NASA missions investigate a ‘TIE fighter’ active galaxy

Not so long ago, astronomers mapped a galaxy far, far away with radio waves and found that it had a strikingly familiar shape. In the process, they discovered the object, called TXS 0128 + 554, experienced two powerful attacks of activity in the last century.

About five years ago, NASA’s Fermi Gamma-ray Space Telescope reported that TXS 0128 + 554 (short TXS 0128) is a faint source of gamma rays, the highest energy form of light. Scientists, meanwhile, have taken a closer look at NASA’s Very Long Baseline Array (VLBA) and Chandra X-ray Observatory.

“After the Fermi announcement, we zoomed in a million times closer to the galaxy with the VLBA’s radio antennas and over time mapped the shape,” said Matthew Lister, a professor of physics and astronomy at Purdue University in West Lafayette, Indiana. “The first time I saw the results, I immediately thought it looked like Darth Vader’s TIE fighter jet from ‘Star Wars: A New Hope’. That was a nice surprise, but the performance on various radio frequencies also helped us more. to learn about how active galaxies can change dramatically on time scales. “

A paper describing the findings, led by Lister, was published in the August 25 issue of the Astrophysical journal and is now available online.

TXS 0128 lies 500 million light-years away in the constellation Cassiopeia, anchored by a supermassive black hole about 1 billion times the mass of the sun. It is classified as an active galaxy, which means that all its stars together can not count the amount of light it emits.

The extra energy of an active galaxy includes redundant radio, X-ray and gamma-ray light. Scientists believe that this emission originates from regions near the central black hole, where a swirling disk of gas and dust accumulates and heats up due to gravity and frictional forces.

Around a tenth of active galaxies produce a pair of jets, beams of high-energy particles that travel at almost the speed of light in opposite directions. Astrophysicists think that these jets produce gamma rays. In some cases, collisions with linear intergalactic gas slow down and stop the outward motion of ray particles, and the material begins to flow back to the center of the galaxy. This results in wide regions, or lobes, filled with fast-moving particles that revolve around magnetic fields. The interactions of particles create clear radio emission.

Fermi has identified more than 3,000 active galaxies with its Large Area Telescope, which examines the entire sky every three hours. Almost all of them are stretched so that one jet points almost directly at the earth, which stimulates their signals. However, TXS 0128 is about 100,000 times less powerful than most. Indeed, although relatively close, Fermi had to collect data from the galaxy for five years before reporting it in 2015 as a gamma-ray source.

Researchers then added the Milky Way to a long-running survey conducted by the VLBA, a network of radio antennas operated by the National Radio Astronomy Observatory that stretches from Hawaii to the U.S. Virgin Islands.

The measurements of the array provide a detailed map of TXS 0128 at different radio frequencies. The radio structure they reveal covers 35 light-years and extends about 50 degrees outside our line of sight. This angle means that the jets are not aimed directly at us and may explain why the galaxy is so dim in gamma rays.

“The real world universe is three-dimensional, but when we look at space, we normally see only two dimensions,” said Daniel Homan, a co-author and professor of astronomy at Denison University in Granville, Ohio. “In this case, we are happy, because the Milky Way is angled in such a way, from our perspective, that the light of the far lobe travels tens of more light-years to reach us than the light of the nearest one. “This means we see the further lobe at an earlier point in its evolution.”

If the galaxy were aligned so that the jets and lobes were perpendicular to our line of sight, all the light would reach the earth at once. We would see both sides in the same stage of development, what they are in reality.

The apparent shape of the galaxy depends on the radio frequency used. At 2.3 gigahertz (GHz), about 21 times the maximum broadcast frequency of FM radio, it looks like an amorphous blob. The TIE fighter format appears at 6.6 GHz. Then at 15.4 GHz a clear gap appears in the radio emission between the core of the galaxy and its lobes.

Lister’s team suspected a lull in the activity of TXS 0128 made this gap. It seems that the galaxies of the galaxy started about 90 years ago, as observed from Earth, and then stopped about 50 years later, leaving the unconnected lobes behind. Then, about a decade ago, the jets turned on again, producing the emission that was seen closer to the core. What caused the sudden onset of these active periods remains unclear.

The radio emission also sheds light on the location of the star’s gamma ray signal. Many theorists have predicted that young, radio-bright active star systems produce gamma rays when their jets collide with intergalactic gas. But in the case of TXS 0128, the particles in the lobes at least do not produce enough combined energy to generate the detected gamma rays. Instead, Lister’s team thinks that the galaxy’s jets produce gamma rays closer to the nucleus, as the majority of active galaxies Fermi sees.

The team observed the galaxy in X-rays with Chandra, looking for evidence of an enclosing cocoon of ionized gas. Although their measurements cannot confirm the presence or absence of a cocoon, there has been evidence for such structures in other active galaxies, as Cygnus A. is consistent with a very skewed viewing angle.

“This galaxy reminds us of the importance of observations at multiple wavelengths, and looks at objects across a wide range on the electromagnetic spectrum,” said Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Fermi, the VLBA, and Chandra each add a layer to our growing image of this object, and discover their own surprises.”

The Fermi Gamma-ray Space Telescope is a partnership for astrophysics and particle physics managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fermi was developed in collaboration with the U.S. Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center monitors science and aircraft operations from Cambridge and Burlington, Massachusetts.