# Galaxy rotation curve of higher mass galaxy with same size

1. Jun 26, 2013

### Philosopha

How would a galaxy rotation curve look if every matter simply had a 6 times larger mass than the visible? (please neglect how that could be)
Wouldn't a same size galaxy then reside in a 6 times larger gravitational well so that the spiral arms would still be in the steep part of the well because the galaxy wasn't 6 times larger but same size?

2. Jun 26, 2013

### Mordred

The amount of visible matter is only a small percentage of the mass. You also have the mass of dark matter which comprises the majority of the mass. More visible mass would affect the rotation rates but it will also affect the amount of dark matter in the halo. Assuming it doesn't and the two scenarios have equal dark matter then I would surmise the inner sphere would have a faster rotation curve with the outer spirals slower, if the dark matter ratio changed as well then the curve would be about the same.
I would have to dig out the rotation curve power law for a spiral galaxy to be sure on that though

the graphs on this site and the formula's on the page will help you understand what I'm getting at better than my explanation.

quote from the Milky way example at bottom

1. There is 10x as much dark matter as 'normal' matter!
2. The mass distribution is very large (rdm = 25 kpc).

Last edited: Jun 26, 2013
3. Jun 26, 2013

### Philosopha

thx for that Mordred - not my point :(.... I'm studying Physics, doing research and am familiar with DM.

What would happen if we would take a galaxy and drop it into the gravitational potential well of a 6 times more massive one? Take volcano out of the equation.

4. Jun 26, 2013

### Philosopha

just not far enough into it yet to tackle this last problem of mine... :(

5. Jun 27, 2013

### Mordred

the link has the formulas, involved so you can calculate with the formulas in section 3 if your including DM. If DM is not involved the keplar formula will work thats also on the article. The part I am not sure on is how universal the formula in section 3 is?. Its designed to work with a spiral galaxy so should be fine for your application, but you may want to confirm that.
If you just increase the mass in the center, then the curve should look more like the Kepler rotation in the article.

non controversial articles on this subject is tricky to find, I may have one though been digging around in my archives. Hopefully I can locate it for you.

or these article

http://cdn.intechopen.com/pdfs/1873...ves_in_the_context_of_lambdacdm_cosmology.pdf
http://ircamera.as.arizona.edu/astr_250/Lectures/Lec_22sml.htm
http://ned.ipac.caltech.edu/level5/March01/Battaner/node3.html
http://ned.ipac.caltech.edu/level5/March01/Battaner/revision.html
http://ned.ipac.caltech.edu/level5/March01/Battaner/node7.html this one shows some calculations
http://www.maths.qmul.ac.uk/~wjs/MTH726U/chap5.pdf [Broken]
http://arxiv.org/abs/1110.1925
http://arxiv.org/abs/astro-ph/9606089
http://vixra.org/pdf/1104.0003v2.pdf

as you can see the distribution of matter is critical, more matter int eh center than the outside would lead to faster spin in the center as opposed to outside.

Last edited by a moderator: May 6, 2017
6. Jul 18, 2013

### Philosopha

Thank you Mordred - you did help me a lot. I picked a simplification to have a look at my issue.

What I find funny is that a theory like the CDM-theory that explains the general picture struggels with the rotation curve - (however DM can nicely be molded where we need it to be, so its a bit of a perfect filler), however a theory like MOND that is to describe the rotation curve doesn't solve the general picture. I nearly wonder if there is maybe more than one thing contributing to the complete picture.

7. Jul 18, 2013

### Mordred

glad to be of help, the values of the LCDM are always being fine tuned. Each small change is increasing the precision of LCDM with regards to rotation curves. As you noticed MOND works great for specific circumstances where LCDM covers a broader range of galaxy rotation curves. Part of the problem is inaccuracies in mass energy distributions. As well as not being to directly measure DM.

8. Jul 20, 2013

### Philosopha

Nobuhiro Okabe et al. 2013 ApJ 769 L35 doi:10.1088/2041-8205/769/2/L35

the study of 50 galaxies with weak lensing where the missing mass was mapped towards the visible light - as they said largely in agreement with the CDM-theory? I thought that the rotation curve would the CDM demand to be located differently than this study suggests it is. Did I get something wrong?

9. Jul 22, 2013

### Mordred

Sorry for getting to your question late didn't notice it till recently. Found the arxiv version of the paper you posted. Thank fully it was public released.

http://arxiv.org/pdf/1302.2728v2.pdf

the paper is concerned mainly with galaxy clusters no individual galaxies. Its more of a calibration, constraint paper at a specific range 0.15 < z < 0.3. Its also a paper on the The Navarro-Frenk-White model. In the paper its NFW. This is a "N body simulation model".

http://en.wikipedia.org/wiki/Navarro–Frenk–White_profile

on that page there are reference links at the bottom of the page for more accurate detail on NFW. Until reading this paper its the first I heard of it. Although I have heard of "n body simulations".

one of the references I read previously http://arxiv.org/abs/astro-ph/9508025

Its a decent article with some good information in it. One thing to keep in mind during the publish date of the last article I posted there was debate on whether dark matter was cold (non relativistic) or hot (relativistic)

Last edited: Jul 22, 2013
10. Jul 22, 2013

### Philosopha

Great info -thx - now I need more time