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kelly
2004-Apr-13, 09:35 PM
Hello, just wondering how one can distinguish between a distant elliptical galaxy <you cant really resolve globular clusters within the galaxy> and a globular cluster. Some ellipticals are easily distinguished as one can observe clusters within them. But say if there is absorption via dust, the distance can be miscalculated. Is there a major visible difference to not mis-label this object, since they look similar in many photos I've found?

Normandy6644
2004-Apr-13, 11:16 PM
Hello, just wondering how one can distinguish between a distant elliptical galaxy <you cant really resolve globular clusters within the galaxy> and a globular cluster. Some ellipticals are easily distinguished as one can observe clusters within them. But say if there is absorption via dust, the distance can be miscalculated. Is there a major visible difference to not mis-label this object, since they look similar in many photos I've found?

Well, i would imagine that the distances would be accurate enough to distinguish the two. Even if some variable altered them, there would be a large enough difference that I would think the uncertainty wouldn't matter if the objective is to decide whether it's one or the other.

Brady Yoon
2004-Apr-14, 01:07 AM
Elliptical galaxies are usually more massive than globular clusters, so mass would distinguish the two.

ngc3314
2004-Apr-14, 01:09 AM
Hello, just wondering how one can distinguish between a distant elliptical galaxy <you cant really resolve globular clusters within the galaxy> and a globular cluster. Some ellipticals are easily distinguished as one can observe clusters within them. But say if there is absorption via dust, the distance can be miscalculated. Is there a major visible difference to not mis-label this object, since they look similar in many photos I've found?

Globular clusters are a lot fainter than luminous elliptical galaxies. So much so that it, say, Palomar Sky Survey pictures, the confusion could set in only for globulars in other galaxies - and by that time an elliotical galaxy, to be so faint, is so distant that not only its spectrum, but its color, gives it away. (Because elliptical galaxies have such red stellar populations, not only the location of their spectral lines, but their color measured between a pair of standard filters, changes strongly with increasing distance).

Some dwarf ellipticals aren't that much brighter than globular clusters, but these are a lot bigger. That said, there are a few things wandering around in some clusters of galaxies that look sort of like disembodied giant globulars, which may be all that's left of former dwarf ellipticals that have been shredded by the tidal influence of larger bullies. In fact, there's some evidence that Omega Centauri in the Milky Way is such a thing. The chemical makeup of its stars shows signs of enrichment through multiple generations of star formation, and ordinary globulars are generally not able to hold on to ejected stellar material with their more modest gravity.

Globulars are the largest systems for which we pretty well understand the dynamics in terms of what we can see - their internal motions fit with the mass of their stars, wthout dark matter beyond what "dead" stars would produce. Maybe that's a difference to key on - galaxies are in pools of dark matter, globulars aren't. Even when they've wandered far from home like NGC 2419...

(Rats. I've been trying not to be professorial, but it just doesn't work).

Kaptain K
2004-Apr-14, 10:41 AM
Although distant ellipticals and local globulars appear similar in small scopes and in pictures in books and magazines, they are so different in scale that the differences are obvious in larger scopes.

Spaceman Spiff
2004-Apr-14, 11:08 PM
Hello, just wondering how one can distinguish between a distant elliptical galaxy <you cant really resolve globular clusters within the galaxy> and a globular cluster. Some ellipticals are easily distinguished as one can observe clusters within them. But say if there is absorption via dust, the distance can be miscalculated. Is there a major visible difference to not mis-label this object, since they look similar in many photos I've found?

The light distributions (surface brightness vs. distance from center) differ as do their stellar spectra (and therefore do their colors, if you do broad band photometry). As mentioned above, elliptical galaxies are generally more massive than globular clusters, so the widths of their stellar absorption lines will be wider (stars move faster because they sit in a deeper grav. well). For that matter, dwarf ellipticals differ greatly from "normal" ellipticals in their properties.

The extinction by dust is recognizable in optical-nearIR spectra and colors(dust leaves its finger prints) and thus removable.

The post by ngc3314, above, says the rest.