Daily Image
18-02-2014
Changing your profile
| Submitter: | Erwin de Blok |
| Description: | One of the most important properties of a galaxy is its mass. This helps us determine, for example, how much dark matter there is in a galaxy. Radio observations are typically used for this, as the neutral hydrogen in galaxies (which we observe in the 21-cm line with radiotelecopes) is very extended, and gives us the clearest picture of the mass distribution. By measuring how fast a galaxy rotates we can then measure its "dynamical mass". Unfortunately, most galaxies are too distant for these kind of detailed observations, but we can use the "global profile" of a galaxy to still give us an estimate. A global profile shows us how much emission there is at each observed velocity in a galaxy. The width of that profile is then used as an estimate of the rotation velocity. The shape of the profile depends not only on the rotation velocity of a galaxy, but also on the distribution of the gas in that galaxy. In the top panel of the picture we see the same galaxy (NGC 3198) in neutral hydrogen gas (left), infra-red stellar light (middle) and molecular gas (CO; right). The hydrogen is more extended than the stars, while the molecular gas is less extended. This affects the shape of the global profile. The bottom panel shows for one model galaxy what its global profile would look like in neutral hydrogen (dark gray, widest profile) and in CO (light orange profile). Other colours show profiles for various other gas distributions. The shape and width of the profiles clearly vary significantly. In a paper that is available at http://arxiv.org/abs/1401.8158 Erwin de Blok (ASTRON) and Fabian Walter (Max Planck Institute for Astronomy, Heidelberg) model many of these profiles and do a detailed study of the impact that these different shapes have on studies of high- and low-redshift galaxies. Observations of gas in galaxies are usually done in neutral hydrogen at low redshift, but in CO at high redshift. The clear difference in profile shape, for the same galaxy, shows that one has to be careful in comparing observations of the dynamics of galaxies at low and high redshift. |
| Copyright: | Erwin de Blok |
| Tweet |  |