Talk:Dark matter halo

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ERROR![edit]

This article is in error, particularly in the section Milky Way dark matter halo, which says "The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center.". Sorry I don't have time to fix it. The dark matter halo is not just surrounding the galactic disk (which has a diameter of ~100,000 ly). It also has to permeate the disk, otherwise it could have little gravitational effect on the disk stars (see Newton's Principia). This is a common mistake due to the awful terminology "halo," which unfortunately has become standard. So the dark matter "covers the space between the galactic center out to 300,000 light-years," and you might check on that 300,000 ly figure (larger?). The disk is embedded in a big, probably spherical blob of dark matter whose density scales roughly as 1/r, r being distance from the galactic center. DCCougar (talk) 11:41, 24 April 2012 (UTC)


NFW Profile[edit]

My understanding is that the NFW profile is no longer considered to be the best representation of dark matter halos. It has been replaced by the "Einasto" profile, which is similar to the Sersic profile that is used to describe ordinary galaxies. Can someone who is an expert in this field please confirm this, and update the article as necessary? 1210Poppy (talk) 11:22, 8 October 2008 (UTC)

Scale of halo[edit]

It is my understanding that there are two uses for the term "dark matter halo". The small-scale use is addressed here, where an individual galaxy can be defined in terms of it's luminous and dark matter components, the latter far greater in mass and volume than the former. The large-scale use of the term is addressed in part in Halo Occupation Distribution (HOD), where the large scale structure of the universe can be characterized as bubbles of dark matter or 'halos' in which form multiple galaxies, the HOD going to describe the galactic mass distribution in relation to an individual 'halo'. I think it would be useful for these concepts to be reconciled with one another to the benefit of the general public. Note that I did not start the Halo Occupation Distribution article, but rewrote it and added references. Regards, --User:Ceyockey (talk to me) 00:48, 16 December 2007 (UTC)

How Does the Dark Matter Make The Outer Stars Move[edit]

Can anyone explain why the dark matter would make the outer stars of the galaxy move faster as you approach the central bulge and then level off in a plateau. If black matter is equally distributed why would there be any difference at all in the rotation speed. I'd appreciate an explanation for a layman. Thanks. Johnor (talk) 08:21, 15 September 2008 (UTC)

Contradictory statement[edit]

"The dark matter halo is the hypothetical gravitational core of a galaxy."

This statement seems contradictory. When is a halo a core?—RJH (talk) 20:26, 2 April 2010 (UTC)

Here's the original version of the text:
Most of the mass of any galaxy is dominated by a component concentrated at the centre of the galaxy but dominating its dynamics throughout, known as the dark matter halo.
At some point, someone tried to make it a bit more concise - which was a good thing - and some of the meaning got lost in translation. How about this:
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. As it consists of dark matter, the halo cannot be observed directly, but its existence can be inferred through its effect on the motions of stars and gas in the galaxy. Dark matter halos play a key role in current models of galaxy formation and evolution.
Cosmo0 (talk) 22:34, 3 April 2010 (UTC)
Actually I like that so much I'm probably just going to go ahead and add it if no-one objects. Cosmo0 (talk) 22:36, 3 April 2010 (UTC)

The image displaying observed vs expected velocities on the galaxy rotation curve seems to be wrong[edit]

Observed velocities on the galaxy rotation curve cannot be lower than expected velocities or dark matter could not be the explanation. Additional (dark or other) matter could only increase the observed velocities and never decrease them below those we conclude from the visible matter creating the gravitational attraction. Yet, in this image, there is a whole section where the blue line is below the red line. RothschildM (talk) 09:20, 17 June 2010 (UTC)

I agree. The standard picture of visible vs dark matter rotation curves is GalacticRotation2.svg (as shown in Dark Matter). The picture is also wrong since the velocity of the Sun is not as expected from visible matter only.71.208.118.191 (talk) 05:36, 1 April 2011 (UTC)
Yes, it is incorrect. Although, perhaps it's worth redoing rather than replacing with the simple diagram as it has some extra detail that might be nice to keep, i.e. real data for the Milky Way from Combes 1991 and error bars showing increasing uncertainty with radius. I don't have the data from Combes 1991, but perhaps something along these lines using the data from Sofue 2012/2009 might be used as a replacement, e.g.. A figure showing the contributions of the separate components to the rotation curve might be useful as well i.e. contributions from the bulge, disk and dark halo. --92.4.177.142 (talk) 18:17, 29 June 2012 (UTC)

Description of Milky Way Galaxy?[edit]

Under the section about the Milky Way Dark Matter Halo, only really the first few sentences are relevant. The rest of the section is a standard description of the MW Galaxy but has nothing to do with the DM halo. Should everything after "The Milky Way is a large galaxy..." be deleted (or at least moved into the article on the Milky Way? 71.208.118.191 (talk) 05:36, 1 April 2011 (UTC)

I suppose they were trying to give some context, but I agree it is way too much detail and something that could probably be replaced with a diagram. --92.4.177.142 (talk) 18:17, 29 June 2012 (UTC)