Questions regarding spiral/elliptical galaxies

[MightyMidget]

1. Are the properties of bulges of spiral galaxies similar to the properties of elliptical galaxies?

2. If so, imagine a powerful god-like creature that's roaming the universe (yes I know, it's silly). One day he sees a spiral galaxy and desides to destroy/remove the entire stellar disk. The next day, the galaxy remnant is discoverd be some earth scientists. Would it be classified as a elliptical galaxy?

3. Can elliptical galaxies (re-)form a stellar disk with spiral arms? (I know this is impossible in areas with a high galaxy density, because interactions will prevent this form ever happening. But what about a elliptical galaxy in a calm environment, rich in cold intergalactic gas?)

4. What about Centaurus A? It has a disk component. Is it possible this structure may be stable on the long term? And what kind of galaxy is it? Elliptical with dustdisk resulting from a merger? Or a spiral galaxy with an especially massive central bulge?

[acey]

1. Are the properties of bulges of spiral galaxies similar to the properties of elliptical galaxies?

They are very similar in appearance, but the big difference between spirals and ellipticals is that spirals contain a lot of inter-stellar dust and gas.

2. If so, imagine a powerful god-like creature that's roaming the universe (yes I know, it's silly). One day he sees a spiral galaxy and desides to destroy/remove the entire stellar disk. The next day, the galaxy remnant is discoverd be some earth scientists. Would it be classified as a elliptical galaxy?

It might be, on the basis of its light curve. But further study would show it to contain an unusual amount of dust and gas, leading astronomers to conclude that it looked like a spiral bulge with its outer halo somehow removed. It would then most likely be classified as "peculiar".

3. Can elliptical galaxies (re-)form a stellar disk with spiral arms? (I know this is impossible in areas with a high galaxy density, because interactions will prevent this form ever happening. But what about a elliptical galaxy in a calm environment, rich in cold intergalactic gas?)

No. To form spiral arms you need gas and dust, plus young, hot stars that will become supernovas, setting up shock waves in the inter-stellar medium. Ellipticals lack both requirements.

On the other hand, if an elliptical merges with another galaxy then this might introduce star-forming material - see next question.

4. What about Centaurus A? It has a disk component. Is it possible this structure may be stable on the long term? And what kind of galaxy is it? Elliptical with dustdisk resulting from a merger? Or a spiral galaxy with an especially massive central bulge?

It seems that Centaurus A is most likely a giant elliptical that has recently merged with a barred spiral.

Galaxy Centaurus A / NGC 5128