Sand and Stars

Centaurus A, Fourcade-Figueroa Shred, NGC 5237

5 May 2019

Centaurus A (left) NGC 5237 (centre) Fourcade-Figueroa (right). Image credits Hubble and DSS

This trio of objects has to offer one of the most extraordinary observations one can make, for when you observe the three of them in conjunction you are looking at the result 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.”

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 they are…

The three objects spectacularly photobombed by Omega Centauri! DSS image annotated

How much more thrilling can astronomy get than sitting at one’s telescope, observing these three absolutely dumbfounding objects, and pondering their history?


Observing this spectacular trio

16″ f/5 Dobs at magnifications of 70x, 150x, 228x and 333x 

10×50 binoculars

These three objects certainly run the gamut of observing – from bright and easy and detailed, to exceeding faint and difficult and not-detailed!

Centaurus A with its beautifully warped dust lane. Image credit Hubble/ESO

Centaurus A (NGC 5128)

RA 13 25 27.6   Dec -43 01 09   Mag 6.8   Size 25.7′x20.0′   SB 13.5      

As galaxies go, Centaurus A (which lies about 12 million light-years away) is 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 heavily warped dust lane draped across its middle showing up very well even in modest telescopes, and displaying some fine detail in larger ones.

Although I must have observed this gorgeous galaxy literally dozens of times, it never fails to take my breath away. In my 10×50 binoculars, this wonderful object looks like a distant streetlight on a dark, wet and misty evening. 

At 70x in the telescope 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.

Cent A
Centaurus A’s dust lane – science and aesthetics as one. Image credit Hubble/ESO

And as for the galaxy’s pièce de résistance – its dust lane – superb!! At 333x 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.

NGC 5237. DSS image

NGC 5237

RA 13 37 38.8   Dec -42 50 51   Mag 12.5   Size 1.9′ x 1.6′   SB 13.6    

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.

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.

Fourcade-Figueroa Shred. DSS image

Fourcade-Figueroa Shred

RA 13 34 47.3   Dec -45 32 51   Mag 11.4   Size 11.5′x1.4′   PA 118°   

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!

The 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 enhanced the ghostly glow 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!

Copyright © Susan Young 2019