109 views

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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1answer
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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
44 views

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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1answer
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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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1answer
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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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2answers
114 views

Dynamical mass of a 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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