What percentage of early-type galaxies (ellipticals) sometimes retain residual gas?
Answer
Around 25%
While the defining characteristic of elliptical galaxies is their lack of cold gas and the resulting cessation of star formation, modern astronomical observations have revealed that this rule is not absolute for every system. A measurable minority of these early-type galaxies, estimated to be around 25 percent, still possess discernible residual gas reservoirs. The presence of this gas allows for low-level, ongoing star formation in these systems, contradicting the 'red and dead' generalization for a significant fraction. The mechanisms allowing gas retention might involve incomplete stripping during mergers or slow accretion from external streams.
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