Galaxy Zoo Talk

Definitions/more explanation of terms needed please

  • kirstenr by kirstenr

    Hi Galaxy Zoo team,

    I think your examples (given while working through the classification scheme) have improved since the last time I spent a lot of time on this project; however, there are still some things that remain unclear to me as a non-astronomer. It would be helpful to have a clear definition of the following terms preferably accompanied by a montage of images showing examples of the term:

    1. elliptical---Is this the same thing as a "smooth galaxy," or does it have to be a particular shape to be elliptical?
    2. disturbed---I see people marking disturbed galaxies on Talk, but either it is a very subtle characteristic or people are marking disturbed galaxies incorrectly. What, exactly, does a "disturbed galaxy" look like?
    3. I think the overlap vs. merger question was answered in the FAQ post: It's difficult to tell, so take your best guess; is that correct?
    4. lenticular galaxy
    5. arc

    Thank you so very much,


  • williamaskew by williamaskew


  • vrooje by vrooje admin, scientist

    Hi Kirsten,

    Just to add some more detail from what wtaskew provided:

    1. An elliptical is generally the same as "smooth and rounded", yes -- it's worded that way so that we don't have to explain the physics behind the difference between ellipticals and disks before we ask people to classify. If you have some astronomical background, the first question is asking you to decide whether you think the galaxy is an elliptical or whether it's a disk galaxy.
    2. Most galaxies should be relatively symmetric unless they're disturbed or disrupted by something, so it's probably asymmetry that people are picking up on.
    3. Yes, definitely just take your best guess -- I agree it's often really hard to tell!
    4. A lenticular is between an elliptical in a disk -- it has elements of both, so it's kind of the middle ground in the classification scheme. These are hard to classify for our classification scheme, actually; we tend to identify them because nobody agrees on what they are in the first question.
    5. This is a specific term related to the signature of a gravitational lens, and they should be very rare.

      I'm going to add one more as I see you've tagged a few galaxies with this (yay! -- these are what I study):

    6. Bulgeless - this term is specific to disk galaxies, and it refers to a galaxy that is only disk.

    Hope that helps! Thanks for visiting us again. ๐Ÿ˜ƒ



  • kirstenr by kirstenr in response to wtaskew's comment.

    Thanks for the input, wtaskew!



  • kirstenr by kirstenr

    Hi vrooje (Brooke),

    Thanks for the added detail. It did help on many fronts.

    I think I've also been marking irregular galaxies incorrectly. So if they're basically symmetrical, but just may be a little raggedy around the edges instead of having a clearly defined smooth edge, they're not irregular, right?

    The wiki article defined lenticular galaxies as disks without spiral arms. I have seen some of these, and I notice this type of galaxy sometimes has rings.

    The moderator Budgieye added to my understanding of a disturbed galaxy by describing it as "having trails of stars coming off of it;" thus creating the asymmetry you mentioned.

    Thanks for cluing me in to the fact that "bulgeless" refers only to disks. This makes sense because you are looking for black holes, so you would expect these galaxies to be rotating, correct? Budgieye also directed me to this link, which prompted a new question about bulgeless galaxies and bulges in general (try to keep a straight face!):

    (Scroll down to the table/chart of bulgeless galaxies.) Most of the galaxies pictured here are ones in which I have been answering "just noticeable" or "obvious" when asked to describe the bulge. So when we are talking about the bulge, does it need to resemble a ball (appear to be 3-D) to actually be considered a bulge? A spherical area in the center of the galaxy that is a different color (usually orange or yellow) than the rest of the galaxy is not in and of itself considered a bulge? If so, I think this needs to be made much more clear in the examples/tutorial.



  • Budgieye by Budgieye moderator

    Agree, I think Dr. vrooje's ability to find bulgeless galaxies borders on the mystical. I would like a flow chart too! ๐Ÿ˜ƒ

    enter image description here potentially bulgeless host galaxies from visual selection.

    How do you decide if a galaxy is bulgeless, when you see it face-on ?


  • kirstenr by kirstenr

    One thing I notice is that many of the above galaxies are actively star-forming. I marked some today that looked like these but weren't nearly as active. Is that one criterion to consider?



  • Budgieye by Budgieye moderator

    Maybe the SDSS images were sharped up for publication?


  • vrooje by vrooje admin, scientist

    I agree each of the galaxies in the mosaic above doesn't appear to be purely bulgeless, but rather it looks like they each have a small central concentration of light in addition to what you'd expect from a disk. The reason they're still potentially bulgeless is that each of those has an AGN in the center, so the light could be coming from the nuclear emission instead of a small bulge. But additional analysis can identify those - and indeed the bottom-right two galaxies in that image turned out to be totally different from what we were looking for.

    Some other (more nearby) examples of bulgeless galaxies are NGC 3621 and 4395:

    NGC 3621

    NGC 4395

    Whereas, here's an example of a face-on spiral galaxy with a central bulge:

    NGC 4385

    It's a relatively small bulge, but it's still a bulge -- the central concentration of light is much stronger and it's also a bit flattened from the inclination (the same as the whole galaxy), so that's unlikely to be AGN emission, which wouldn't be affected by that.

    I should also say 2 other things:

    1. "Bulgeless" is a shorthand for scientists but it's really more of a continuum, and sometimes "bulgeless" is defined as having a bulge that's no more than (for example) 5% of the total light or mass of the galaxy's stars. So "bulgeless" can really mean "a bulge so small it's basically insignificant".

    2. Galaxies of all shapes and sizes have central supermassive black holes... the rotation (or lack thereof) of the stars doesn't have anything to do with whether there's a black hole or an AGN, as far as we know.

    Happy hunting! ๐Ÿ˜„