This image of the relatively nearby radio galaxy Centaurus A, shows its giant lobes of radio emission in violet; the main galaxy and its nucleus are at the center. The center shows the central galaxy (massive jet below). New infrared observations of distant radio galaxies have found some with multiple black holes at their nuclei. Centaurus A, also known as NGC 5128, is the closest so-called radio galaxy to Earth, located about 12 million light-years away in the constellation Centaurus.
Centaurus A is also an active galaxy, which means that its central region that exhibits strong emission across many wavelengths of the light spectrum. Active galaxies are powered by an actively feeding black hole millions of times more massive than the sun.
Radio galaxies beam as much as one trillion solar-luminosities of radiation into space at radio wavelengths. They are therefore cosmic beacons, and the light from the most distant ones known was emitted back when the universe was only a few billions of years old (compared with its age today of about 13.7 billion years).
The origin of this intense emission is thought to lie in the hot environment of a massive black hole at the galaxy's nucleus, with the radio emission being produced by electrons moving rapidly in strong magnetic fields. Astronomers seeking to better understand galaxies in general, and the context of the Milky Way's origins, want to know when and how radio galaxies formed, how they evolved, and how they impact their environments.
Luminous radio galaxies of course also contain stars. The relationship between the development of a radio galaxy's stars and its nuclear black hole is still very uncertain. CfA astronomers Steve Willner and Giovanni Fazio, together with nine colleagues, used the Spitzer Space Telescope and its infrared cameras to study the warm, infrared bright dust in a large sample of seventy radio galaxies whose light has been traveling towards Earth for times spanning a range of between seven and one-half to twelve billion years -- the cosmic epochs when astronomers think most galaxy maturation occurred.
The astronomers report that most of these galaxies completed the process of making the majority of their stars when the universe was only about two and one-half billion years old. They also find that differences in the infrared emission of the sample is consistent with the idea that their active nuclei are obscured by dust disks being observed at various angles.
The team reaches one more notable conclusion. Four of the seventy galaxies show evidence in the images for a second supermassive black hole, suggesting that the origin of these galaxies (and perhaps the others as well during some earlier stage of development) was caused by the merger of two smaller galaxies.
Centaurus A, which is one of the brightest sources of radio waves in the sky, also radiates extremely high-energy gamma-rays, recent observations from NASA's Fermi Gamma-Ray Telescope have found.
"This is something we've never seen before in gamma-rays," said Teddy Cheung, a Fermi team member at the Naval Research Laboratory in Washington, D.C.Scientists think these gamma-rays, the highest-energy form of light in the universe, began as remnant radiation from the Big Bang thought to be the beginning of our universe about 14 billion years ago. But this radiation has been amped up to higher energy levels by the energetic particles zooming around magnetic fields in huge lobes that extend from either side of the galaxy, researchers said. This ramp-up in energy of photons is thought to be a fairly common process in the universe, but is the first time it has been observed in this particular situation.
In a radio galaxy, "the black hole somehow diverts some of the matter falling toward it into two oppositely directed jets that stream away from the center," explained Yasushi Fukazawa of Hiroshima University in Japan, and a member of the team that studied the gamma-ray emission.
These jets contain magnetized particles that move near the speed of light. Over the course of tens of millions of years, the jets expand out into two large lobes that straddle the central source of the galaxy and extend out into space about 1 million light-years. The radio waves from the lobes arise as high-speed electrons spiral through the lobes' tangled magnetic fields.When Fermi turned its eye on Centaurus A during its first 10 months of data collection, it found quite a surprise: In addition to the radio waves known to emanate from the lobes, Fermi also detected gamma-rays radiating from the galaxy.
"Not only do we see the extended radio lobes, but their gamma-ray output is more than 10 times greater than their radio output," Cheung said.
The gamma-rays arise through a process that involves the spiraling magnetic fields of the radio lobes and other low-energy radiation that permeates the universe: Photons from the cosmic microwave background (the remnant radiation of the Big Bang), as well as infrared and visible light from stars and galaxies, can collide with the high-energy particles zooming through the lobes of Centaurus A."When one of these photons collides with a super-fast particle in the radio lobes, the photon receives such an energy boost, it becomes a gamma ray," said team member Lukasz Stawarz of the Japan Aerospace Exploration Agency (JAXA) in Japan.
The process that accelerates the photons up to gamma-rays is called inverse Compton scattering and is a common way of making cosmic gamma-rays.
The process had been known to produce X-rays in dozens of active galaxies, but Centaurus A is the first case where astronomers have solid evidence that that microwaves can be boosted up to gamma-ray energies. Cheung had theorized before Fermi launched (in June 2008) that the process would work and produce gamma-rays in this way for galaxies like Centaurus A.
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