IC 1101

[TranslightDefender]

I searched the Internet for largest known galaxy, and its IC 1101.
Wikipedia puts its diameter at 5-6 million lightyears.
In Celestia, its radius is 1.832e+05 ly, which would only be a diameter of 366,400 ly.
It seems that there's a little discrepancy here.
So which is wrong?

[t00fri]

I searched the Internet for largest known galaxy, and its IC 1101.
Wikipedia puts its diameter at 5-6 million lightyears.
In Celestia, its radius is 1.832e+05 ly, which would only be a diameter of 366,400 ly.
It seems that there's a little discrepancy here.
So which is wrong?

The linear diameter in ly of a galaxy is measured by means of its angular size in radian and it's distance in ly:

angular size =diameter/distance


Both the reliable RC3 galaxy catalog and the SIMBAD world database give a similar value for the angular size ~ 1.2 - 1.4 arcmins. However, the distance of IC 1101 is largely unknown! The paper that is referred to in the IC 1101 Wikipedia article (that you presumably used) is not a standard accepted scientific reference. Correspondingly, there is no distance value of IC1101 in the NED1d catalog on galaxy distances. As a rough estimate of the distance I therefore used Hubble's law for Celestia's data base (galaxies.dsc). The resulting diameter and distance are consistent with the observed angular size.

If you want a much larger diameter, the distance must be also much larger, in order to match the reasonably well known angular size. In any case there are very large uncertainties involved. Since IC 1101 is the central object in a galaxy cluster, the distance from Hubble's law (and the spectroscopically determined redshift z) might receive large corrections due to so-called peculiar (i.e. non-radial) velocity components induced by other gravitating cluster members!

Fridger

[TranslightDefender]

I thought it might be something like that, that the distance isn't known very well.

It seems that the Google results say a diameter of 5-6 million lightyears frequently.
A blog, answers.com, and Wikipedia say that its diameter is ~5 million ly, yet at the same time say its 1 billion ly away, which is the distance Celestia has it at.

[t00fri]

I thought it might be something like that, that the distance isn't known very well.

It seems that the Google results say a diameter of 5-6 million lightyears frequently.
A blog, answers.com, and Wikipedia say that its diameter is ~5 million ly, yet at the same time say its 1 billion ly away, which is the distance Celestia has it at.

I am only using scientific resources, i.e those that professional astronomers consult. Numbers are worth nothing in this context as long as you don't know about their uncertainties as well! The fact that the NED1d data base for galaxy distances doesn't have an entry for IC 1101 means that there is NO serious distance value available, where the uncertainty can be sensibly estimated.

Once more: the ratio of diameter/distance is relatively well known.

When you read 5-6 million ly as diameter, you always need to consult the original scientific source NOT some obscure webpage that copied such numbers from somewhere. The essential question is whether there are several independent scientific sources for a distance leading to 5 million ly diameter by means of the reliable angular size of 1.2-1.4 arcmins! The only alternative method of extracting a IC 1101 distance is via Hubble's law (that I used). Possibly some of your resources also used this method whence the agreement.

Fridger