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What is the Size of the Giant Galaxy IC1101?

-- | September 28, 2014

Question: I’ve been looking at information about the largest known galaxy, IC 1101. Most sources say it is ~6 million light years in diameter but I can’t find any primary sources for this size estimate.

I was looking through research papers and the closest I could find to a direct measurement was 600kpc (so only about 2 million light years). There are lots of other figures mentioned around the internet but they just seem to have been pulled out of thin air!

I even tried measuring it myself using an image from Chandra, but I don’t think the usual trigonometry calculations work when objects are so far away and redshifted (I got a much smaller 150,000 light years as a result!)

I suppose my question is, how big is IC 1101 really, and how do we know?

Many thanks,
Jackie

Answer: You can get a pretty complete listing of the information available on just about any galaxy from the NASA Extragalactic Database (NED) at http://ned.ipac.caltech.edu/. You can use the object name search to look up the information for IC1101, where it lists in its “basic data” section that IC1101 has a major axis diameter of 1.2 arcminutes, which for a distance of 328 Mpc yields a major axis diameter of about 114 kpc. Note, though, that this is likely a size based on optical measurements of just the stars in this galaxy. I was not able to find any references to actual measurements of this galaxy which confirm the claimed sizes in the 6 million light year (about 2 million parsec) range. Note, though, that the actual diameter, which would include matter that is “dark”, could easily be 10 times larger than its optical size. So, since you have clearly done a bit more digging into the research literature than I have, your 2 million light year estimate is probably a pretty good estimate of this galaxy’s size.

Jeff Mangum

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