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.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory in Cambridge, Mass., controls
Chandra's science and flight operations.
Fast Facts for NGC 1399:
Scale: Image is 6.5 arcmin across. (about 130,000 light years)
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 03h 38m 29.08s | Dec -35º 27' 02.67"
Constelattion: Fornax
Observation Dates: 3 pointings between 18 Jan 2000 and 08 Jun 2008
Observation Time: 40 hours 32 min (1 day 16 hours 32 min)
Obs. IDs: 319, 4172, 9530
Instrument: ACIS
References: Werner, N. et al, 2014, MNRAS 439, 2291-2306; arXiv:1310.5450
Color Code: X-ray (Blue)
Distance Estimate: About 65 million light years
Fast Facts for NGC 4472:
Scale: Image is 6.5 arcmin across. (about 100,000 light years)
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000: RA 12h 29m 46.90s | Dec +08º 00' 13.00
Constellation: Virgo
Observation Dates: 12 Jun 2000
Observation Time: 11 hours 6 min
Obs. IDs: 321
Instrument: ACIS
References: Werner, N. et al, 2014, MNRAS 439, 2291-2306; arXiv:1310.5450
Color Code: X-ray (Blue)
Distance Estimate: About 55 million light years
Fast Facts for NGC 4636:
Scale: Image is 6.5 arcmin across. (about 95,000 light years)
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 12h 42m 49.87s | Dec +02° 41' 16.01"
Constellation: Virgo
Observation Dates: 3 pointings between 26 Jan 2000 and 15 Mar 2003
Observation Time: 55 hours 57 min (2 days 7 hours 57 min)
Obs. IDs: 323, 3926, 4415
Instrument: ACIS
References: Werner, N. et al, 2014, MNRAS 439, 2291-2306; arXiv:1310.5450
Color Code: X-ray (Blue)
Distance Estimate: About 50 million light years
Fast Facts for NGC 5044:
Scale: Image is 6.5 arcmin across. (about 190,000 light years)
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 13h 15m 23.97s | Dec -16º 23' 08.00"
Constellation: Virgo
Observation Dates: 2 pointings on 19 Mar 2000 and 07 Mar 2008
Observation Time: 28 hours 39 min (1 day 4 hours 39 min)
Obs. IDs: 798, 9399
Instrument: ACIS
References: Werner, N. et al, 2014, MNRAS 439, 2291-2306; arXiv:1310.5450
Color Code: X-ray (Blue)
Distance Estimate: About 102 million light years
Source: NASA’s Chandra X-ray Observatory
