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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Why are jets not commonly found in spiral galaxies?

Jets and radio lobes are generally associated with elliptical galaxies. Why not spirals?
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What is the distribution of Population I and II stars in the Milky Way galaxy?

I have been trying to find out the distribution of Population I and II stars in the Milky Way. The distribution I mean is the percentage of each population to the total stars in the galaxy. So in ...
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Why do galaxies “disappear?”

So, this bit of information confused me lately. Before, I figured galaxies were no longer visible by us because their luminosity decreased in an inverse square manner. However, while watching a movie ...
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Direct Dark Matter Detection: relative velocity between WIMPs & Nuclei

In direct dark matter detection it is said that the relative velocity between the WIMPs, which form a DM halo, and the target nuclei on earth is of order $100 \frac{\text{km}}{\text{s}}$. How does one ...
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When will galaxies become unstable due to dark energy?

When will galaxies become unstable due to dark energy? What will it be like for a civilization to observe their own galaxy going through this period of instability?
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Olbers Paradox Solution

Olbers’ Paradox says that in an infinite universe every line of sight will end on a star. Surface brightness is independent of distance (moving a star further away makes it smaller and reduces its ...
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Baryonic Missing Mass

A recent article from a popular astronomy website tells of discovery of missing mass (not dark matter) that has puzzled astronomers for some time. Apparently, the discovery involves enhanced electron ...
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Gamma Ray Bubble at the center of our galaxy seen by Fermi Telescope

How could we measure high energy photons, whithout measuring them ? I can't understand how we can "see" those Gamma Ray Bubbles if they are not reaching here In this graph from Nasa you can see ...
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Frame dragging resulting in an orbital plane?

In astrophysics today we talked about spinning black holes, ring singularities, and frame dragging. Is this also (to some degree) the cause of the milky way being as flat as it is? Does the spin of ...
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What sets the lower mass limit for dark matter dominated structures?

Large galaxies appear to be dominated by dark matter, as do dwarf spheroidal galaxies with masses of perhaps $10^6 {\rm M}_{\odot}$. But (as far as I'm aware) there is no evidence for dark matter in ...
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Thought experiment: Tethered galaxies - to the extreme [duplicate]

If two rocks were tied together with a tight, absurdly long, non-elastic rope, and placed on planets at either end of Earth's observable universe - or beyond - What would happen? Is the structural ...
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Accuracy and assumptions in deriving the Tully-Fisher relation

I understand the mathematical derivation of the Tully Fisher relation from basic physics formulas, as shown on this site. However, after using the physics equations, it seems that several assumptions ...
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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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What happens in the event that the cooling radius is shorter than the virial radius of a Cold Dark Matter Halo?

The cooling radius of a cold dark matter halo is defined to be the time at which the cooling time $t_{cool} = t_{free fall}$ where $$t_{cool}=\frac{\rho \varepsilon }{\Lambda \left ( T \right )n_{H}...
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Current constraints on Dark Matter self-interaction from galactic profiles

The self-interaction of dark matter may be small but it cannot be negligible if it is able to dissipate energy to relax into galactic clumps (necessary to explain galaxy rotation curves). According ...
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Why is the Milky Way flat? [duplicate]

I read recently that the galactic "flatness" of the Milky Way is due to the rotation of the galaxy combined with a vast stretch of time. Yet, I also read where 1) the Milky Way rotates once every 225 ...
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How can a black hole zap a galaxy into existence?

I am referring to this picture published here. Apparently super massive black holes emit radiation and matter in astrophysical jets. And these jets can form galaxies. I have some questions: Isn't ...
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How are galaxy filaments formed? And do they have any analogues in stellar formation?

In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like ...
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Why don't black holes within a galaxy pull in the stars of the galaxy?

visit http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-a-black-hole-k4.html If black holes can pull even light, why cant they pull the stars in the galaxy?
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How is a star's parent galaxy recognized?

A star is probably visible/detected by it's radiation. But that star may or may not belong to our own galaxy ... yet news reports speak of detecting a star/nova in a distant galaxy. How does one ...
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“Mass Discrepancy-Acceleration Relation” in ΛCDM Paradigm

Rotation curves of galaxies seem to require more mass than observed - the conventional solution is the introduction of Dark Matter. Reading some articles on this topic (in particular a recent ...
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Measuring more accurately the distance of remote galaxies

From what I read in Wikipedia, the velocity of a Galaxy has two components: one is due to Hubble's law for cosmic expansion, and the other is the peculiar velocity of the galaxy. Since the peculiar ...
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Frame dragging — is there a “non-tiny” example?

Now. As I understand it, in fact, the earth (10^25 kg) creates a very small, very tiny, frame dragging effect. Indeed, we have measured this using satellite experiments. So, the Earth (10^25 kg) ...
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463 views

Sampling a distribution (from a galaxy model)

I am reading the following article: http://www.kof.zcu.cz/st/dis/schwarzmeier/galaxy_models.html and am currently at section 5.6 (positions of bodies in a galaxy). I am trying to redo the simulations ...
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Dark Matter and Rotational Velocity Curves of Galaxies and Stellar Systems

The way dark matter is introduced to documentary watchers such as myself is to account for the difference between the rotational velocity of the stars in a galaxy compared to that of planets in ...
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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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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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Why sun has a velocity with respect to the dark matter halo?

I was reading that Sun has a velocity with respect to the local dark matter halo of about 244 km/s. Both sun and the dark matter halo revolve around the Milky way. Since the local dark matter halo ...
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What is a “scale length”, and how do I calculate it for galaxies?

I'm trying to work through, understand, and apply concepts regarding mass models of galaxies. Looking at the Hernquist model, I'm finding the equation $$Φ(r)=−\frac{GM}{r+a},$$ where a is the scale ...
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How can the Andromeda Galaxy alone have more mass than the Local Group?

I was reading about the Local Group, and it is stated that the total mass of the group is $1.29±0.14 × 10^{12}$ Solar masses. The number actually felt a bit low to me because I know the Milky Way ...
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What is the Schechter luminosity function's domain of support?

I'm trying to fit a Schechter luminosity function to some data points, but it's not clear from this definition what the domain of support of the PDF should be. I'm familiar with the standard Pareto ...
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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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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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Why did the universe continue to expand from inertia after inflation stopped, if galaxies have no motion but space between them expands?

I’ve read many answers on this site about space expansion and I will write from what I understand so please correct me if I’m wrong. The space between galaxies expands but the galaxies themselves are ...
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Is there 'gravitational force of repulsion'?

According to hubble's law of universal expansion, the velocity of a galaxy moving away from ours is directly proportional to the distance between the two. Now velocity is increasing in direction away ...
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1answer
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Where are all the Population III galaxies?

We can now see back to 400 million years after the Big Bang. Yet we still haven't seen any Population III galaxies. Why is this? How do metal-rich galaxies evolve in just 400 million years?
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Are we closer to an accurate 3 D image of our galaxy using the Gaia billion star data set?

From the BBC: Report on Gaia Mission Data 14th September 2016 Astronomers working on the Gaia space telescope have released a first tranche of data recording the position and brightness of over a ...
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Is there a possibility of anti-dark matter?

Based on Galactic Gamma Ray Emission My question is simply put. Although dark matter itself is almost a complete mystery, has anything been published which postulates the existence of anti-dark ...
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1answer
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Looking for a resource that explains all colors and shapes of stars and galaxies

I've been looking at some of the astronomy pictures of the day from NASA. They all have content that is indicative of certain situations or events. Supernovas have the scattered lines look, new stars ...
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Could there be any alternative to a supermassive black hole that might explain Sgr A*?

I am aware of How do we know the stars orbiting Sgr A* are orbiting a supermassive black hole and not just the center of mass of the Milky Way galaxy?, which asks why the motions of stars near the ...
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A paradox about distant galaxies [duplicate]

When we observe a galaxy farther than 13 billion light years away, we see that galaxy as it was 13 billion years ago. But back then, that galaxy was much closer to us ,if indeed we live in an ...
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Potential of an axisymmetric disc with constant rotation velocity

I am having trouble understanding why the form of the 3D potential for a disc with a constant rotation velocity for circular orbits of stars within the disc \begin{equation} v(R) = v_0, \tag{1} \end{...
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What is the average mass of galaxies according to Hubble Deep and Ultra Deep field observations?

It is very widely known among people interested in astronomy that there are 100-400 billion stars in the Milky Way galaxy and there are ~ 100 billion galaxies in the observable universe, which is ...
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Does the rate of expansion of the universe exceed the rate at which the local group is being pulled toward the Great Attractor?

I understand that the expansion rate of the universe driven dark energy exceeds the rate at which the local group is being pulled toward The Great Attractor (Hydra-Centaurus Supercluster). Does this ...
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Hypervelocity black holes

Is it possible for a black hole to be ejected from a galaxy after an encounter with the central supermassive black hole? What would be the impact of the passage of the hyperV-BH through the galaxy? ...
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Galaxies Center

It is believed (to my understanding) that at the center of all large galaxies are super massive black holes. Why is it then when you see photos of galaxies that the center is extremely bright if a ...
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What determines a progenitor's fate as a spiral or elliptical?

I was thinking about my answer to Are the inner planets on planar orbits because there was more dust in the inner solar system (early on in planetary accretion)? - when it occurred to me that maybe I ...
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What forms first: the dark matter halo or the galaxy?

I read that in the early Universe dark matter forms halos, and dust and gas inside those halos formed the first ever galaxies. I am curious to know whether the dust and gas formed the galaxy inside a ...
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Could the estimated stellar mass for the Milky Way galaxy include brown dwarfs?

Trying to find an estimate for the stellar mass of the MW galaxy, I found this paper and the estimated stellar mass is $~6.5 \times 10^{10} M_{\odot}$. I was also trying to understand the methods used ...
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Distance from redshift

I am looking for a exact derivation of a relation between redshift $z$ and distance $d$. What I know is the definition $$z=\frac{\lambda_{\text{observed}}}{\lambda_{\text{unshifted}}}-1=\sqrt{\frac{1+...