Scientists have used Chandra to make a detailed study of an enormous cloud of hot gas enveloping two large, colliding galaxies. This unusually large reservoir of gas contains as much mass as 10 billion Suns, spans about 300,000 light years, and radiates at a temperature of more than 7 million degrees Kelvin.
This giant gas cloud, which scientists call a "halo," is located in
the system called NGC 6240. Astronomers have long known that NGC 6240 is
the site of the merger of two large spiral galaxies similar in size to
our own Milky Way. Each galaxy contains a supermassive black hole at its center. The black holes are spiraling toward one another, and may eventually merge to form a larger black hole.
Another consequence of the collision between the galaxies is that the
gas contained in each individual galaxy has been violently stirred up.
This caused a baby boom of new stars that has lasted for at least 200
million years. During this burst of stellar birth, some of the most
massive stars raced through their evolution and exploded relatively quickly as supernovas.
The scientists involved with this study argue that this rush of
supernova explosions dispersed relatively high amounts of important elements
such as oxygen, neon, magnesium, and silicon into the hot gas of the
newly combined galaxies. According to the researchers, the data suggest
that this enriched gas has slowly expanded into and mixed with cooler
gas that was already there.
During the extended baby boom, shorter bursts of star formation have
occurred. For example, the most recent burst of star formation lasted
for about five million years and occurred about 20 million years ago in
Earth's timeframe. However, the authors do not think that the hot gas
was produced just by this shorter burst.
What does the future hold for observations of NGC 6240? Most likely the two spiral galaxies will form one young elliptical galaxy
galaxy over the course of millions of years. It is unclear, however,
how much of the hot gas can be retained by this newly formed galaxy,
rather than lost to surrounding space. Regardless, the collision offers
the opportunity to witness a relatively nearby version of an event that
was common in the early Universe when galaxies were much closer together
and merged more often.
In this new composite image of NGC 6240, the X-rays from Chandra that
reveal the hot gas cloud are colored purple. These data have been
combined with optical data from the Hubble Space Telescope, which shows
long tidal tails from the merging galaxies, extending to the right and
bottom of the image.
A paper describing these new results on NGC 6240 is available online
and appeared in the March 10, 2013 issue of The Astrophysical Journal.
The authors in this study were Emanuele Nardini (Harvard-Smithsonian
Center for Astrophysics, or CfA, Cambridge, MA and currently at Keele
University, UK), Junfeng Wang (CfA and currently at Northwestern
University, Evanston, IL), Pepi Fabbiano (CfA), Martin Elvis (CfA),
Silvia Pellegrini (University of Bologna, Italy), Guido Risalti
(INAF-Osservatorio Astrofisico di Arcetri, Italy and CfA), Margarita
Karovska (CfA), and Andreas Zezas (University of Crete, Greece and CfA).
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 controls Chandra's science and
flight operations from Cambridge, Mass.
Scale: Image is 3 arcmin across (About 290,000 light years)
Category: Black Holes, Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 16h 52m 59s | Dec +02° 24' 01.70"
Constellation: Ophiuchus
Observation Date: 4 pointings between July 2001 and May 2011
Observation Time: 136 hours 6 min (5 days 16 hours 6 min)
Obs. ID: 1590, 6908, 6909, 12713
Instrument: ACIS
References: Nardini, E et al, 2012, ApJ 765, 141; arXiv:1301.5907
Color Code: X-ray (Purple); Optical (Red, Green, Blue)
Distance Estimate: About 330 million light years (redshift = 0.0245)
