disks and spirals
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kirstenr
Can someone clarify for me the difference between a smooth galaxy that is not completely round and a galaxy containing a disc? The examples aren't all that clear to me.
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jxfrost
A smooth galaxy, like this one, is smooth throughout. A galaxy has arms that you can make out. For the disc galaxy, the smoothness is not throughout meaning the smoothness does not go out to the edge and usually they have some kind of bulge in the middle of the disc. A smooth galaxy, even a cigar shape, is smooth throughout to the edges, usually without a bulge. I do not think there are any 100% rules to use since each galaxy is so unique. The idea is to get the classification as close as you can to one or the other, and through multiple people classifying, the truth will emerge. Any mis-classification does not condemn the galaxy to be misunderstood forever.
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jxfrost
Correction: "A galaxy has arms that you can make out." should read 'A disc galaxy has arms that you can make out."
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kirstenr
Doesn't only a spiral disc galaxy have arms? Thanks for your help. I realize the project does not hang on my classifications, but I still want to do my best.
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klmasters
scientist, admin
There are disc galaxies without arms, but not elliptical galaxies with them. So "Smooth" should pick up all the elliptical galaxies, and some discs without arms or other features, while if you see any features it's better to click "features" and then identify what you see. Sorry our examples were not clear for you.
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suelaine
Can it be said that ALL disks are actually spirals viewed edge on, or nearly edge on ?
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Capella05
moderator
We seem to have several threads on this subject, and Scientist Karen replied in one of them, so I am going to merge.
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JeanTate
in response to suelaine's comment.
Can it be said that ALL disks are actually spirals viewed edge on, or nearly edge on ?
The terms are a bit tricky; as I understand it, a top-level classification is elliptical/disk/irregular, with merger and overlap sorta there too (the unruly children, who are not always acknowledged).
Disk/spiral is tricky:
- disk galaxies can appear featureless, but imaging in different wavelengths (or using techniques to enhance features) can show them as having arms (or other features)
- the features of a disk galaxy with them may not include arms; e.g. ring galaxies, barred galaxies without arms, very distinct bulge+disk, 'tadpole' galaxies, ...
- inclination, as you said, can make features visible (or invisible) that would not be at a different viewing angle
- scale: some ellipticals have inner/nuclear spiral disks (e.g. M87?), which are too small to be seen in any SDSS/UKIDSS image
- (there's probably more too)
The expert on galaxy morphology is Ron Buta; his latest paper is quite accessible (I'll dig up a link to it later), and reading this you quickly learn just how rich the variety of features (etc) of galaxies is! 😄
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