Massive systems held together by gravitational attraction, consisting of stellar associations, star systems, star clusters and types of interstellar clouds, stellar remnants, gasses, cosmic dust and an important but not yet completely understood component called dark matter.

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What is the meaning of a negative Galactic longitude?

What is the meaning of negative longitude in Galactic coordinate system? Does the longitude $-65^\circ$ equal $295^\circ$?
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57 views

Why does most galaxies resemble a 2 dimensional plane? [duplicate]

Older galaxies mostly tend the revolve forming a disk. Why are the stars not revolving around the center of the galaxy like a sphere instead similar to electron in an atom?
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7k views

Why don't galaxies orbit each other?

Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered ...
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1answer
34 views

What is exactly the “progenitor bias”?

I am taking a course in astrophysics and my teacher mentioned different biases that are present when taking a sample of galaxies: the progenitor bias and the Malmquist bias. I understand very well the ...
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1answer
293 views

Relation between isophotal radius and virial radius in spiral galaxies?

Is there any (proposed) relation between the $B$-band isophotal radius of a spiral galaxy and its virial radius ($R_{200}$)? If you know of such a relation, please post a reference paper.
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1answer
57 views

Need help with determining the total mass using the NFW profile

My review assignment has a question that asks us to use the Navarro–Frenk–White (NFW) profile to find total mass in the galaxy using $$\rho(R)=\frac{\rho_0}{1+\frac{R}{R_c}}$$ then taking a triple ...
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1answer
805 views

If we were to travel through space (sci-fi style), how close to the false-color images would the galaxies we see be?

I understand that the black-and-white images you see looking through a household telescope are only like that due to the intensity of the light that reaches us, and that most of the astronomy images ...
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5answers
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2answers
53 views

Why do galaxies in the Local Group not obey the Hubble law?

The way I understand it is that the galaxies in the Local Group have a gravitational attraction that causes a relative velocity inconsistent with the recessional velocity predicted in the Hubble law. ...
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1answer
271 views

About the hump on galaxy rotation curves

The past days I have been studying the rotation curves of disk galaxies and I am currently trying to understand how we can extract information about the dark matter of a galaxy by looking its rotation ...
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111 views

Relation between dark matter and supermassive black holes located in the center of galaxies

This research (see press article and technical paper), conducted on 3000 elliptical galaxies (which are typically galaxy merger results), reports finding "distinct relationship between the mass of the ...
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0answers
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Observationally distinguishing a galaxy of antimatter from a galaxy of matter [duplicate]

I was just wondering how, observationally, we would distinguish a distant galaxy of "normal" matter from one of antimatter. Maybe there is a simple answer but I don't see it. Once I started thinking ...
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1answer
44 views

Do satellite galaxies have the same proportion of dark matter as “ordinary” galaxies

My question is relatively straightforward: Do we know if satellite/ dwarf galaxies contain the same proportion of dark matter to ordinary matter as "regular" sized galaxies? The Milky Way, for ...
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1answer
32 views

Do AGN produce protons that are relativistic enough to collide with the CMB and make pions?

AGN (Active Galactic Nuclei) produce protons in their jets and they are relativistic. I was reading about photo-pion production, where a proton and photon annihilate to produce a pion. Could this ...
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1answer
107 views

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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2answers
113 views

How are stellar and galactic ages determined?

Being a 4th year undergraduate physicist, you'd think I'd know this! But it's never really taught in any detail, just vague mentions of metallicities and, in galactic case, redshift. So how exactly ...
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2answers
130 views

Relativity and Galaxy Rotation Speed

If time travels slower nearer gravity wells, why can't the galaxy rotation speeds being faster on the outer edges than the inner areas be explained by relativity? What necessitates dark matter?
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0answers
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About the use of Newtonian Relations for the movement of stars in the Galaxy [duplicate]

From a General Relativity point of view Gravity is given as the result of spacetime curvature interacting with energy-mass density. To get to the Newtonian limit one needs to take a) Non-...
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1answer
44 views

Surface brightness

I have to compute the surface brightness as a function of the radius from the following set of data: {right ascension ($\alpha$), declination ($\delta$), magnitude (m)}. I also know the center of the ...
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0answers
53 views

The reason for the fading of galaxies?

Galaxies that are greater than a distance of c/H metres from Earth have recessional velocities exceeding the speed of light and begin to fade. Thus, theoretical astronomers 3 trillion years the future ...
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3answers
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Gross “temperature” of a globular cluster

Globular clusters can be very large, which means we can do statistics about the stars in them. And that means we can try matching their star-as-particle potential/kinetic energy distribution against ...
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1answer
1k views

If matter and antimatter repel, how do we know other galaxies aren't made of antimatter? [duplicate]

Wikipedia's article on antimatter says this: There is considerable speculation as to why the observable universe is composed almost entirely of ordinary matter, as opposed to a more even mixture ...
2
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1answer
33 views

Stars more affected by gravity of stars further away, need help understanding argument why

In the first chapter of Binney and Tremaine's "Galactic Dynamics" there is a statement that I don't yet understand. The first chapter of the book is linked here, straight from the publisher. The ...
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0answers
45 views

Gravity's effects on time

So, in the movie Interstellar they say that one year by the black hole is about 35 years back on Earth (excuse any lack of accuracy in the numbers, I haven't seen the movie in over a year). Now, the ...
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2answers
130 views

Which direction does a galaxy move in respect to its axis of rotation (Black Hole)

As Galaxies travel through the universe, how do they orient? And, does this orientation apply to stars and their satellites? that is to ask if the movement of a galaxy or star is perpendicular to ...
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2answers
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Density of stars near the center of the Milky Way

At night, I can look up and see the Milky Way across the sky. My question is, supposing our solar system was, instead of way out on an 'arm' of the galaxy, if we were near the galactic center. Would ...
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0answers
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Binding energy of a galaxy?

I'm a novice in physics and new to the forms, so please forgive me if this is a dumb question or if this is in the wrong section to be posted, but how would one find the gravitational binding energy ...
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1answer
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Why don't the black holes appear black in color in images of galaxies taken from HST?

According to NASA a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area According to the documentary Space Unraveling The Cosmos about ...
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2answers
77 views

The force causing galaxies to accelerate away from each other?

The galaxies in the universe appear to be accelerating away from each other according to the Doppler shift that is present in the light we receive from other galaxies. According to Newtonian ...
4
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1answer
48 views

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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0answers
48 views

Rotational axes of galaxies

The spiral shape is a preferred form for galaxies due to angular momentum, and the axis of rotation indicates a locally preferable orientation for the angular momentum. Why do galaxies in Hubble ...
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4answers
77 views

Structure formation: Galaxies as different entities and stars as groups

Why do stars exist in groups like galaxies rather than individual stars, and why do galaxies exist as different entities without continuity and with large intergalactic spaces?
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0answers
25 views

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 $10^6 M_{\odot}$. But (as far as I'm aware) there is no evidence for dark matter in ...
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0answers
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Where can I find data for the masses/component velocities of galaxies in the Local Group?

I am trying to run an N body simulation to see how galaxies in the Local Group gravitationally interact for my thesis, but I can't find any data for the masses of the galaxies in the Local Group, or ...
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1answer
139 views

How many galaxies could be the source of the recent LIGO detection?

The recent LIGO detection is pretty exciting, and a lot of people are asking whether there is a chance of optical detection of the black hole pair that created the signal. From a cursory reading of ...
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5answers
288 views

Is dark matter really there? [closed]

Dark matter has been proposed to explain two phenomena - Uniform velocity curve of spiral galaxies Observed gravitational lensing. If something explains these two, would we still need dark matter?...
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1answer
56 views

Questions about shape of dark matter orbits in spiral galaxies

Could someone please describe how are the proposed (calculated) orbits of dark matter? Are they exactly as those of baryonic matter (i.e. spiral), or they are different from those of baryonic matter? ...
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3answers
120 views

If our eyes see at the speed of light, how do we know about the current states of stars and galaxies far away?

As you guys know that we see at the speed of light, it means that we see the past of stars and galaxies. So say a star went supernova right now, how are we able to know current state of that star that ...
3
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1answer
131 views

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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5answers
9k views

Why aren't there spherical galaxies? [duplicate]

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out ...
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1answer
83 views

Mass and time relativity [duplicate]

In the space between galaxies in the absence of matter, would time run faster than inside a galaxy? A black hole can slow time. Is there a cosmic opposite of that effect on time? Like the following ...
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3answers
472 views

Are there models/simulations of antigravitational antimatter-galaxies?

In the comments to another question's answer, I started wondering: Assuming antimatter possessed negative gravitational mass§ (which is not proven impossible to date, though deemed unlikely), ...
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0answers
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How to determine the size of other galaxy's/The dark matter problem

From what i understand there are two methods (well as far as i know) to determine how much mass there is in a distant galaxy. First, Luminosity. By measuring the amount of light produced from a ...
5
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1answer
190 views

What is the limit for the size of a galaxy?

The largest known galaxy is just 100x more massive than Milky Way. I wonder if there is some physical mechanism (some equilibrium) which limits the size of galaxies or if it is just because of limited ...
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1answer
74 views

What is the limit of the deep field exploration?

The deep field exploration is quite fascinating, first the Hubble Deep Field (HDF) showing a small piece of the Univers one billion year after the Big-Bang, next the Hubble Ultra-Deep Field (HUDF), ...
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0answers
40 views

How are planets distributed in elliptical galaxies?

I'm trying to figure out if planets are equally distributed in galaxies (I'm assuming they aren't) or if there is some logic behind their distribution.
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1answer
848 views

$N$-body simulation in General Relativity

How would one perform an $N$-body simulation in General Relativity (GR) for something like galaxy formation or galactic dynamics? Suppose one wants to simulate the rotation curve $v(r)$ for galaxies ...
2
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1answer
169 views

Do the arms of a spiral galaxy spin around like a candy bar in long spirals?

Because the arms of a spiral galaxy have a cylindrical-linear center, perhaps they have a cyilindrical rotation around their center same as a vortex of smoke? Have similar movements within galaxies ...
0
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1answer
95 views

Bigger Galaxies = Faster Clocks [closed]

There are in all about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. We have Blue-shifted galaxies are in our own local group, and are all bound to ...
2
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3answers
109 views

How big bang could produce later collisions

The Milky Way and the Andromeda galaxies are supposed to "collide" in 4 billion years (collide in the sense of overlapping space, but nothing is really supposed to contact anything else). Assuming: ...