The rotation of our galaxy and many others have been measured using Doppler shifts of the 21cm (radio) line of hydrogen
|
The rotation velocity of a galaxy can be measured from Doppler shifts
of its spectral lines. With the distance of the galaxy known, we can use Kepler/Newton
laws to compute the mass of the galaxy; in fact, we can map the mass as a function of the
distance from the center of the galaxy. The rotation of our galaxy and many others have been measured using Doppler shifts of the 21cm (radio) line of hydrogen |
 |
 |
| If the mass followed the "normal" matter -- stars and gas -- the rotation speed would drop like the "Keplerian motion" line, like for the planets. | Instead, the rotation curve is nearly flat with increasing radius. Evidently there are huge amounts of unseen "dark" matter in the outer parts of the galaxy. (Figures from The Essential Cosmic Perspective, by Bennett et al.) |
 |
Like the Milky Way, virtually all galaxies have flat rotation curves to well beyond where they have many stars, indicating that they are all surrounded by large halos of dark matter. (From The Essential Cosmic Perspective, by Bennett et al.) |
When we account carefully for the mass in stars in a galaxy, it turns out to be much less than the mass we measure from Newton's laws! In addition, there appears to be mass we can't see outside the region occupied by the stars. As much of 90% of galaxies may be in some form of unseen mass.
We have no good idea of what galaxies are mostly made of!! Could they have many
brown dwarfs? Or is there some basic particle of physics that we don't know about that
accounts for the unseen mass? This is evidently the dark matter we know played such a
central role in shaping the Universe, but all we know about local examples is from galaxy
rotation curves.
A good link for
further information is at http://www.eclipse.net/~cmmiller/DM/.
Test your understanding before going on
 Marvel Quasar Comic,
http://www.coincidental.net/comics/series/quasar/1-033.html |
|
Pillars in M16, NASA, HST, J. Hester, P. Scowen, APOD http://antwrp.gsfc.nasa.gov/apod/ap951106.html |
Click to return to syllabus |
||
| Click to return to Active Galactic Nuclei | hypertext © G. H. Rieke |
Click to go to the Interstellar Medium |
