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Fraser
2004-Oct-26, 03:39 PM
SUMMARY: Astronomers using the Chandra X-Ray Observatory have discovered a huge halo of dark matter around an isolated elliptical galaxy; an object that shouldn't have such a halo, according to optical observations. The galaxy, NGC 4555, is unusual that it's a large elliptical galaxy which isn't part of a larger cluster of galaxies. It's surrounded by a cloud of gas, twice the size of the galaxy itself, that's been heated to 10-million-degrees Celsius. This gas could only get that hot if it was being constrained by a halo of dark matter ten times the mass of the stars in the galaxy.

What do you think about this story? Post your comments below.

John L
2004-Oct-26, 03:41 PM
This story clearly proves that we still don't have a clue what dark matter is, how it works, or even if it really exists. These guys did not see any dark matter. All they saw was the hot, x-ray emitting gas surrounding the galaxy. They are assuming that the only mechanism for this cloud of gas to exist is dark matter. And since this finding directly contradicts another recent study I have to conclude that they don't have a clue about the mechanisms resposible for these observations.

antoniseb
2004-Oct-26, 03:59 PM
NGC 4555 could be the last galaxy in a small cluster. All other galaxies in the cluster having previously been absorbed by NGC 4555. In such a relatively unusual case, there should be a cluster's worth of dark matter surrounding the object.

This observation is only strange if you assume that all elliptical galaxies should be the same, and must have formed the same way. They aren't and didn't.

Guest
2004-Oct-26, 04:16 PM
So you say, Antoniseb, that there is a reasonable explanation for this? Interesting...I was going to suggest that DM was not responsible for this, and that it was something else (what it is I could not be certain).

I abstain. ;)

lswinford
2004-Oct-26, 05:12 PM
I am inclined to agree with the guest above, though for personally intuitive reasons. As I was reading the article, my eyes kept bouncing back to the picture and a recollection from reading of a rather phenomenal human by the name of Nickolai Tesla. Tesla was doing a stage show (not all geeks and nerds are introverts it seems). Some four years before the florescent light tube was invented, Tesla stood on stage with a large metal emitter panel on either side of him and the tube would glow with its own light. Among the other parlor tricks he was making a rarified gas glow. I have this stupid suspicion that something else, perhaps a phenomenon more common, cooks this elliptical galaxy's halo.

VanderL
2004-Oct-26, 09:11 PM
NGC 4555 could be the last galaxy in a small cluster. All other galaxies in the cluster having previously been absorbed by NGC 4555. In such a relatively unusual case, there should be a cluster's worth of dark matter surrounding the object.

This observation is only strange if you assume that all elliptical galaxies should be the same, and must have formed the same way. They aren't and didn't.



Is that your own interpretation or is this something that is agreed upon. I haven't seen anything other that the fact that this is an isolated galaxy with a large hot gas (or plasma) envelope. The conflict in data is with a similar isolated galaxy (also without galaxies anywhere near) that does not show such an envelope.
So either the idea of dark matter haloes after merging is incorrect (there are no galaxies anywhere near), or the other galaxy has somehow lost it's dark matter halo altogether. Either way, I agree with John L.

Cheers.

Greg
2004-Oct-27, 06:52 AM
This kind of speculation simply demonstrates that we still have a long way to go in the field of cosmology when something as basic as dark matter is still so poorly understood. The more questions like this that come up, however, the closer we get to the real answers. So we are getting somewhere, albeit slowly.

GOURDHEAD
2004-Oct-27, 12:44 PM
I don't remember the article addressing the heat source nor the details of the spectroscopic analysis on which the observation is based. If a similar galaxy had no envelope of gas or one that had not been heated to such temperatures, the dark matter constraining force would be more difficult to detect. I am surprised that no mention was made of confirmation of the gas escape restraining effect by observation of the orbiting velocities of the peripheral stars.

Gas at the reported temperature should be plasma and could be constrained by a magnetic field giving rise to some sort of feedback loop that would sustain the effect.

I would like to indict frame dragging if I were capable.

Eric Burgess
2004-Oct-27, 12:52 PM
I know there are much greater minds than mine. However, this dark
> matter halo story puzzles me. They are they talking about dark matter
> keeping the halo at high temperatures but not gravity? If the mass is
> much larger than predicted (maybe several large black holes at the
> center of the galaxy), why can't that pull the gas back towards the
> center and produce the elevated temperatures? Why must it always be
> something we can't measure as yet. Frankly, I hope there is a bunch of
> dark matter. Warp drive here we come!
>
> Eric Burgess

Fraser
2004-Oct-28, 06:10 PM
Here's an official answer Eric.


The answer is that the Chandra observations show that the dark matter is distributed, so we know so we know that the mass is not all clumped in the centre. Our best estimate is that about 2/3 of the mass in the system is outside a radius of 60,000 light years [ie outside the optically luminous part of the galaxy), and we don't see anywhere near enough material out there to explain this. Just to touch a couple of other points in the forum thread, we're pretty sure the galaxy isn't the remnant of a group or cluster of galaxies (it would be more X-ray luminous and have more neighbouring dwarf galaxies), and we can't back up the X-ray mass estimate with orbital measurements because the stars don't go far enough out and it's too distant for globular clusters or planetary nebulae to make good targets.

Ewan O'Sullivan
Principal Investigator