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)?
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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.