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Structure formation
Structure in the Universe — galaxies, galaxy groups, clusters, and superclusters — forms from regions of the Universe which are denser than the average; dense enough the overcome the expansion of the Universe and collapse gravitationally.
Whereas it was previously thought that the largest structures collapsed first, later fragmenting to ...
9
Other answers have provided general ideas about how to confirm that individual sources are distant galaxies and not clusters, so I'll focus on the question of how astronomers in the Dark Energy Survey (DES) might be confident about galaxy identifications for the hundreds of millions of sources in their catalogs -- particularly as the overwhelming majority of ...
8
Short answer: both.
HUDF is representative on the type of galaxies we can find at that distance (and remember that distance also means time ago).
On the other hand, we do not really know how the galaxies are grouped beyond certain level, but we think they are grouped on so-called filaments, leaving enormous empty voids.
In this sense, HUDF's density does ...
8
As you point out, in an accelerating Universe, large scale structures will become more and more isolated. So at a certain point you will have gravitationally bound superclusters separated by very large voids and less and less filamentary structures.
Once isolated, we can then study the dynamics of these independent superclusters. On very large time scales, ...
7
This depends where the cluster is. Large scale features of the universe (both observed and simulated) consist of filaments and voids. Below is a map produced by the Sloan Digital Sky Survey (SDSS) showing these features.
Each dot here is an individual galaxy.
Simulations show remarkably the same thing. Here is a video of the Millennium Simulation. It ...
7
I'm sorry for the long answer. The result is in the bottom. :)
The number of galaxies in the "wall", as in any volume of space, is the number density $n_{\mathrm{gal}}$ times its volume $V_{\mathrm{wall}}$. However, the number density of galaxies depends on their masses/sizes: Small galaxies are much more common than large galaxies. Usually we ...
7
Yes.
Theoretically, structure is expected to form first on small scales (stars and stellar clusters), and later on increasingly larger scales — galaxies, groups, and eventually galaxy clusters (see e.g. Longair 2006). This is confirmed, at least to some extent, observationally. For instance, galaxies have been detected out to a redshift of $z=11.2$ (400 ...
7
We measure the distance.
A cluster is a group of stars in close proximity to each other. For relatively near stars you can directly measure the distance via the parallax. That's what the Gaia observatory has been doing with unprecedented accuracy during the last years. Thus anything which we can measure a parallax for is known - anything we don't measure a ...
6
Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal.
The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to massive objects between the source and the observer. This also applies to GWs. The result will be similar - the direction of the waves can be changed, resulting ...
6
I don't want to make any assumptions here regarding the Milky Way's presence in the Laniakea Supercluster simply because of how recent the discovery is. The findings could very well be accurate, but I don't want to base this answer off of them. Fortunately, I've found a few papers that get us around that little issue, as well as the University of Hawaii's ...
6
The Local Group contains 54 plus galaxies.
Don't know that that counts as much of a cluster.
Next up in scale, the Milky way is part of the Laniakea Supercluster
That contains about 100,000 galaxies, so it's a bit on the large size to call a simple cluster.
The well known Virgo Cluster contains about 1300 galaxies.
So it's clearly a thing of intermediate ...
6
The milky way is not part of a galaxy cluster. The local group is on a lobe of the Virgo supercluster.
Not all galaxies are members of large clusters, ours is in a small group. However the only difference between "group" and "cluster" is the size.
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OK, guessing rather wildly in the absence of solid information....
The press-release page is here; that provides a bit more information
than the news article, and includes two images of (different regions of) the cluster and a hint that the cluster is named MACS0940.
I think the main image is a color composite from HST imaging. It shows a galaxy cluster (...
6
I'm assuming you're talking about physical distances (as opposed to any of the other distance measures in cosmology).
The comoving distance to a galaxy at redshift $z$ is
$$
d_C(z) = \frac{c}{H_0}\int_0^z
\frac{dz}{\sqrt{
\Omega_r(1+z)^4 +
\Omega_m(1+z)^3 +
\Omega_k(1+z)^2 +
\Omega_\Lambda
}},
$$
...
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No, it doesn't contradict the paradigm of the universal inflation. It would merely mean that the example galactic cluster that you mention has its proper (own) motion at a smaller velocity relative to the proper motion of the Milky Way than the speed of the universal inflation at the distance between them. I.e. it would move towards us, if the Universe wasn'...
5
The answer to your first question is "Yes, the temperature referred to is the 'normal' temperature, reflecting the average kinetic energy of the gas particles".
The answer to your second question is a bit more complex:
Cooling function
Gas cools by various processes, with an efficiency depending on the temperature, the density, and the composition of the ...
5
I'm going to break your question down into two sections.
Do galaxy clusters have measurable peculiar velocities?
The answer to this depends slightly on what is meant by "peculiar velocity". In the most general sense, the peculiar velocity of some object is the velocity it has with respect to some standard of rest. In the cosmological sense, the peculiar ...
5
I think the key point is how the velocity dispersion in a given structure compares with the velocity dispersion you would expect from the difference in the Hubble flow across the linear extent of the object. Or, equivalently, what range of distances (according to Hubble's law) are implied by the velocity dispersions and how does this compare with the actual ...
5
What is the mathematical difference between their refraction indices?
The conventions on the signs of the direction of magnetic field vectors and of charges.
The tutorial specifies several things that can have a sign;
direction of the light propagation
direction of the magnetic field
sign of the particle's charge (electrons have negative charge)
Let's ...
4
I think this might answer your question, I have quoted the important paragraph below.
If peculiar velocities could have any value, then this would make Hubble’s law useless.
However, peculiar velocities are typically only about 300 km/sec, and they very rarely exceed
1000 km/sec.
Hubble’s law therefore becomes accurate for galaxies that are far away,...
4
@AlexeyBobrick is correct. To add to his answer:
The scale of galaxy clusters are on the order of ~1Mpc (~3.14 Million light years in size), and are therefore much smaller than the cosmological horizon.
The cosmological principle is extraordinarily important for cosmologists, and makes the assumption about the universe's global properties (which turn out ...
4
No, the structures are not of the size of the observable Universe. In fact modern cosmolgical models rely on the Universe being homogeneous over scales above $100 \textrm{Mpc}$. This is an observational fact, which serves as a base for the so-called cosmological principle. See also a wiki article on large-scale structure, where the scale is mentioned in the ...
3
I second Pela's answer, apart from one addition. The CMB and the intracluster gas do interact with each other via the inverse Compton effect, whereby electrons in the gas cool by giving their energy to CMB photons, increasing their energies.
This process, when applied to hot gas in the intracluster medium, is also known as the Sunyaev-Zel'dovich effect. ...
3
The Laniakea Supercluster contains several galaxy filaments, even just reading the wikipedia articles you provided you can see the difference in size of the two:
Galaxy Filament
They are massive, thread-like formations, with a typical length of 50 to 80 megaparsecs, (163 to 261 million light years) that form the boundaries between large voids in the ...
3
BTW if anyone wants a quick and fast query to solution do the following:
Go to https://skyserver.sdss.org/dr12/en/tools/search/sql.aspx. Paste a query like this:
SELECT
s.specobjid, s.ra, s.dec, s.z
FROM SpecObj as s
WHERE
s.z > 0 AND s.z < .18 AND s.ra > 0 AND s.ra < 50 AND s.dec > 0 AND s.dec < 30
Then after downloading a csv file ...
3
SDSS DR12 Catalog Data
looks like a good starting point,
apparently pretty open to those willing and able to figure it out.
Their
SciServer Compute
site hosts Jupyter notebooks to query CasJobs in SQL.
The Large Scale Structure galaxy catalog under
BOSS value added catalogs
may also be relevant.
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Schindler et al. 1999: Morphology of the Virgo Cluster: Gas versus Galaxies has details for $\beta$ model fits for Virgo and its subclusters.
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The point of the two-point correlation function (pun not intended) is to describe how clustered the galaxies in the universe are. Astronomers want to know if they're all bunched up in tight bundles with huge voids, or maybe they're more or less uniformly distributed.
The two-point correlation function is a way of measuring the distribution of these galaxies ...
3
Short answer, no, they don't.
Longer answer, it's complicated. There are, in essence, five different measures for the centre of a galaxy cluster, based on different physical properties of the cluster, and occasionally not in agreement with each other.
BCG, the brightest cluster galaxy. It should sink to the bottom of the gravitational well, but that's only ...
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