# Thread: Distances in the PGC?

1. Newbie
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## Distances in the PGC?

Does the Principal Galaxies Catalog contain enough information to determine an approximate distance to the galaxies it contains?
My first thought was just to divide a galaxy's given radial velocity by the Hubble constant, but it's my understanding that this doesn't produce very good results once the velocity gets into the high thousands of km/s. Is it even a reasonable approximation?

2. For many purposes, the method you describe is just fine. The method will produce poor results when a galaxy is very close to the Milky Way -- closer than the Virgo Cluster, say -- due to peculiar motions. It will also produce poor results when the recession velocity grows to a significant fraction of the speed of light.

For many purposes, it's good enough. What's your purpose? Why do you need to know the distances to these galaxies? If you tell us, maybe we can provide some additional guidance.

3. Newbie
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Just curiosity. I took some images in the region around M59 and M60 towards Virgo, and was trying to figure out what the faintest objects in the image were. I was able to confirm 3 PGC objects fainter than mag 20: 3551250, 3551277, and (barely) 3551270. I thought it'd be interesting to try and figure out how far away these guys really were!

4. To do better than cz/H at large redshifts, you have to fold in a cosmological model. One place to see the results is Ned Wright's Javascript calculator. There are also iPhone/iPod apps oike CosmoCalc to do this - you could make a fine-grained table and interpolate from that for higher-z galaxies if there are too many to conveniently look up one at a time.

5. Newbie
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Originally Posted by ngc3314
To do better than cz/H at large redshifts, you have to fold in a cosmological model. One place to see the results is Ned Wright's Javascript calculator. There are also iPhone/iPod apps oike CosmoCalc to do this - you could make a fine-grained table and interpolate from that for higher-z galaxies if there are too many to conveniently look up one at a time.
Thanks for the information!
And to convert the radial velocity into redshift, the formula is:

z = sqrt((1 + v/c)/(1 - v/c)) - 1

Is this correct?

6. Don't use the relativistic form to go from velocity to redshift. All the calculations work directly from redshift z which has the enormous virtue of being a non-arguable observable parameter. The standard catalogs simply use v = cz.

(This wording has been carefully edited to avoid the ire of GR aficionados as to whether the relativistic Doppler form applies in the cosmological context; it for sure doesn't for catalog values. Except when IRAF was used to measure a cross-correlation velocity.)

7. Order of Kilopi
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Originally Posted by ngc3314
(This wording has been carefully edited to avoid the ire of GR aficionados as to whether the relativistic Doppler form applies in the cosmological context; it for sure doesn't for catalog values. Except when IRAF was used to measure a cross-correlation velocity.)
Thanks for the silly giggle on this end, Bill.

8. Newbie
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Originally Posted by ngc3314
Don't use the relativistic form to go from velocity to redshift. All the calculations work directly from redshift z which has the enormous virtue of being a non-arguable observable parameter. The standard catalogs simply use v = cz.
Sounds simple enough. Thanks again!

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