Giant elliptical galaxies are the most puzzling type of galaxy in the Universe. Since they mysteriously shut down their star-forming activity and remain home only to the longest-lived of their stars – which are low-mass ones and appear red – astronomers often call these galaxies ‘red and dead’.

Up until now, it was thought that red-and-dead galaxies were poor in cold gas – the vital raw material from which stars are born. While cold gas is abundant in spiral galaxies with lively star formation, the lack of it in giant ellipticals seemed to explain the absence of new stars.

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While the six galaxies with plenty of cold gas harbour moderately active black holes at their centres, the other two show a marked difference. In the two galaxies without cold gas, the central black holes are accreting matter at frenzied pace, as confirmed by radio observations showing powerful jets of highly energetic particles that stem from their cores.

The jets could be an effect of the hot gas cooling down, and flowing towards the centre of the galaxies. This inflow of cold gas can boost the black hole’s accretion rate, launching the jets that are observed at radio wavelengths.

The jets, in turn, have the potential to reheat the galaxy’s reservoir of cold gas – or even to push it beyond the galaxy’s reach. This scenario can explain the absence of star formation in all the galaxies observed in this study and, at the same time, the lack of cold gas in those with powerful jets.

“These galaxies are red, but with the giant black holes pumping in their hearts, they are definitely not dead,” comments Werner.

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