Astronomers using Atacama Large Millimeter/Submillimeter Array (ALMA) have captured a new image of the centre of the distinctive galaxy Centaurus A.
This image of Centaurus A combines ALMA and near-infrared observations of the massive elliptical radio galaxy (ALMA / ESO / NAOJ / NRAO / Y. Beletsky)
Centaurus A is a massive elliptical radio galaxy – a galaxy which emits strong radio waves – and is the most prominent, as well as by far the nearest, radio galaxy in the sky. Centaurus A has therefore been observed with many different telescopes. Its very luminous centre hosts a supermassive black hole with a mass of about 100 million times that of the Sun.
In visible light, a characteristic feature of the galaxy is the dark band that obscures its centre. This dust lane harbors large amounts of gas, dust and young stars. These features, together with the strong radio emission, are evidence that Centaurus A is the result of a collision between a giant elliptical galaxy, and a smaller spiral galaxy whose remains form the dusty band.
To see through the obscuring dust in the central band, astronomers need to observe using longer wavelengths of light. This new image of Centaurus A combines observations at wavelengths around one millimeter, made with ALMA, and observations in near-infrared light. It thus provides a clear view through the dust towards the galaxy’s luminous centre.
The new ALMA observations, shown in a range of green, yellow and orange colors, reveal the position and motion of the clouds of gas in the galaxy. According to ESO, they are the sharpest and most sensitive such observations ever made.
ALMA was tuned to detect signals with a wavelength around 1.3 millimeters, emitted by molecules of carbon monoxide gas. The motion of the gas in the galaxy causes slight changes to this wavelength, due to the Doppler effect. The motion is shown in this image as changes in color. Greener features trace gas coming towards us while more orange features depict gas moving away. We can see that the gas to the left of the centre is moving towards us, while the gas to the right of the centre is moving away from us, indicating that the gas is orbiting around the galaxy.
