Estimating diameter of red-shifted galaxy

In summary, the question asks for an estimate of the diameter (in kpc) of a red-shift=6.56 galaxy at the moment it emitted light. The assumption is made that early galaxies with similar red-shifts have the same number of stars and masses as present-day galaxies. After considering various equations and information, it is determined that the density and mass of the galaxy can be used to calculate its volume, giving the final estimated diameter in kpc.
  • #1
bobo1455
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Q: Estimate the diameter in kpc of the red-shift=6.56 galaxy at the moment when it emitted light. We're assuming that early galaxies with similar red-shifts have the same number of stars to galaxies as in the present state of the universe.

I've taken a look at the small angle formula a = s / d where a = angular size, d = distance and s = galaxy true size (diameter). But for that I need the angular size and distance. I'm not sure if if there is a way to calculate that using the information given in the question. Other than the given information, I have the average density and scale-factor as well.
 
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  • #2
Average density of what?

You need something more than the redshift value, because there are galaxies of different size. The problem statement mentions "early galaxies with similar red-shifts", which suggests to compare this galaxy to some other galaxies at that redshift.
 
  • #3
I have the average density of the z= 6.56 galaxy. The question says that all galaxies around the time of the red-shift = 6.56 have similar masses and number of stars to galaxies in the present universe. I'm assuming that is supposed to make the question easier and overall I'll need less variables to estimate the diameter, but I'm not sure what to do next.
 
  • #4
Okay, so you know the density, and you can take the mass and number of stars from a current galaxy. That alone should be sufficient to get the volume, and if you assume that the shape is the same you can also get the diameter.
 
  • #5
I've read a bit more about this and found this page: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/hubble.html#c3

If you put in a wavelength it can give you the distance to the galaxy. I also found out that c * z = v is the velocity of the galaxy with a red-shift wavelength z. So I now I have the velocity of the galaxy as well. I calculated distance using v = H0 x d OR d = v / H0 and I also have its distance in Mpc. Not sure what to do at this point. I've looked at a lot of equations now and am confused.
 
  • #6
bobo1455 said:
I also found out that c * z = v is the velocity of the galaxy with a red-shift wavelength z.
There is no unique meaningful definition of a velocity for galaxies far away from us.
bobo1455 said:
So I now I have the velocity of the galaxy as well. I calculated distance using v = H0 x d OR d = v / H0 and I also have its distance in Mpc.
That does not work, neither for the current nor for the past distance.

If I understand the problem correctly, you are overthinking this massively. You have the mass and the density, calculate the volume.
 
  • #7
Ok I finally got the value I was looking for. I really was overthinking it for no reason. The final distance in Kpc worked out to be the correct one. Thanks.
 

FAQ: Estimating diameter of red-shifted galaxy

What is a red-shifted galaxy?

A red-shifted galaxy is a type of galaxy that is moving away from us at high speeds, causing the light emitted from it to shift towards the red end of the spectrum. This phenomenon is a result of the expansion of the universe.

How is the diameter of a red-shifted galaxy estimated?

The diameter of a red-shifted galaxy can be estimated using a method called the Tully-Fisher relation. This method involves measuring the red-shift of the galaxy and using it to calculate its distance from Earth. The apparent size of the galaxy can then be measured and used to estimate its diameter.

Why is estimating the diameter of red-shifted galaxies important?

Estimating the diameter of red-shifted galaxies is important because it allows us to understand the size and scale of the universe. It also helps us to study the evolution and formation of galaxies and their relationship with the expansion of the universe.

What challenges are involved in estimating the diameter of red-shifted galaxies?

One of the main challenges in estimating the diameter of red-shifted galaxies is the accuracy of distance measurements. The farther a galaxy is, the more difficult it is to accurately measure its distance and therefore its diameter. Another challenge is the possibility of obscured or distorted images due to the presence of dust or other objects in the line of sight.

Can the diameter of a red-shifted galaxy change over time?

Yes, the diameter of a red-shifted galaxy can change over time as it continues to expand and evolve. This is especially true for younger galaxies that are still in the process of forming and growing. However, the change in diameter may not be noticeable in a short period of time and requires long-term observations to accurately measure.

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