Angular size of redshifted galaxies?

[vlaiv]

I was trying to figure out furthest galaxies that one can observe / image with amateur equipment in terms of angular size. Not in terms of brightness, but rather angular size.

If we assume that average galaxy is about 50,000Ly across and we take distance of 1Bly we would get angular size of 10" - which is pretty much in domain of resolution of amateur equipment.

But then it hit me - these objects will be red shifted due to expansion of universe which means that they were closer at the time light started its journey.

Since galaxy was closer at that time and since scale factor is equal in all directions (space expands equally in all 3 dimensions of space), which ought to preserve angles - are we going to observe such galaxy with angular size equal to that of what it was at the moment the light started its journey (making it larger in angular size compared to distance obtained from red shift)?

[andrew s]

Hi vlaiv, if you Google cosmological distance measures and look at the wikipedia entry I think it will give you the answer.

Regards Andrew 

[vlaiv]

1 minute ago, andrew s said:

Hi vlaiv, if you Google cosmological distance measures and look at the wikipedia entry I think it will give you the answer.

Regards Andrew 

Yep, just was doing that - it appears that I'm right, but actual dependence is more complicated - trying to figure out at what distances / redshifts it's becoming significant

 

[andrew s]

2 minutes ago, vlaiv said:

Yep, just was doing that - it appears that I'm right, but actual dependence is more complicated - trying to figure out at what distances / redshifts it's becoming significant

 

Good luck. What are you trying to do? Are you trying to resolve the galaxy or just capture it ? 

Regards Andrew 

[vlaiv]

2 minutes ago, andrew s said:

Good luck. What are you trying to do? Are you trying to resolve the galaxy or just capture it ? 

Regards Andrew 

This was more of a pondering what's possible. Was looking at large scale features - voids and large galaxy clusters and I came across Abell 2218 with its gravitational lensing and thought to myself - now this is interesting target to image. I did quick search on internet and found one amateur image of it - most galaxies in image were tiny dots (just few pixels across).

All of that got me thinking about average galaxy sizes, what can be "resolved" in terms being more than 4-5px in image and obviously a galaxy rather than star, with amateur equipment - and while calculating angular size vs distance it just occurred me that we are not only looking at distance but in past as well and that those objects have been closer to us in past at the time light was emitted from them.

I was just unclear about angles and would we see it as larger because it was indeed "larger" in the past (closer to us). I was actually confused because expansion of space acts on intergalactic space and not galaxies them selves - no galaxy is "swollen" due to expansion of space (gravity dominates there). On the other hand I reasoned, once light is emitted - it is no longer tied to physical size of object and travels thru space that is being expanded. Expansion is equal in all directions - hence it should preserve angles ....

 

[vlaiv]

For anyone interested in this (with couple of bonus details ), this page explains it pretty much completely (in light of lambda CDM):

https://en.wikipedia.org/wiki/Distance_measures_(cosmology)

With transverse comoving distance being the same as comoving distance according to latest data (omega being very close to 1 - flat universe).

 

[Bukko]

Hi Vlav,

A few years ago, my youngest son pointed our 12" f4 Newt at the hercules cluster and got about 25 hours of luminence under less than ideal conditions. From memory, the galaxies are in the region of 200 to 400Mly distant so hopefully the attached will help support your concept of looking back around 1Bly distant.

A bigger (16"+?) in much better skies might manage it and in Luminence, I don't think the redshift is big enough to spoil the show...

Hope this helps.

Gordon.