Clean up "Irregular galaxies," submitted by Sarah Weissman (STScI) 8/8/2016

[katieefrey]

Cleaned up the "Irregular galaxies" category. The morphological classification of irregular galaxies typically contains three types Type 1 / magellanic irregular, type 2 / amorphous irregular and Dwarf Irregular. "I galaxies" isn't meaningful according to our scientists. We could add "Irr I galaxies" as a synonym for magellanic irregular and "Irr II galaxies" as a syonym for amorphus. (It's possible that "I galaxies" was supposed to mean "Irr I galaxies" but it's not clear to me.) See: https://ned.ipac.caltech.edu/level5/CLASSIFICATION/miag.html https://en.wikipedia.org/wiki/Irregular_galaxy Binney and Merrifield, Galactic Astronomy https://en.wikipedia.org/wiki/Hubble_sequence

Irregular galaxies-->Magellanic irregular galaxies-->Magellanic Clouds-->Large Magellanic Cloud Irregular galaxies-->Magellanic irregular galaxies-->Magellanic Clouds-->Magellanic Stream Irregular galaxies-->Magellanic irregular galaxies-->Magellanic Clouds-->Small Magellanic Cloud Irregular galaxies-->Magellanic irregular galaxies-->Magellanic Clouds Irregular galaxies-->Magellanic irregular galaxies

[katieefrey]

Could the concept "Magellanic irregular galaxies" include concepts other than "Magellanic Clouds"? Or should one be a non-preferred version of the other?

[seweissman]

Yes. Magellanic irregular galaxies can include other concepts. The Magellanic Clouds are specific objects that are often cited as examples of irregular galaxies because they are visible to the unaided eye from earth.

You can see a whole list of magellanic irregular galaxies at NED: http://ned.ipac.caltech.edu/cgi-bin/OBJatt?delimiter=bar&nondb=row_count&crosid=objname&position=ra%2Cdec&enotes=objnote&position=pretype&position=z&position=zflag&attdat_CON=M&attdat_CON=S&attdat=attned&M=1149

[seweissman]

Actually, according to this Wikipedia article (https://en.wikipedia.org/wiki/Irregular_galaxy#Magellanic_Clouds) the Large Magellanic Cloud might not actually be irregular. Possibly another example of the science moving beyond the terminology. This needs more research.

[katieefrey]

I'm also thinking that the Magellanic Stream is also not really an irregular galaxy either, right?

How about this? Local Group -->Magellanic Clouds-->Large Magellanic Cloud Local Group -->Magellanic Clouds-->Magellanic Stream Local Group -->Magellanic Clouds-->Small Magellanic Cloud Local Group -->Magellanic irregular galaxies-->Small Magellanic Cloud Irregular galaxies-->Magellanic irregular galaxies-->Small Magellanic Cloud

[seweissman]

Magellanic irregular galaxies should go under Irregular galaxies, because this is one of the main types of irregular galaxies. https://ned.ipac.caltech.edu/level5/CLASSIFICATION/miag.html. Magellanic irregular are also known as Type I and Amorphous as Type II. It may be better to use Type I and Type II as the preferred terms. I'm not 100% sure.

Magellanic irregular galaxies should not go under Local group because Magellanic irregular is a classification of galaxies and Local group is a specific group of galaxies.

You can see a use of Magellanic irregular galaxies referring to a general class of galaxies and not specifically the LMC or SMC in this paper: https://ui.adsabs.harvard.edu/#abs/1993PASP..105..374H/abstract

[katieefrey]

Ah, thanks for clarifying, I didn't quite catch the nuance of how "Magellanic irregular" was a descriptive class. I'll look into the pref label issue you brought up, that's an easy change to make down the line, if need be.

Revised hierarchy:

Local Group -->Magellanic Clouds-->Large Magellanic Cloud Local Group -->Magellanic Clouds-->Magellanic Stream Local Group -->Magellanic Clouds-->Small Magellanic Cloud Irregular galaxies-->Magellanic irregular galaxies (or Type I irregular galaxies) -->Small Magellanic Cloud Irregular galaxies-->Amorphous irregular galaxies (or Type II irregular galaxies)

[katieefrey]

Above hierarchy implemented.