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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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Is the Maffei 1 galaxy gravitationally bound to the Milkway Galaxy?

I watched a Kurzgesagt video explaining how the limit human exploration is the local group, because outside galaxies are accelerating away from us faster than the speed of light, (or faster than we ...
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326 views

Is there a theoretical maximum “size” for 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 ...
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1answer
99 views

Effects of dark matter

I was wondering, when sims of galaxies show that we need dark matter to account for the movement of outer stars, are models taking into account relativistic effects on gravity? The tips of spiral ...
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1answer
65 views

Do attraction between two galaxies count for dark matter?

We know that the uniform rotation curve of individual spiral galaxies does require presence of some form of dark entity (matter). Does the attraction between two (or more) spiral galaxies also require ...
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1answer
35 views

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 ...
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1answer
91 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'....
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1answer
53 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 ...
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1answer
36 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 ...
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1answer
51 views

Could space repel mass?

I'm not an advanced physics person, so any explanations need to be reasonably low-level... I was looking at oil/water bubbles (hydrophobic vs hydrophylic molecules) and it got me thinking about the ...
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1answer
34 views

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 ...
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416 views

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?
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54 views

Estimating total mass inside the Sun's orbit

I'm trying to estimate the local density of dark matter at a given radius, $r=R_0=8\text{kpc}$. I also have values for the core radius, $a=5\text{kpc}$, and circular velocity of the Sun, $v_{\text{...
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66 views

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 ...
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1answer
40 views

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 ...
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1answer
65 views

Galaxy equillibrium

I know this is a very dumb question, but, to maintain a galaxy as a bounded system, the total energy and the total angular momentum of every particle component of the galaxy (suppose made of gas and ...
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1answer
92 views

Has galaxy merger and universe expansion been proven?

I understand the basic argument for hierarchical merging of galaxies during expansion of the universe. I also understand peculiar motion and the Hubble flow. So I understand what's going on with ...
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1answer
65 views

Synthesising types of galaxies using various stellar spectra

I have been given the task of synthesising an elliptical galaxy, a starburst galaxy and a spiral galaxy (considering the arms and the bulge separately and adding them) using stellar spectra which I ...
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127 views

Rotational axes of galaxies

The disc shape is a preferred form for galaxies due to angular momentum, and the axis of rotation indicates a locally preferred orientation for the angular momentum vector. Why do galaxies in Hubble ...
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146 views

Spatial dimension of galaxy filaments

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 ...
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60 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}...
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54 views

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 ...
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0answers
88 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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0answers
41 views

Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
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0answers
81 views

Distribution function independent of $\boldsymbol\theta$ from Jeans theorem?

In Binney and Tremaine, 2008 2nd ed pg418 the authors state: According to the Jeans theorem, the equilibrium DF may depend only on the actions; thus $f_0 =f_0(\mathbf{J})$. I have problems with ...
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148 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 ...
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0answers
124 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 (...
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0answers
448 views

Calculating photon number density inside a volume on whose surface the energy flux is known

I have the power spectrum for a galaxy of radius $R$, which we can approximate as a sphere at some distance $D$. Now, on earth we can measure the power spectrum which is given as a list of tuples, ( $...
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358 views

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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0answers
140 views

Bulge-Disk decomposition in galaxies

I'm starting a new project on dark matter astrophysics in which I have to extract from some data (I will write some below) the surface brightness, to later construct the mass density profiles of de ...
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0answers
290 views

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 ...
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0answers
56 views

How to derive the gravitational potential on an infinitely thin disc of finite radius?

I am stuck on a problem that requires me to calculate the gravitational potential on an infinitely thin disc of finite radius $R$ with mass $M$. This infinitely thin disc models a galaxy. Once I have ...
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0answers
39 views

Does a disk galaxy wobble (galactic wobble)?

The center of mass of the solar system occasionally wanders up to 1/2 the solar diameter outside the sun. The sun rotates around this CM. Does something similar occur in spiral galaxies? If so, ...
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0answers
19 views

Population histograms and relationship between size of galactic disk and amount of dark matter

I am interested in seeing a plot of dark to baryonic matter ratios in terms of the galactic disk size (when it applies, as not all galaxies fit into disk shapes, and probably analyzing disk galaxies ...
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0answers
38 views

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

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 ...
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0answers
134 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 ...
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0answers
61 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{...
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0answers
61 views

Could black hole mergers be the source of density waves that show up as galaxies' spiral arms?

Consider the following mechanism: a black hole occasionally falls into the supermassive black hole in the centre of a galaxy. This causes gravitational waves to be emitted that take e.g. 100,000 years ...
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0answers
53 views

Number of stars with apparent flux

The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. ...
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0answers
59 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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0answers
31 views

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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0answers
116 views

Cumulative mass & mass distribution of a disc galaxy

Hernquist has a great formula to represent the cumulative mass of a spherical galaxy (8): http://star-www.st-and.ac.uk/~hz4/contempastro/hqm.pdf 1) Is there any simple formula for disc shaped galaxy ...
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12 views

How well supported by observations is the hypothesis that concentrations of dark matter are essential to galaxy formation?

Much work regarding the formation and early evolution of galaxies seems to assume that galaxies form based on clumps dark matter. The following observations indicate that some galaxies started with ...
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36 views

Gravitational redshift and mass decrease

If there are two distant gravitationally unbound galaxies and the observer galaxy loses its mass twice faster than the distant galaxy. So gravitational blueshift decreases in the observer galaxy. ...
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0answers
12 views

How do Oort's constants relate to shear and vorticity?

I am told to show that Oort's constants represent shear and vorticity in the velocity field of an orbiting star. I know that Oort's constants are defined as: $A = \frac{1}{2}(\frac{v_{\bot,0}}{R_{0}}...
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0answers
35 views

Gravitational potential energy of an isothermal sphere

I am trying to calculate the gravitational potential energy, W, defined as: $W = -\frac{1}{2}\int\rho(r)\Phi(r)d^{3}r$ for an isothermal sphere. I am given that the density profile varies with r as:...
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0answers
25 views

Jeans equation for a fully isotropic velocity dispersion tensor

Given a known spherically symmetric gravitational potential, $\Phi(r)$, I need to calculate the value of the velocity dispersion tensor $\sigma_{rr}$. To calculate $\sigma_{rr}$ I am using Jean's ...
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0answers
35 views

Does dark matter experience drag?

Is dark matter dragged around the galaxy by gravity as stars and groups of stars orbit the galaxy, and, if so, is its shape/distribution deformed? Similarly, would the Oort cloud be deformed by being ...
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0answers
47 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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0answers
78 views

How does mirror matter account for the faster than expected spin of gallaxies?

To explain dark matter, some physicists have postulated the existence of mirror matter as an alternative to WIMPs, where mirror matter particles could interact with one another in the same way as ...