SALT - Southern African Large Telescope

Discovery of a Triple Galaxy Merger: The Bird

The Bird in colour: the blue shows where new stars are forming vigourously; the green indicates areas rich in gas clouds (both from Hubble images); the red shows where either dense dust clouds or many old stars can be found. (NASA/STSci/Vaisanen/ESO)
Infrared image of the Bird, labelled and scaled. The wings span 100000 light years. (Vaisanen/ESO).

The power of SALT and the European Southern Observatory's Very Large Telescope (VLT) in Chile has led astronomers to discover and understand the chaotic aftermath of a three-galaxy collision where nearly 200 times the mass of our sun is going into the birth of new stars each year!

Prior to the SALT and VLT observations the rather strange-looking object on the right was known simply as a 'luminous infrared galaxy'. Upon closer inspection however, it was revealed to be interacting triplet of galaxies 650 million light years away, radiating strongly at infra-red wavelengths.

Surprises came from the VLT and SALT. Firstly, infrared images using adaptive optics (for ultrasharp images) at the VLT let astronomers peer through the dense dust clouds to see this system clearly – and the third galaxy of the triplet was discovered. As you can see the whole system resembles a bird, the main components were named the 'Heart', 'Body' and 'Head', with fainter 'Wings' and 'Tail' completing the picture by the astronomers who made the observations.

Scientists at SALT led by Dr Petri Vaisanen and Dr Alexei Kniazev then analysed light from the 'Bird' using the Robert Stobie Spectrograph, an instrument which breaks light up into its component colours – this made it possible to study the physical conditions and motions of the three colliding galaxies in detail.

The brightest component (the 'Body') is a rather disturbed-looking galaxy, part of which extends about 20000 light years South to form the 'Tail'. It and the 'Heart' both weigh about 30-70 billion times as much as our Sun, making them roughly a tenth as massive as our own Milky Way, and they appear to have first collided about 200-300 million years ago. Their gravitational interaction pulled some of their stars and gas out into 'tidal tails' which are the 'Wings' extending across 100000 light years. The 'Head' is a third galaxy, which we see as it makes a first high-speed fly-by through the system at a speed of more than 400 km per second! The collision has so disturbed and compressed the gas clouds in the 'Head' that nearly 200 times the mass of our Sun is being formed into new stars each year. Why most of the star formation is in the Head, with much less happening in the Heart or the Body, still remains a mystery however.

The Heart and the Body are already on their way to merging together, and in half a billion years or so all three galaxies will probably have merged into a featureless 'elliptical' galaxy with all the dust and gas involved in the spectacular action we now see either expelled into space or turned into stars.

The original press release is available here.