The Southern Pinwheel Galaxy (M83) is an astrophotography favourite at this time of year. The galaxy lies at a distance of around 15 million light years in the southern sky constellation of Hydra, and I’ve recently learned that it has a few tricks up its sleeves when subjected to longer exposure times, especially from a dark(ish) imaging location.
A 1997 research paper by astronomers David Malin (Anglo-Australian Observatory, NSW) and Brian Hadley (Royal Observatory, Edinburgh) describes unusual faint features that can be revealed in very deep images of even well-known galaxy targets, including M83. They present image data collected by the UK Schmidt Telescope at the high-altitude Siding Spring Observatory in Coonabarabran NSW, including a deep image of M83 which reveals “an enormous loop around the NW quadrant of the galaxy.” They also make mention of a “very extensive H1 halo with 80 percent of the detected H1 beyond the Holmberg radius [ie what is usually regarded as the galaxy’s “edge”], much more than is normal in such galaxies.”
[Ref: Malin, D & Hadley, B 1997, ‘HI in Shell Galaxies and Other Merger Remnants’, Publications of the Astronomical Society of Australia, vol. 14, no. 1, pp. 52–58.]
My image consists of about 20 hours of OSC broadband colour data combined with about 2.5 hours of narrowband hydrogen and oxygen (via a OSC dual band filter). The extended “halo” is quite obvious, though a little more work is required to see the loop. I’ve included here the full field-of-view, an inverted (and significantly stretched) image to better showcase the “loop”, a crop to show the full-res detail of galaxy core, and finally some relevant excerpts from the Malin and Hadley paper.
Skywatcher 200PDS, Sharpstar 0.95x MPCC, QHY294C, NEQ6
Optolong UV-IR, 243 x 5min subs
Optolong L-Extreme, 14 x 10min subs
Image data collected from Strathalbyn SA. Processed with Pixinsight.
full res:
https://www.astrobin.com/ogs7rg/
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