Harry observes the “Woomera” galaxy pair – NGC 1532 & 1531


 NGC 1532 & 1531, commonly called the “Woomera”, reminds Harry of a Mimi spear thrower from the Arnhem Land area. Unless otherwise noted sketch and copyright Harry Roberts ©, all rights reserved

Regular correspondent and solar observer Harry Roberts reports on his observations of the galaxys NGC 1532 & 1531.

NGC1532: ‘Woomera’

Summer in the southern hemisphere: the South Galactic Pole is overhead and nights are short. With the winter Milky Way gone and the Orion ‘arm’ rising, it’s time for viewing the ‘realm of the galaxies’! Fainter constellations like Sculptor, Cetus, Fornax etc, beckon those who would see galaxies by the dozen.

Most galaxies are ‘poker-faced’ – giving little away: featureless ovals with faint nuclei; but some offer a glimpse of a more dramatic existence –when galaxies collide.  One such is NGC1532 (aka Dunlop 600) in the constellation of Eridanus, the River.

On first impression this is a big edge-on spiral with, seemingly, two nuclei! What’s going on? Higher power shows a small bright elliptical ‘object’ almost in contact with the edge-on’s nucleus. Are these interacting galaxies? References suggest they are: although there is no extended disc to be seen around the small galaxy, NGC1531. The common name for this galaxy pair is ‘Woomera’, the long thin Aboriginal spear-thrower.

In fact, this object seems to still have the ‘spear’ attached! Repeat viewings showed a thin strand of galaxy ‘arm’- aimed at a nearby field star. The whole scene was much like a ‘Mimi’ art figure from Arnhem Land in Australia’s north (Fig above).

Wiki hasn’t much to say: “NGC1532 may possess several dwarf companion galaxies. The galaxy is clearly interacting with one of these, the amorphous dwarf galaxy NGC1531. Tidal forces from this interaction have created unusual plumes above the disk of NGC1532. NGC1532 is also an outlying member of the Fornax Cluster”.

NGC1532 is 5.6-arcmin x 1.8-arcmin in size, with yellow magnitude (approx. visual) of 10.6: a fairly bright galaxy.

Blue arm. While NGC1532 is largely reddish/yellow older population II stars, there is an arm segment of blue ‘young’ stars on the SW edge of the big Sb’s disk: this was just visible – perhaps because it was so blue – and curved toward the 10th mag field star (arrows in Fig). This ‘blue’ arm is likely a burst of new star formation caused by the close interaction of 1532 with 1531. The latter may once have had a surrounding disc of younger stars – but perhaps they have been stripped off by the gravity of 1532, and the 1531 nucleus is all that is left. It’s likely 1531 is merging with 1532.

Dotted in the sketch is where faint streaks were sensed. We see them clearly on Gendler’s ‘deep’ picture of the pair on the wiki page. The ten inch ‘Dob’ was used for the sketch and I wished for more aperture!

All in all NGC1532 is an amazing galaxy pair –one with its own story to tell. Take a look when next under the deep dark skies of Eridanus.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.


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