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'. The Herschel Space Telescope has discovered massive elliptical galaxies in the nearby Universe containing plenty of cold gas, even though the galaxies fail to produce new stars. Comparison with other data suggests that, while hot gas cools down in these galaxies, stars do not form because jets from the central supermassive black hole heat or stir up the gas and prevent it from turning into stars.
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.
Astronomers have long been debating the physical processes leading to the end of their star formation. They speculated that these galaxies somehow expelled the cold gas, or that they had simply used it all to form stars in the past. Although the reason was uncertain, one thing seemed to have been established: these galaxies are red and dead because they no longer possess the means to sustain the production of stars.
"We looked at eight giant elliptical galaxies that nobody had looked at with Herschel before and we were delighted to find that, contrary to previous belief, six out of eight abound with cold gas", explains Norbert Werner from Stanford University in California, USA, who led the study.
This is the first time that astronomers have seen large amounts of cold gas in red-and-dead galaxies that are not located at the centre of a massive galaxy cluster.
The cold gas manifested itself through far-infrared emissions from carbon ions and oxygen atoms. Herschel's sensitivity at these wavelengths was instrumental to the discovery.
"While we see cold gas, there is no sign of ongoing star formation," says co-author Raymond Oonk from ASTRON, the Netherlands Institute for Radio Astronomy. "This is bizarre: with plenty of cold gas at their disposal, why aren't these galaxies forming stars?"
The astronomers proceeded to investigate their sample of galaxies across the electromagnetic spectrum, since gas at different temperatures shines brightly at different wavelengths. They used optical images to probe the warm gas – at slightly higher temperatures than the cold one detected with Herschel, and X-ray data from NASA's Chandra X-ray Observatory to trace the hot gas, up to tens of millions of K.
"In the six galaxies that are rich in cold gas, the X-ray data show tell-tale signs that the hot gas is cooling," says Werner.
This is consistent with theoretical expectations: once cooled, the hot gas would become the warm and cold gas that are observed at longer wavelengths. However, in these galaxies the cooling process somehow stopped, and the cold gas failed to condense and form stars.
In the other two galaxies of the sample – the ones without cold gas – the hot gas does not appear to be cooling at all.
"The contrasting behaviour of these galaxies may have a common explanation: the central supermassive black hole," adds Oonk.
In some theoretical models, the level of a black hole's activity could explain why gas in a galaxy is able – or not able – to cool and form stars. And this seems to apply for the galaxies studied by Werner and his colleagues, too.
While the six galaxies with plenty of cold gas harbour moderately active black holes at their centers, 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.
The study presented here is based on observations performed with the Photodetector Array Camera and Spectrometer (PACS) on board ESA's Herschel Space Observatory.
In addition, the astronomers also used optical observations from the Southern Observatory for Astrophysical Research (SOAR) telescope in Chile and archival X-ray data from NASA's Chandra X-ray Observatory.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Herschel was launched on 14 May 2009 and completed science observations on 29 April 2013.
N. Werner, et al., "The origin of cold gas in giant elliptical galaxies and its role in fuelling radio-mode AGN feedback", 2014, Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stu006.
The image at the top of the page shows a composite view of the giant elliptical galaxy NGC 5044.
The stellar component, as observed at optical wavelengths, is shown in white at the centre of the image. The other stars scattered around the image are foreground stars from our own Galaxy.
The galaxy is embedded in a hot atmosphere of ionised hydrogen gas, which is shown in blue. With temperatures up to tens of millions of K, the hot gas shines brightly in X-rays and was observed using NASA's Chandra X-ray Observatory.
Observations show that some of the hot gas cools down and flows towards the centre of the galaxy. There, the cold gas may condense and form stars, unless it is reheated or expelled from the galaxy by other agents.
The filamentary network shown in red is warm hydrogen gas, as observed in the H-alpha emission line at a rest wavelength of 656.28 nm with the Southern Observatory for Astrophysical Research (SOAR) telescope in Chile.
Image credit: Digitised Sky Survey/NASA Chandra/Southern Observatory for Astrophysical Research/Very Large Array (Robert Dunn et al. 2010)