Stellar Populations in Ellipticals

Some facts:

Elliptical galaxies are much redder than spirals.
Elliptical galaxies have much less atomic and molecular hydrogen gas than spirals.

What kind of stars would you expect to find in elliptical galaxies?

Here's a spectrum of an elliptical galaxy. What do we notice?

Conventional thinking (as of 10 years ago or so): Elliptical galaxies are old, single-burst population galaxies.

Stellar population synthesis tracks: B-V color vs age, for a "single burst" population of evolving stars of different metallicities:

So two things make a stellar population red: old age, and high metallicity. The colors of a galaxy cannot distinguish between the two.

Review question: why would high metallicity make stars redder?

Elliptical galaxies (like spirals) show a color-luminosity relationship: brighter galaxies are redder. This is well-established to be a metallicity effect, not age. So brighter galaxies are more metal-rich.

(galaxy photometry in the cluster Abell 901/2; Wolf et al 2005)

Also, ellipticals are not devoid of gas. But it's not neutral hydrogen, or molecular hydrogen. Where is it?

Look at the elliptical galaxy M49 in optical and X-ray:

(Thanks to Beth Brown, University of Michigan)

There is extended, X-ray emitting gas. How hot would it have to be to emit X-rays? Set the energy of an X-ray photon equal to the thermal energy of particles:

Solving for T, we get a temperature of ~ 10⁶ K or so.

Mass estimates range from 10⁸ to maybe as much as 10⁹ M_sun of hot gas in ellipticals.

Questions:

Where did this gas come from?
Why is it hot?
What holds it up?