# galaxies's questions - English 1answer

384 galaxies questions.

### 1 How do we know that galaxies are not in the process of flying apart?

A major reason for believing in the existence of dark matter is that the stars in the outer regions of galaxies are moving too fast to remain bound to their galaxies, given only the estimated mass of ...

### 6 Why doesn't the light from galaxies appear stretched? [duplicate]

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this: Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years ...

### 1 What is Cosmic Downsizing?

I've had a quick look at a few lecture pdfs and papers as supplementals to my own given lecture notes, but I can't seem to get a proper explanation for what cosmic downsizing is. The closest ...

### 34 What is the evidence for a supermassive black hole at the center of Milky Way?

Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific ...

### 2 Intuitive understanding of the virial radius/mass

1 answers, 35 views cosmology galaxies virial-theorem
I'm currently trying with no avail to understand the intricacies that define the virial radius or mass in a way that is different from the actual galactic radius/mass. I understand it is derived in ...

### In theory, can we count every galaxy in the observable universe one-by-one?

Suppose I have a very advanced telescope that can detect long wavelengths of light, so I can deal with extreme redshift. I am also a very quick counter, so I can get through all of the galaxies before ...

### 2 Understanding the tidal force tensor

I'm just looking to gain a basic intuitive understanding of the forces at work in tidal stripping and their direction/motivation etc. I'm struggling to grasp the underlying principles and it is making ...

### 1 Mass-to-light ratio and rotation curve from brightness profile

This should probably be basic but I've been looking for days and I can't find how to (I'm probably over complicating, but still). I want to calculate a rotation curve for some spiral galaxies. From ...

### 3 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 ...

### 16 How did we 'discover' dark matter? [closed]

I'm an astrophysics student and I've been researching this topic and there is one point that keeps eluding me. How did the scientific community realize that there had to be dark matter in the ...

### What is the meaning of 'systemic velocity' of galaxies?

I came across the term systemic velocities of galaxies. Can you please explain what it means?

### -1 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 ...

### 4 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 ...

### Estimating the age of the Fermi Bubbles

Assuming the Milky Way’s Fermi Bubbles were caused by a single event near the galactic center, what are the limits to when that event could have happened? The bubbles reach 25,000 light years high, ...

### 2 What is the current shape of the Milky Way?

1 answers, 80 views astrophysics galaxies milky-way
The things in the universe which are thousands of light years away from Earth can be seen or pictures can be taken by Hubble Telescope because those stars, galaxies etc emitted light thousand, ...

### 3 Is the gravitational effect of distant galaxies lost forever?

Hubble's law is usually expressed by the equation $$v = H_0D$$ According to this equation, the space between us and very distant galaxies, is expanding with a speed greater than the speed of light ...

### 4 New observation of a Galaxy without dark matter: what is(are) the implication(s)?

1 answers, 130 views cosmology astronomy dark-matter galaxies
This recent paper in Nature (also see here$^1$) claimed to have discovered a galaxy without dark matter. However, in the Standard paradigm of cosmology dark matter played a crucial role in the ...

### Are galaxies merely groups of nearby stars that collected together, or do they have a central birth? [duplicate]

The news, that some galaxies have dark matter and others do not, leads to the question of how this could be possible. If galaxies are merely groups of stars that gathered together (under ...

### 1 Is there a relationship between a stellar-mass black hole and its host galaxy?

0 answers, 33 views black-holes astrophysics galaxies
Out of curiosity, are there any relationships between stellar-mass black holes and their host galaxy? I am asking because I have read about how there is a relationship between a supermassive black ...

### 2 Convert from Relative Magnitude to Mass

I have data which gives me the magnitude density (${\rm mag}\,{\rm arcsec}^{-2}$) of M31 as a function of radius. How can I convert these data to the (enclosed) mass at a given radius (for velocity ...

### -1 Sun-path on the sky [duplicate]

2 answers, 41 views gravity earth sun galaxies
We are standing on the Earth and we see Sun on the sky. We see it walking in a certain direction, with predictable moving. Earth is a spheroid, as we all know. It is circling around the Sun, and also ...

### 9 How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?

The actual number: How far apart are galaxies on average? An attempt to visualize such a thing: If galaxies were the size of peas, how many would be in a cubic meter?

### Original distance from us to galaxy at redshift of z=6 if we know the current distance and taking into account universe expansion?

So I'm given that a galaxy cluster is currently at redshift z = 6 and the question asks "what was its distance from us at the time the light left the cluster?" We only know the redshift, but the ...

### 9 What is the density of stars near the center of the Milky Way?

2 answers, 6.815 views astronomy galaxies 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, near the galactic center, would the night ...

### 2 How are velocity and dispersion maps of galaxies made?

How can I get the velocity dispersion and velocity maps of galaxies given 3D data cubes obtained using an integral field spectrograph?

### 12 If our galaxy's dark matter halo is so large and diffuse, why is the ordinary matter in it so much more localised and compactly located?

I just read that our galaxy's dark matter halo is estimated to be 1.5m ly across, compared to the visible galaxy's 100k ly across, needed to explain stellar rotation curves. Why would this be? By ...

### Would calculating the relativistic mass of every particle in a Galaxy account for dark matter?

1 answers, 46 views mass dark-matter galaxies
I have seen explanations elsewhere that stars' relativistic velocities in a Galaxy would not account for the missing mass of what we call "dark matter", but in order to get a total relativistic mass ...

### Extragalactic Magnetic Fields - General Information That I'm Missing

I have started studying Extragalactic Magnetic Fields, and mainly focusing on Biermann Battery Theory. I have gathered much information on other Dynamo Theories, as well as about our current ...

### Dark matter halo distribution for simple galaxy model

It is a sufficiently well-known fact that the rotation curves of spiral galaxies are approximately flat in the middle to far regions of the disk. This is in apparent contradiction with the fact that ...

### 5 AGN accretion disk vs. torus

2 answers, 474 views galaxies
The torus is the donut of dust encircling the Active Galactic nucleus. The accretion disk is inside the torus. Is there a boundary between the two? At what point does a torus become an accretion ...

### If the Universe is expanding, then why will Andromeda collide with the Milky Way? [duplicate]

Andromeda is moving towards the Milky Way Galaxy at around 250,000 mph, but I wonder what is the direction of our Milky Way Galaxy? Are these two galaxies heading towards each other or Andromeda will ...

### 4 Gravothermal catastrophe: looking for simple explanation

I am beginning to try to understand the gravothermal catastrophe. I was hoping someone could provide an explanation to help me understand what the gravothermal catastrophe is and why it is important, ...

### 4 What objects/states of objects with absolute magnitude do we know of?

For measuring distances the knowledge of absolute magnitude or luminosity is often crucial, especially for very big distances. Unfortunately we can't measure the diameter of far distant objects and ...

### 8 Why does Omega Centauri have a distinct chemical signature from the rest of the Milky Way?

1 answers, 152 views astronomy galaxies elements star-clusters
In answering a question about the orbital path of Omega Centauri, I learned that it has a distinct chemical signature from the rest of the Milky Way. Basically, it is very rich in s-process elements, ...

### 3 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 ...

### 3 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?

### 4 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 ...

### 44 Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?

Question: As we know, (1) the macroscopic spatial dimension of our universe is 3 dimension, and (2) gravity attracts massive objects together and the gravitational force is isotropic without ...

### 2 Galaxies and projected radius vs arcmins

I have two sets of data (globular clusters), one for the Andromeda Galaxy and one for the Milky Way. I want to compare the distribution of globular clusters between the two galaxies according to ...

### 7 Applying the Maxwell–Boltzmann statistics to astrophysical objects

Quoting Wikipedia: In statistical mechanics, Maxwell–Boltzmann statistics describes the statistical distribution of material particles over various energy states in thermal equilibrium, when the ...

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

1 answers, 26 views galaxies star-clusters
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 ...

### 4 Dimension of galaxy filaments

0 answers, 96 views cosmology galaxies structure-formation
Galaxy filaments are "amongst the largest known cosmic structures in the Universe. They [...] form the boundaries between large voids in the Universe." As their name suggests, filaments are ...

### 8 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 ...

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

1 answers, 141 views universe collision galaxies
Scientists estimate that Andromeda and The Milky Way are going to collide in about 2.5 million years, how accurate is that calculation?

### 1 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 ...

### 8 Is there a theoretical maximum “size” for galaxies?

1 answers, 201 views mass astrophysics galaxies
I am aware of upper limits to the mass of burning stars and neutron stars, beyond which they degenerate into neutron stars and black holes respectively. And also, if I understand it right, there is a ...

### 12 How Does Hubble's Expansion Affect Two Rope-Tied Galaxies?

Suppose we have two galaxies that are sufficiently far apart so that the distance between them increases due to Hubble's expansion. If I were to connect these two galaxies with a rope, would there be ...