MessageToEagle.com - Supernova explosions and the jets of a monstrous black hole are scattering a galaxy's
star-making gas like a cosmic leaf blower, a new study finds.
The findings, which relied on ultraviolet observations from NASA's Galaxy Evolution Explorer and a host of
other instruments, fill an important gap in the current understanding of galactic evolution.
It has long been known that gas-rich spiral galaxies like our Milky Way smash together to create elliptical
galaxies such as the one observed in the study. These big, round galaxies have very little star formation.
The reddish glow of aging stars comes to dominate the complexion of elliptical galaxies, so astronomers
refer to them as "red and dead."
The process that drives the dramatic transformation from spiral galactic youth to elderly elliptical is
the rapid loss of cool gas, the fuel from which new stars form. Supernova explosions can start the decline
in star formation, and then shock waves from the supermassive black hole finish the job.
Now astronomers think they have identified a recently merged galaxy where this gas loss has just gotten underway.
"We have caught a galaxy in the act of destroying its gaseous fuel for new stars and marching toward being
a red-and-dead type of galaxy," said Ananda Hota, lead author of a new paper in the Monthly Notices of the
Royal Astronomical Society.
"We have found a crucial missing piece to connect and solve the puzzle of this phase of galaxy evolution," said Hota,
an astronomer in Pune, India, conducted the study as a post-doctoral research fellow at the Institute
of Astronomy & Astrophysics at Academia Sinica in Taipei, Taiwan.
Time is running out for the galaxy NGC 3801, seen in this composite image combining light from across
the spectrum, ranging from ultraviolet to radio. NASA's Galaxy Evolution Explorer and other instruments
have helped catch the galaxy NGC 3801 in the act of destroying its cold, gaseous fuel for new stars.
Astronomers believe this marks the beginning of its transition from a vigorous spiral galaxy to a quiescent
elliptical galaxy whose star-forming days are long past. Image credit: NASA/JPL-Caltech/SDSS/NRAO/ASIAA
The supermassive black holes that reside in the centers of galaxies can flare up when engorged by gas during
galactic mergers. As a giant black hole feeds, colossal jets of matter shoot out from it, giving rise to
what is known as an active galactic nucleus. According to theory, shock waves from these jets heat up and
disperse the reservoirs of cold gas in elliptical galaxies, thus preventing new stars from taking shape.
The galaxy Hota and his team looked at, called NGC 3801, shows signs of such a process. NGC 3801 is unique
in that evidence of a past merger is clearly seen, and the shock waves from the central black hole's jets
have started to spread out very recently. The researchers used the Galaxy Evolution Explorer to determine
the age of the galaxy's stars and decipher its evolutionary history.
The ultraviolet observations show that NGC 3801's star formation has petered out over the last 100 to
500 million years, demonstrating that the galaxy has indeed begun to leave behind its youthful years.
The lack of many big, new, blue stars makes NGC 3801 look yellowish and reddish in visible light, and thus middle-aged.
Astronomers believe they have caught the galaxy NGC 3810 at a critical point in its history, just as
it is making the transition from a vigorous spiral galaxy to a quiescent elliptical galaxy whose star-forming
days are long past. This diagram sketches out the stages leading up to its current observed state and beyond.
Image credit: NASA/JPL-Caltech
What's causing the galaxy to age and make fewer stars?
The short-lived blue stars that formed right after it merged with another galaxy have already blown up
as supernovae, triggering a fast outflow of heated gas from NGC 3801?s central regions and banishing the reserves
of cold gas for star making.
Some star formation is still happening in NGC 3801, as shown in ultraviolet wavelengths observed by the Galaxy
Evolution Explorer, and in infrared wavelengths detected by NASA's Spitzer Space Telescope.
But that last flicker of youth will soon be extinguished by colossal shock waves from the black hole's jets, seen in X-ray light by NASA's
Chandra X-ray Observatory.
At nearly 900 kilometers per second, these blast waves rushing outward from the galactic center will reach the outer
portions of NGC 3801 in about 10 million years, scattering any remaining cool hydrogen gas and rendering the galaxy truly red and dead.
Astronomers think the transition captured early-on in the case of NGC 3801 -- from the merger of gas-rich galaxies
to the rise of an old-looking elliptical -- happens very quickly on cosmic time scales.
"The quenching of star formation by feedback from the active galactic nucleus probably occurs in just a billion years.
That's not very long compared to the 10-billion-year age of a typical big galaxy," said Hota.
"The explosive shock wave event caused by the central black hole is so powerful that it can dramatically
change the future course of the evolution of an entire galaxy."
These big round galaxies have very little star formation. We have caught a galaxy in the act of destroying its gaseous fuel
for new stars and marching toward being a red-and-dead type of galaxy,” Hoda said.
Hoda is the lead author of the the paper published in the current issue of the Letters of Monthly Notices of the
Royal Astronomical Society, U.K. He conducted part of the study while he was a post-doctoral research fellow at
the Institute of Astronomy & Astrophysics at Academia Sinica in Taipei, Taiwan.
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