APOD: 2021 August 4 - EHT Resolves Central Jet from Black Hole in Cen A

Good morning, Rallypoint. Today's Astronomy Picture of the Day (APOD) is titled "EHT Resolves Central Jet from Black Hole in Cen A." How do galaxies, black holes, and jets relate to each other? And what does that have to do with an event horizon? Clearly there is a lot to unpack here.

We suspect a black hole is present at the center of most galaxies. We can also observe jets of material ejected along the axis of rotation of the suspected black hole. An event horizon is a boundary beyond which anything - including light and gravity - can affect an observer. That's where the Event Horizon Telescope comes in.

Recall that the speed of light is constant, so we can use the travel time of light to accurately measure distances. When two antenna dishes look at an object in the sky, they view it from slightly different positions. This means light from the object reaches one antenna a bit before it reaches the other. The time difference is only a tiny fraction of a second, but it means the timing of the two signals are a bit off. But by precisely timing the signal from each antenna, astronomers can correlate them so they don’t interfere. This is known as interferometry.

The Event Horizon Telescope (EHT) is a worldwide consortium of radio telescopes that use the principles of interferometry to image distant objects. Centaurus A (also known as Cen A, NGC 5128 or Caldwell 77) is a galaxy that lies 4,000 light years away in the constellation of Centaurus, and was discovered by Scottish astronomer James Dunlop in 1826. It is one of the closest radio galaxies to Earth, and the central black hole in Cen A ejects a relativistic jet that is the source of observed emissions in the radio (as well as X-ray) wavelengths. The EHT allows an effective magnification of 60,000,000 times into the heart of Cen A. As you can see, the results are mesmerizing.