A distant galaxy was trapped in the act of closing.
The galaxy, called CQ 4479, is still forming many new stars. But it also has a supermassive black hole that actively feeds at its center and will stop star formation within a few hundred million years, astronomers reported Jan. 11 at the American Astronomical Society's virtual meeting. Studying this galaxy and the like will help astronomers find out exactly how such stops occur.
“How galaxies die is precisely an open question,” says astrophysicist Allison Kirkpatrick of the University of Kansas at Lawrence. "That could give us a lot of insight into that process."
Astronomers think that galaxies usually start making new stars with a passion. Stars form from cold gas bags that contract under their own gravity and ignite thermonuclear fusion at their centers. But at some point, something interrupts the cold fuel that forms stars and sends it into the supermassive black hole in the galaxy's core. That black hole thickens the gas, heating it white. An actively feeding black hole can be seen billions of light years away and is known as a quasar. The radiation from the hot gas pumps extra energy to the rest of the galaxy, blowing or heating the remaining gas until the star-forming plant closes permanently (SN: 3/5/14).
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That image fits with the types of galaxies astronomers typically see in the universe: “blue and new” star formers and “red and dead” latent galaxies. But as they examined data from major sky surveys, Kirkpatrick and colleagues noticed another type. The team found about two dozen galaxies that emit energy X-rays characteristic of an actively swallowing black hole, but also glow with low-energy infrared light, revealing that there is still cold gas in some part of the galaxies. Kirkpatrick and colleagues christened these galaxies as "cold quasars" in a September 1 issue of the Astrophysical Journal.
“When you see a black hole actively augmenting the material, you expect star formation to have already stopped,” says co-author and astrophysicist Kevin Cooke, also of the University of Kansas, who presented the research at the meeting. "But the cold quasars are at a strange time when the black hole in the center has just begun to feed."
To investigate individual cold quasars in more detail, Kirkpatrick and Cooke used SOFIA, an aircraft equipped with a telescope that can see in a series of infrared wavelengths that the original observations of the cold quasar did not cover. SOFIA analyzed CQ 4479, a quasar cold about 5.25 million light-years away, in September 2019.
Observations have shown that CQ 4479 has about 20 billion times the mass of the sun in the stars and is adding about 95 suns a year. (It's a furious rate compared to the Milky Way; our home galaxy builds two or three solar masses of new stars a year.) The central black hole of CQ 4479 is 24 million times more massive than the sun and is growing at a 0.3 solar masses per year. In terms of percentage of their total mass, stars and the black hole grow at the same rate, Kirkpatrick says.
K.C. Cooke et al / arxiv.org 2020, Sloan Digital Sky Survey
That kind of “step-by-step evolution” goes against theories of how galaxies darken and shrink. “You should have all your stars finish growing first and then grow your black hole,” Kirkpatrick says. "This (galaxy) shows that there is a period when they really grow together."
Cooke and colleagues have estimated that in half a trillion years the galaxy will host 100 billion solar masses of stars, but its black hole will be passive and quiet. All cold star-forming gas has been heated or ruined.
Observations from CQ 4479 support broad ideas of how galaxies die, says astronomer Alexandra Pope of the University of Massachusetts Amherst, who did not participate in the new work. Since galaxies disable their star formation, it makes sense that there should be a transition period. The findings are a "confirmation of this important phase in the evolution of galaxies," she says. Looking more closely at the colder quasars will help astronomers discover how quickly galaxies die.