This is yet another fairly bright galaxy with a complicated discovery history. It was first observed by William Herschel on September 13, 1784. However, Heinrich d'Arrest observed it on August 23, 1862, and later it was failed to realise that these were observations of the same object. So the d'Arrest object became NGC 523 and the Herschel object NGC 537. Due to its very strange shape, Arp put this galaxy in the category Galaxies (not classifiable as S or E); Disturbed with interior absorption.
NED has these notes on this galaxy:
- "Post-eruptive, blue compact with 3 compact knots connected by a bright bar, fan-shaped jets and matrix."
- "Post-eruptive, blue, 3 compact knots connected by bright bar, fan-shaped jets and matrix (CGPG)"
- "Possibly a colliding or strongly interacting pair. Compact core."
- "Peculiar spiral, with two nuclei on sides (east-west). Chincarini G., and Heckathorn, H. M. 1973, Pub. A.S.P., 85, 568. claim that the west nucleus is a foreground star, but our measured redshift is similar to that of the east one."
This paper is about neutral hydrogen detection in the system. The abstract of the paper reads
"… This is a disturbed system with distinct optical knots connected by a linear structure embedded in luminous material. There is also a diffuse spray to the southeast. The HI seems to be made up of three distinct, kinematically separate systems. Arp 158 bears a certain optical resemblance to NGC 520 (Arp 157), which has been identified as a mid-stage merger. From our 21 cm observations of Arp 158, we also see a comparable HI content with NGC 520. These similarities suggest that Arp 158 is also an intermediate stage merger."
So, we are dealing here with a triple system that is in collision. "What does it look like?" you may ask. Here is a picture of the galaxy.
I observed this galaxy on September 9, 2016, from SQM 21.5 skies in the south of France using my 20" telescope. Please note that I am not working my way through the Arp galaxies. I am merely trying to observe as many NGC and IC objects as possible. Imagine my surprise when I came across this galaxy! My notes read
"At 320x very intriguing. A kind of bar with a star superimposed on the western end, a faint stellar nucleus and a condensation to the east. The bar is fairly thin. Between the star and the nucleus another very faint bar starts running to the south east. The central bar continues west of the star but very faintly."
Of course this was before I read up on this galaxy so the star to the west actually turns out to be the western nucleus!
As always,
"Give it a go and let us know!
Good luck and great viewing!"
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