The Stack Overflow podcast is back! Listen to an interview with our new CEO.

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].

Filter by
Sorted by
Tagged with
0
votes
2answers
388 views

Why is dark matter called dark matter?

It has been observered that spiral arm formation galaxies exist. This is a problem because due to the laws of gravitation, stars close the galactic center should orbit with a much higher angular ...
4
votes
0answers
62 views

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}...
4
votes
2answers
162 views

Does gravity keep nearby galaxies from flying apart in space expansion?

On a large scale, the universe expansion pulls galaxies apart while gravity keeps galaxies from expanding. So there seems to be a certain scale, at which the expansion and gravity roughly cancel each ...
0
votes
2answers
109 views

Evolution of the virial ratio in $N$-body sim

I have run an 2D $N$-body sim to simulate the cold collapse of a galaxy. Initial conditions: $N=500$, all velocities=0, positions generated randomly within a circle of radius $R$. The system is ...
2
votes
1answer
185 views

Why elliptical galaxies congregated toward the cluster center?

When we look at a rich galaxy cluster, why is it that elliptical galaxies tend to live near the center while spiral galaxies tend to live at the outskirt?
1
vote
1answer
632 views

How to use a galaxy's redshift to measure its distance

I know how we can use the spectrum emitted by a galaxy to measure whether it is redshifted/blueshifted, but out of curiosity, how can the redshift of a galaxy be used to determine its distance from us?...
3
votes
1answer
153 views

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 ...
3
votes
1answer
153 views

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 ...
1
vote
1answer
294 views

How to use parallax and Cepheids together to measure distances?

I can't quite grasp the simple version of how the Cepheid method uses the parallax method to measure the distance to distant galaxies. What I know is that Cepheids exhibit a period vs luminosity ...
1
vote
1answer
101 views

Rotation of our Galaxy's inertial frame

Suppose in the universe, there are inertial frames in the vicinity of galaxies. Suppose also that these frames rotate slightly with respect to each other - that the universe is not quite a 'mill pond'....
0
votes
2answers
82 views

Spin of Supermasive Blackhole

How long does it take for the black hole at the center of our galaxy to make 1 full rotation?
1
vote
2answers
1k views

How the Hertzsprung-Russell Diagram allows us to calculate distance to stars

I understand how to interpret a H-R diagram, in the sense that I know that the upper right top corner is occupied by cool stars, but they are very luminous so they must be big; and the bottom left ...
1
vote
4answers
141 views

Can a super-duper massive black hole eventually eat an entire galaxy?

Might there be some roaming black galaxies out there eating up other galaxies?
0
votes
1answer
176 views

Why does dark matter form a halo, Unlike normal matter? [duplicate]

Hi I am curious to know why dark matter forms a halo? Or why doesn't normal matter form a halo.What is the difference between the two My level is amateur
1
vote
0answers
138 views

Why do galaxies have a super massive black hole at their center?

I know this has now been a common understanding that every galaxy has a supermassive black hole at their centers, But how does this understanding came into existence? Even though these massive black ...
0
votes
1answer
146 views

What models do physicists use in predicting the movement of stars in a galaxy that has lead to a wide-spread conviction that Dark Matter exists?

My (possibly poor) understanding of the argument for Dark Matter's existence is that stars in a large galaxy move more slowly than "they should" (presumably due to either some simplified model of the ...
2
votes
1answer
156 views

Coriolis force effect on spiral galaxies

I know the Coriolis force explains the counterclockwise rotation of the hurricanes hitting the U.S. I wonder if a Coriolis effect also determines (or at least influences) the direction of rotation ...
-2
votes
1answer
81 views

Can we see galaxies that tell us what the Milky Way will be like in future time?

When we are looking at any galaxy we look usually at past time. As in case of Andromeda Galaxy we are looking at 2.5 million years back. In the same way do we have galaxy which will show future ...
0
votes
1answer
225 views

Is it possible for the Oort cloud to account for the Dark Matter issue?

I'm no cosmotolicist, so forgive me if I'm completely off the mark here. A few years back, I read that there is not enough matter in the universe to account for the gravitational forces at play, ...
2
votes
3answers
126 views

Is it possible to estimate the number of galaxies in the universe?

According to this article the observable universe may contain 2 trillion galaxies. Assuming we know the large-scale curvature of the universe thanks to standard model of cosmology, can we estimate ...
1
vote
1answer
57 views

What is “first passage” in orbital mechanics?

Sometimes people talk about the "first passage" of an object falling in toward or beginning an orbit around a more massive object. I'm specifically thinking about this phrase in the context of ...
2
votes
0answers
151 views

Why are all galaxies roughly the same size?

what determines the stable mass of galaxies, stars, star clusters or even galaxy clusters? Is there some obvious way to determine the classes of stable mass clusters from the initial conditions of the ...
6
votes
1answer
149 views

How do we know the actual position of the Andromeda galaxy, if we are seeing 2.5 million years in the past?

Scientists estimate that Andromeda and The Milky Way are going to collide in about 2.5 million years, how accurate is that calculation?
2
votes
0answers
126 views

How does dark matter distribution help to identify it's composition

A question regarding this recently released data, and the paper https://arxiv.org/abs/1708.01538 that details it's distribution: Abstract: We use 26 million galaxies from the Dark Energy Survey (...
1
vote
1answer
37 views

Why is the specific frequency of globular clusters in cD galaxies so high?

I understand that cD galaxies, very large and bright galaxies, have more globular clusters than other galaxies. For example, by calculating the specific frequency for a cD galaxy, the number is ...
3
votes
3answers
531 views

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 ...
2
votes
1answer
97 views

Does the Milky way have relativistic mass against galaxies which are moving away from it at high speeds?

According to the article cited behind, and to a post in Astronomy SE , there are galaxies moving away from Milky Way faster than light, even at speeds of 2.3c . According to this article Can two ...
0
votes
1answer
28 views

Why do galaxies over time become more refined? [duplicate]

Why do galaxies over time become more refined, ordered and defined instead of more random and disordered?
1
vote
2answers
145 views

Can the centers of galaxies act as particle accelerators?

At the centers of galaxies, the amount of dark matter is much higher than that of ordinary matter. So the centers can effectively gravitationally attract and accelerate ordinary matter- without much ...
0
votes
2answers
95 views

Does the orbital velocity of stars in a galaxy equal the rotational velocity of the SMBH at the centre?

Pretty straightforward, couldn't find anything on Google, was hoping someone might know. Thanks in advance.
1
vote
1answer
225 views

Why isn't Dark Matter distributed among ordinary matter?

DM: dark matter; OM: ordinary matter I know that DM has a distribution which is mostly concentrated in the center of galaxies. This was mainly deduced by accounting for gravitational effects on the ...
16
votes
1answer
1k views

When galaxies collide, what happens to dark vs. light matter?

I have read that galactic collisions have been used as evidence demonstrating the effects of dark matter. In particular, that luminous matter slows down and interacts while dark matter shoots through, ...
9
votes
3answers
283 views

Why are stars spherical whereas (some) galaxies are disks?

I read here that galaxies become disks if there is a lot of gas in them, since their angular momentum is conserved while their energy decreases due to collisions of the gas particle. I have two ...
7
votes
1answer
120 views

Do galaxies have a halo of neutrinos and cosmic microwave background?

If virial arguments as in "How can dark matter collapse without collisions or radiation?" allow concluding that dark matter could collapse to galactic halos purely gravitationally, then is this true ...
4
votes
3answers
157 views

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 ...
3
votes
1answer
106 views

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 ...
2
votes
1answer
278 views

Is the expansion of space universal or local?; does matter impede the expansion of space?

Michael Strauss in his recent book "Welcome to the Universe" with authors Tyson & Gott describes that space is NOT expanding within galaxies but rather between galaxies. So then the expansion is ...
1
vote
2answers
270 views

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?

It is my understanding the best evidence we have for Sgr A* being the black hole at the center of our galaxy is the incredible velocities of the stars orbiting around it. But wouldn't the stars ...
3
votes
1answer
724 views

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 ...
-3
votes
1answer
400 views

What is the percentage of dark matter in the galaxy? [closed]

Assuming the mass of the stars and gas within NGC 5055 is $8\times10^{10} M_{sun}$. What is the percentage of the dark matter in it?
1
vote
2answers
79 views

From a planet in one of our satellite dwarf galaxies, would one be able to see the Milky Way?

Say there was a lifeform not unlike ours native to a planet somewhere in one of our nearest satellite galaxies, say Sagittarius, during their nighttime would they be able to see the Milky Way brightly?...
1
vote
0answers
62 views

Thin Disk Potential-Density Relation

Suppose that there is an infinitesimally thin disk in the plane $z=0$ with surface density $\Sigma(x,y)$. How do you show that the potential $\phi(x,y)$ in the disk satisfies $$\Sigma(x',y') = \frac{...
0
votes
1answer
71 views

Orbit of stars in a galaxy

Do all stars orbit around the galactic center? If yes what makes them orbit around the center , what object creates such a massive force which makes stars millions of light years far away from the ...
5
votes
1answer
244 views

Distribution of orbital velocities in a disk galaxy for N-body simulation?

I'd like to write an N-body simulation in which I collide two disk galaxies. To give you an idea of the accuracy I'm trying to achieve, I'm aiming to make this my screensaver at 30fps on my work ...
4
votes
1answer
135 views

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 ...
2
votes
1answer
200 views

In large scale cosmological structure, are filaments, walls, sheets and nodes different terms for the same things?

As the question says - are these all essentially synonyms referring to the same phenomenon (described with different words depending on the specific superficial appearance)? Or do they convey a real ...
5
votes
3answers
528 views

Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?

Galaxy one is located in a dense area of the universe and galaxy two is located in a less dense part of the universe. Would galaxy one appear red-shifted to galaxy two? Is the mass density at our ...
1
vote
1answer
31 views

Criterion for galaxy mergers, negative E?

I'm writing about galaxy mergers and came across this section in a book describing criteria for a merger. I don't understand how there can be a negative $\hat{E}$ yet this is the region where mergers ...
2
votes
1answer
184 views

On the trajectory of the Andromeda-Milky Way collision

The Andromeda-Milky Way collision is going to happen in approximately 4 billion years. What trajectory would the Andromeda galaxy follow on its path to collision with the Milky Way? How could this be ...
0
votes
1answer
320 views

Flat rotation curves and gravitational potential

I have been reading about spiral galaxies rotation curves, and I have a question I would like to clarify. For example, many of them have flat rotation curves after some characteristic distance $r>...