It is widely known that elliptical galaxies have relatively little or no measurable gas, but how is this determined? What is the amount of gas? Is there a known ratio of stellar mass to gas for ellipticals? How does this vary between normal ellipticals and those that were formed by mergers (or still look like merger remnants)?
There is an ongoing survey of molecular gas in elliptical galaxies, and the first results are summarized in The ATLAS3D project - IV. The molecular gas content of early-type galaxies.
Some key points:
- Molecular gas is detected in only about 20% of galaxies they observed. They are looking specifically for CO, which is the most easily detected molecule, and their upper limit for the 80% of non-detections is roughly 10 million solar masses of molecular gas. Compare this to roughly a billion solar masses of molecular gas for the Milky Way, and a median stellar mass of 30 billion solar masses for the galaxies in the sample, and you can see that molecular mass is a negligible component of many elliptical galaxies.
- There are some elliptical galaxies with considerable molecular mass. A few galaxies have molecular masses of order a billion solar masses, comparable to the Milky Way.
- The most isolated elliptical galaxies tend to be the most gas rich. There are also some dynamic trends that indicate that galaxies with a more active merger past have less remaining molecular gas.
- The molecular gas in these galaxies appears to be forming stars. So, while elliptical galaxies are generally assumed to not be forming stars, there is a non-negligible population of elliptical galaxies that are in the process of star formation.