Elliptical Galaxies

[Rhodesign]

Hi Everyone,

Genuine query.

Are elliptical galaxies real or just circular galaxies viewed at an oblique angle?

(Apart that is from colliding pairs of galaxies.)

I wonder because I understand that anomalously rapid rotational speed of galaxies is considered a supporting argument for dark matter.

But if galaxies are commonly elliptical then as it would not be possible to tell which axis is pointing towards us (when we lie in its rotational plane), if it is the long one, higher velocities would be seen.

Or perhaps I have misunderstood the term 'elliptical' as applied to galaxies.

Thanks for any comments in reply.

[wstevenbrown]

Rhodesign:

While it is true that a circular disk when tilted presents a elliptical appearance, a sphere looks like a sphere from any perspective. From the standpoint of standard English, the use of "elliptical" to mean "oblate spheroidal" is incorrect. But astronomers don't speak standard English, and the term of art for an ellipsoidal shape is "elliptical"-- it's an error that has been around for too long to change.

On the scientific half of your question: There are occasional instances of tilted spiral disk galaxies being mistaken for elliptical (ellipsoidal) galaxies, but this is very rare. Typically, the way in which their brightness drops off from center to edge is very different. The nearest example where the ambiguity you noted has surfaced AFAIK, is M110, a satellite of the Andromeda Galaxy. It is usually described as an elliptical with an anomalously bright, dense nucleus (ultracompact dSph galaxy with diffuse envelope). Some, however, have claimed to see spiral structure, but they are very much in the minority.

Best regards-- Steve

[Rhodesign]

Many thanks for the reply Steve, I am a little more enlightened now.

So which type(s) of galaxy give the information of anolmalously high rotational velocities?

Steve R

[Nereid]

Originally posted by Rhodesign@Jul 3 2005, 06:40 PM
Many thanks for the reply Steve, I am a little more enlightened now.

So which type(s) of galaxy give the information of anolmalously high rotational velocities?

Steve R

Do you mean, rotation curves which lead astrophysicists to conclude there's a lot of non-luminous mass in the outskirts of the galaxies (i.e. the curve 'flattens&#39?

[Rhodesign]

Hi Nereid,

Simplistically, I thought that it was just a matter of a mass deficit implied by the speed of rotation and size of a given galaxy. I had meant to ask how the mass was determined at some stage but thought I'd go slowly so as not to get myself too confused.

Perhaps I could impose on you to point me to a suitable publication or post that might serve as a guide and glossary.

Best Regards,

SteveR