Question

Draw the rotation curve of two spiral galaxies in one figure. Each galaxy has a stellar disk that extends out to 20 kpc. Galaxy A has twice as much mass within a radius of 20 kpc. Galaxy B shows evidence for the presence of dark matter. Explain and be quantitative?

Answer 1

Rotation curves are also the major tool to derive the mass distribution in external galaxies, and are usually derived from Doppler velocities of the emission lines such as Hα. The entire rotation curve of the Galaxy is obtained by combining the observed rotation velocities from the various methods. However, when they calculate the rotation velocities from the data, the observers often adopt different parameters. This had led to rotation curves of the Galaxy in different scaling both in R and V (R).

Figure shows the progress in the determination of the rotation curve of the Milky Way Galaxy

The ubiquitous presence of dark matter in spiral galaxies is well established , as is the increase of the dark matter fraction with decreasing luminosity. However, crucial issues at the heart of galaxy formation theories are still open, such as the actual density profile of the dark halo from the centre of the galaxy out to its virial radius, or the nature of the dark matter. In particular, the cold dark matter (CDM) theory predicts a well-defined radial density profile for the collisionless particles which make up the dark halo.

The contributions of the different mass components to the observed circular velocities are shown: gas (dotted line), stars (dashed line), and DM halo (dash-dot line). The full thick line shows the total contribution to the rotation velocity.