Around five years ago, NASA’s Fermi Gamma-ray Space Telescope detected high-energy gamma rays coming from TXS 0128+554, an elliptical galaxy located some 500 million light-years away in the constellation of Cassiopeia. Purdue University’ Professor Matthew Lister and colleagues have since taken a closer look using NSF’s Very Long Baseline Array (VLBA) and NASA’s Chandra X-ray Observatory.
“After the Fermi announcement, we zoomed in a million times closer on the galaxy using the VLBA’s radio antennas and charted its shape over time,” Professor Lister said.
TXS 0128+554 hosts a supermassive black hole of around one billion solar masses, and is classified as an active galaxy, which means all its stars together can’t account for the amount of light it emits.
An active galaxy’s extra energy includes excess radio, X-ray, and gamma-ray light. Astronomers think this emission arises from regions near the central black hole, where a swirling disk of gas and dust accumulates and heats up because of gravitational and frictional forces.
Around one-tenth of active galaxies produce a pair of jets, beams of high-energy particles traveling at nearly the speed of light in opposite directions.
Scientists think these jets produce gamma rays. In some cases, collisions with tenuous intergalactic gas eventually slow and halt the outward motion of jet particles, and the material starts to flow back toward the galaxy’s center.
This results in broad regions, or lobes, filled with fast-moving particles spiraling around magnetic fields. The particle interactions create bright radio emission.
Using the VLBA, a network of radio antennas stretching from Hawaii to the U.S. Virgin Islands, Professor Lister and colleagues created a detailed map of TXS 0128+554 at different radio frequencies.
The radio structure they revealed spans 35 light-years across and tilts about 50 degrees out of our line of sight. This angle means the jets aren’t pointed directly at us and may explain why the galaxy is so dim in gamma rays.
“We’re lucky because the galaxy is angled in such a way, from our perspective, that the light from the farther lobe travels dozens more light-years to reach us than the light from the nearer one,” said Denison University’s Professor Daniel Homan, co-author of the study.
“This means we’re seeing the farther lobe at an earlier point in its evolution.”
“If TXS 0128+554 was aligned so the jets and lobes were perpendicular to our line of sight, all the light would reach Earth at the same time. We would see both sides at the same stage of development, which they are in reality.”
The galaxy’s jets appear to have started around 80 years ago, as observed from Earth, and then stopped about 50 years later, leaving behind the unconnected lobes.
Then, roughly a decade ago, the jets turned on again, producing the emission 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 galaxy’s gamma-ray signal.
Many theorists predicted that young, radio-bright active galaxies produce gamma rays when their jets collide with intergalactic gas.
But in TXS 0128+554’s case, at least, the particles in the lobes don’t produce enough combined energy to generate the detected gamma rays.
Instead, the authors thinks the galaxy’s jets produce gamma rays closer to the core, like the majority of active galaxies Fermi sees.
The team also observed TXS 0128+554 in X-rays using Chandra, looking for evidence of an enveloping cocoon of ionized gas.
While their measurements couldn’t confirm the presence or absence of a cocoon, there has been evidence for such structures in other active galaxies, like Cygnus A.
The observations do indicate TXS 0128+554 has a large amount of dust and gas…