Stellar populations and ISM

Stars in Es tend to be older and more metal rich than those in Ss, and as a consequence Es are redder than Ss. You can easily understand why this is so. We discussed this before: if the atmosphere of a star contains more metals, then the dust and the gas will scatter preferentially the blue light hence the stars appears redder. That's one. Secondly, if you have young stars in a stellar population, then the more massive stars may still be alive^7.2, and these are hot and hence blue. So when a given stellar population gets older, it will become redder as well. And in an E galaxy, both effects occur.

The fact that a stellar population appears blue either because it is young, or because it is metal poor, is called the age-metallicity degeneracy: just observing the colour of a population, you cannot tell a young but metal rich population apart from an old but metal poor one. To make sure, you have to have independent constraints on th metallicity $Z$, for example from stellar spectra, or, the astronomer's favourite, from HII regions.

Dust and gas Although I said that a characteristic difference between Es and Ss is that Es don't contain dust or gas, and have no star formation, this is of course not complete true, they usually have some, but much less than Ss. Many Es contain strong dust lanes, the Sombrero galaxy is a beautiful example (Fig. 7.6). Often, there is almost no connection between the orientation of the dust lane, and any other of the galaxy's parameters. It is thought that such dust lanes are evidence that the E recently swallowed a small galaxy. In fact, one sometimes observes very faint rings of stars around an E, and also this could be the result of `galactic cannibalism'. A good example is shown in Figure (7.7).

Figure 7.6: Image of the `Sombrero' galaxy, with its striking dust lane.

Figure 7.7: Image of NGC 3923 taken by David Malin. Several faint shells of stars appear in the outer parts of the galaxy. (see http://www.ast.cam.ac.uk/AAO/images/general/ngc3923.html)