Gravitationally bound galaxies

[nmsurobert]

How do we know if galaxies are gravitationally bound?

I'm guessing it obviously has something to do with the mass of each galaxy in the cluster, but is there an equation that is used to determine when they are bound to each other? Is there some kind of measurement made regarding the velocity of the galaxy?

Thanks.

 

[davenn]

How do we know if galaxies are gravitationally bound?

I'm guessing it obviously has something to do with the mass of each galaxy in the cluster, but is there an equation that is used to determine when they are bound to each other? Is there some kind of measurement made regarding the velocity of the galaxy?

typing your Q into google got this from Wiki ...

Observational methods[edit]

Galaxy Cluster LCDCS-0829 acting like a giant magnifying glass. This strange effect is called gravitational lensing.
Clusters of galaxies have been found in surveys by a number of observational techniques and have been studied in detail using many methods:

• Optical or infrared: The individual galaxies of clusters can be studied through optical or infrared imaging and spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar redshift. Infrared searches are more useful for finding more distant (higher redshift) clusters.
• X-ray: The hot plasma emits X-rays that can be detected by X-ray telescopes. The cluster gas can be studied using both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along with AGN are the brightest X-ray emitting extragalactic objects.
• Radio: A number of diffuse structures emitting at radio frequencies have been found in clusters. Groups of radio sources (that may include diffuse structures or AGN) have been used as tracers of cluster location. At high redshift imaging around individual radio sources (in this case AGN) has been used to detect proto-clusters (clusters in the process of forming).
• Sunyaev-Zel'dovich effect: The hot electrons in the intracluster medium scatter radiation from the cosmic microwave background through inverse Compton scattering. This produces a "shadow" in the observed cosmic microwave background at some radio frequencies.
• Gravitational lensing: Clusters of galaxies contain enough matter to distort the observed orientations of galaxies behind them. The observed distortions can be used to model the distribution of dark matter in the cluster.

 

[nmsurobert]

typing your Q into google got this from Wiki ...

Ah awesome. I tried googling some answers but I couldn't find anything. Thanks.

 

[davenn]

Ah awesome. I tried googling some answers but I couldn't find anything. Thanks.

did that give you the answer you were after ?

 

[nmsurobert]

did that give you the answer you were after ?

yeah... to an extent. it'll do though.

I teach a high school astro class. while i was setting up an assignment i could already hear a couple of my students asking what i asked lol.

 

[nmsurobert]

did that give you the answer you were after ?

The section on dark matter in clusters helped answer my question to when discussing escape velocities

https://web.njit.edu/~gary/202/Lecture25.html

 

[lpetrich]

How do we know if galaxies are gravitationally bound?

One can address that issue by working out how long it would take a galaxy to fall apart if it was not gravitationally bound. One finds for most galaxies a time much less than the age of the Universe. Since galaxies can be seen over most of the age of the Universe, one concludes that their persistence means that they are gravitationally bound.

 

[davenn]

One can address that issue by working out how long it would take a galaxy to fall apart if it was not gravitationally bound. One finds for most galaxies a time much less than the age of the Universe. Since galaxies can be seen over most of the age of the Universe, one concludes that their persistence means that they are gravitationally bound.

Did you actually read the OP ?

It is about 2 or more galaxies being gravitationally bound to each other, not gravity within a particular galaxy

so this comment of yours is totally irrelevant ...

One can address that issue by working out how long it would take a galaxy to fall apart if it was not gravitationally bound.

Dave

 

[lpetrich]

The same argument applies for galaxies in a cluster. In the absence of gravity, how long would it be before the clustered galaxies disperse?

 

[davenn]

In the absence of gravity, how long would it be before the clustered galaxies disperse?

that doesn't really make sense, because if they weren't gravitationally bound, there would be no cluster, to start with, to disperse

 

[jim mcnamara]

This thread has answers, let's tie it off at this point. Thanks everyone.