I'm confused a bit

[Faiien]

They say that there are thousands(or is it hundreds of thousands) of Galaxies out there and that most of them indeed are galaxies.

 

I'm wondering how can they tell for sure if they are galaxies or not? Main reason I'm confused is this, if 1 very distant object(star/galaxy) can through gravitational lensing appear to us for example as 10 separate sources of light, etc.

 

I guess to rephrase that, taking Lensing into consideration, how do we know which are really stars/galaxies?

[Martin]

They say that there are thousands(or is it hundreds of thousands) of Galaxies out there and that most of them indeed are galaxies.

 

I'm wondering how can they tell for sure if they are galaxies or not? Main reason I'm confused is this, if 1 very distant object(star/galaxy) can through gravitational lensing appear to us for example as 10 separate sources of light, etc.

 

I guess to rephrase that, taking Lensing into consideration, how do we know which are really stars/galaxies?

 

You need quite special circumstances for multiple images to be produced by gravitational lensing, involving a massive object in the foreground (between you and the object being multiply imaged) positioned just right. Since it is a rare and exciting thing to discover, astronomers doing surveys can be expected to be on the lookout for signs of multiple imaging. The multiple images will be arranged around the central object in the foreground whose gravity is doing the lensing, and they will have identical spectra. Unfortunately I don't remember ever seeing any estimate of how many cases surveys may have missed. One could make some guess if one had a count of the number of times a survey team has initially failed to spot a case which someone else then later pointed out. I assume the number of mistakes like that is small, but i can't give you a firm conclusive answer on how small.

Edited November 1, 2008 by Martin