Chandra Helps Explain “Red and Dead Galaxies”
NASA's Chandra X-ray Observatory has shed new light on the mystery of why
giant elliptical galaxies have few, if any, young stars. This new evidence
highlights the important role that supermassive black holes play in the
evolution of their host galaxies.
Because star-forming activity in many giant elliptical galaxies has shut down
to very low levels, these galaxies mostly house long-lived stars with low masses
and red optical colors. Astronomers have therefore called these galaxies "red
and dead”.
Previously it was thought that these red and dead galaxies do not contain
large amounts of cold gas − the fuel for star formation − helping to explain the
lack of young stars. However, astronomers have used ESA's Herschel Space
Observatory to find surprisingly large amounts of cold gas in some giant
elliptical galaxies. In a sample of eight galaxies, six contain large reservoirs
of cold gas. This is the first time that astronomers have seen large quantities
of cold gas in giant elliptical galaxies that are not located at the center of a
massive galaxy cluster.
With lots of cold gas, astronomers would expect many stars to be forming in
these galaxies, contrary to what is observed. To try to understand this
inconsistency, astronomers studied the galaxies at other wavelengths, including
X-rays and radio waves. The Chandra observations map the temperature and density
of hot gas in these galaxies. For the six galaxies containing abundant cold gas,
including NGC 4636 and NGC 5044 shown here, the X-ray data provide evidence that
the hot gas is cooling, providing a source for the cold gas observed with
Herschel. However, the cooling process stops before the cold gas condenses to
form stars. What prevents the stars from forming?
A strong clue comes from the Chandra images. The hot gas in the center of the
six galaxies containing cold gas appears to be much more disturbed than in the
cold gas-free systems. This is a sign that material has been ejected from
regions close to the central black hole. These outbursts are possibly driven, in
part, by clumpy, cold gas that has been pulled onto the black hole. The
outbursts dump most of their energy into the center of the galaxy, where the
cold gas is located, preventing the cold gas from cooling sufficiently to form
stars.
The other galaxies in the sample, NGC 1399 and NGC 4472, are also forming few
if any stars, but they have a very different appearance. No cold gas was
detected in these galaxies, and the hot gas in their central regions is much
smoother. Additionally, they have powerful jets of highly energetic particles,
as shown in radio images from the National Science Foundation's Karl G. Jansky
Very Large Array. These jets are likely driven by hot gas falling towards the
central supermassive black holes. By pushing against the hot gas, the jets
create enormous cavities that are observed in the Chandra images, and they may
heat the hot, X-ray emitting gas, preventing it from cooling and forming cold
gas and stars. The centers of NGC 1399 and NGC 4472 look smoother in X-rays than
the other galaxies, likely because their more powerful jets produce cavities
further away from the center, where the X-ray emission is fainter, leaving their
bright cores undisturbed.
A paper describing these results was published in the February 25, 2014 issue
of the Monthly Notices of the Royal Astronomical Society and is available online. The first author is Norbert
Werner from Stanford University in California.
Image credit: X-ray: NASA/CXC/Stanford Univ/N.Werner et al; Optical: DSS
NASA
Guillermo Gonzalo Sánchez Achutegui
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