Anyone who has looked at Centaurus A will agree that it is absolutely spectacular! As galaxies go, it’s a very odd-looking garbled object – both in photographs and, for us observers, in our telescopes. In fact, its unique appearance makes it one of the few galaxies that is instantly recognizable from its images when you see it through your telescope – stunningly bright and exquisitely beautiful, its famous dust lane across its middle showing up very well even in modest telescopes, and displaying some fine detail in larger ones.
Photographs show that the dust lane encircling Centaurus A is heavily warped, suggesting that something strange has taken place here… and it has! Indeed, spectacular as Centaurus A is, it becomes spectacularly spectacular when you observe it in conjunction with the nearby Fourcade-Figueroa shred and the dwarf elliptical galaxy NGC 5237, and consider the extraordinary thought that you are looking at the product of a galaxy shredding encounter… a violent merger between an elliptical galaxy and a smaller companion spiral galaxy that took place some 500 million years ago (a theory first proposed in 1954 by Walter Baade and Rudolph Minkowski).
In a 1992 paper, Galaxy shredding. I – Centaurus A, NGC 5237, and the Fourcade-Figueroa shred, R.J. Thomson began with a very succinct explanation of galaxy shredding, “Galaxy shredding occurs when a spiral galaxy undergoes a strong prograde interaction with a massive galaxy. The fragile disc of the spiral galaxy is disrupted (shredded) during the encounter and the massive galaxy captures up to half of the disc material. The rest is ejected from the system as a non-rotating shred of dusty, gas-rich disc material which appears as a blue irregular/starburst galaxy. The robust bulge of the spiral progenitor is relatively undisturbed by the encounter and emerges from the shredding episode as a dwarf elliptical galaxy.”
Thomson went on to write, “A case study of Centaurus A suggests a shred-forming encounter with a spiral galaxy (about the same size as our own galaxy) took place some 500 million years ago. The associated bulge and shred remnants have been identified as the nearby dwarf elliptical galaxy NGC 5237, and the Fourcade-Figueroa shred. The captured disc material now forms the conspicuous ring of gas and dust which girdles Centaurus A, and probably provides the rule that powers the radio emission we see today.”
And here they are…
The paper concluded, “The shredding model described here provides a constant picture of many aspects of the Centaurus A systems, including the relative positions and velocities of Centaurus A, NGC 5237 and the Fourcade-Figueroa shred; the orientation and sense of rotation of the dust lane in Centaurus A; the peculiar nature of the interacting dwarf elliptical galaxy NGC 5237; and the orientation and observed non-rotation of the Fourcade-Figuero shred.The complex shell system of Centaurus A can also be understood in terms of a two-component model: density waves in a thick disc induced by the interaction, plus sharp-edged features in the accretion disc.”
And here we can sit with our telescopes and observe these three absolutely dumbfounding objects, and ponder their history!! How much more thrilling can astronomy get?
The extraordinary Fourcade-Figueroa shred was discovered on May 27, 1970 at the Cerro Tololo Inter-American Observatory, Chile, by the Argentinian astronomer Carlos Raúl Fourcade (1927-1993), along with his night assistant, the Chilean Edgardo Javier Figueroa. They had taken a photograph of the Centaurus A region using the Curtis-Schmidt camera, and when checking the plate Figueroa noticed a large (~6.5’x1.5′) and hitherto un-catalogued object in the corner… the shred! NGC 5128 was discovered by James Dunlop on 29 April, 1826 using his 9-inch f/12 speculum reflector from his home in Parramatta, in New South Wales, Australia. And John Herschel discovered NGC 5237 on 3 June, 1834 using his 18-inch f/13 speculum telescope at the Cape of Good Hope, South Africa.
I observed these three stupendous objects with my 16″ Dobs, and they certainly run the gamut of observing – from bright and easy and detailed, to exceeding faint and difficult and not-detailed!
Mag 6.8 Size 25.7′ x 20.0′ SB 13.5 PA 35d
Although I must have observed this gorgeous galaxy literally hundreds of times, it never fails to take my breath away. In my 10×50 binoculars, this wonderful object looks like a streetlight on a wet and misty evening. I can’t make out its dust lane, although, thanks to being so familiar with the sight of this singular galaxy, I sense a very vague less-brightening across its middle, but this may very well be because I know it’s there… not because it is there. The galaxy lies in a rich and beautiful field of stars, making this a truly gorgeous binocular view. Moving to the telescope, even at 70x the galaxy is gorgeously large and bright, its dust lane appears as a dark vacancy that appears as if it is cleaving the galaxy in half, giving it a curiously biological appearance… it looks like a luminous cell splitting in two. The brightest section of the galaxy is oriented southeast to northwest, and the dust lane runs across the middle of the face in the same direction. At 228x the galaxy is spectacular. The southwestern hemisphere is both larger and brighter, and a bright star studs the silky nebulosity like a small glittering diamond. The northwestern hemisphere is not only smaller and less prominent, it also has a somewhat flatter look. The way the brilliant glow of both hemispheres fades away is gorgeous… fading first to a beautiful translucency against the rich starry background before softly disappearing into the dark sky. And as for the galaxy’s pièce de résistance – its dust lane – superb!! It has a beautiful wavy edge and averted vision reveals some very subtle mottled uneveness. Most wonderful is a very tenuous and very faint streak of very hazy paleness near the centre of the dust lane – that beautiful bright detail so obvious on images!! West of centre, a mag 12 star is superimposed on the dust lane; its bright little glitter accentuating the dark rift beautifully. It staggers the imagination to look at Centaurus A’s dark rift and think about what R.J. Thomson wrote, “The captured disc material now forms the conspicuous ring of gas and dust which girdles Centaurus A, and probably provides the rule that powers the radio emission we see today.” Good grief.
Mag 12.5 Size 1.9′ x 1.6′ SB 13.6 PA 128d
At 228x this galaxy appears as a pretty bright, fairly small, not-quite-round glow, smoothly bright with no noticeable core. It forms the southern vertex of a striking isosceles triangle with two lovely bright yellowy stars – mag 7.0 HD 118483 7′ NE, and mag 7.4 HD 118337 7′ NW. However, this tiny dwarf elliptical galaxy isn’t about what I am seeing in the eyepiece… but what I am actually seeing in the eyepiece… the robust bulge of the spiral progenitor that was relatively undisturbed by the violent encounter and emerged from the shredding episode as a dwarf elliptical galaxy. Not something you see every night, that’s for sure.
Mag 11.4 Size 11.5′ x 1.4′ PA 118
The Fourcade-Figueroa shred was as tough as I expected it to be! And words cannot convey the sense of wonder I felt when, at 150x, I saw the extremely faint, narrow, elongated glow… a ghost of a streak whose extremely low surface brightness made it only-just visible in the rich star field. Looking at the DSS image, it appears that I could only see the brightest part of the central section. I find it almost impossible to describe the colour of any object, never mind a wraith-like streak that was almost translucent against the rich starry background, but this tiny shred is the softest wash of faintest pearl-coloured light. It is very wispy and has no edges, just melts very rapidly into the sky. Averted vision brightened the ghostly glow up slightly (but when what you’re looking at is very ethereal, slightly makes a big difference) but it remained exactly what it is… the merest shred of faintest light. Magnificent!
All in all, just about the most stunningly fascinating trio of objects I have observed!
Image credits: Centaurus A – Hubble ESO; F-F and NGC 5237 – DSS images
Copyright © Susan Young 2018