Up in Ursa Minor is NGC 5144, a galaxy over 150 million light-years away. Also known as UGC 8420, Mrk 256, or Zw VII 511, it is a very small spiral galaxy. At 68x in my 16-inch, the galaxy is a very small glow while at 300x it's a round glow of fairly even surface brightness with a faint star just in from its south edge. Upping the magnification to 440x, a diffuse core is seen for moments while the star is still brighter and still stellar.
What's interesting is that on images, that "star" I was seeing is a knot in one of the galaxy's arms. However, it doesn't have much for an H-II presence associated with it. Now, I've gotten good at being able to pick out what an H-II/SFR looks like on SDSS and PanSTARRS images. And I have a fair idea of what an interacting dwarf companion would look like (case in point, Mrk 1379a in NGC 5534 below).
Screenshot 2023-04-27 9.42.04 AM.png
NGC 5534 PanSTARRS DR1
But this thing is different. It's not a young H-II/SFR because there is very little nebula. But if it's a dwarf galaxy, it's been 90% integrated into NGC 5144's spiral pattern (because it fits in perfectly in one of its arms). So, it's a star-forming region for sure. But I am having trouble deciding if it is an internal creation or from the merging of a dwarf companion galaxy a long time ago.
Screenshot 2023-04-27 9.42.19 AM.png
NGC 5144 PanSTARRS DR1
Steve Gottlieb observed it seven years ago with his 24-inch and wrote, "This appears to be another post-merger system with two nuclei though it could be a single highly disrupted system. The general glow is moderately bright, irregularly round, fairly small, 0.7'x0.6', mottled or uneven surface brightness but no distinct core. Situated at the midpoint of a mag 11.8 star 1.9' NNW and a mag 13.7 2.0' SSW. [The second nuclei] appeared as a quasi-stellar knot (less than 6" diameter) at the south edge of the halo, just 18" from the center of the main galaxy!"
On his website, Courtney Seligman wrote, "UGC lists this as a superimposed companion of NGC 5144, but given the complex structure of NGC 5144 it is probably just a star-forming region in that galaxy." However, Steve notes that "NGC 5144 probably has a merged companion at the south edge, catalogued as NGC 5144 NED01 as well as as LEDA 200298."
Any thoughts would be appreciated. I'm not planning on including it into any future Sky & Telescope articles or anything, but I'm personally leaning towards describing it as a large, bright star-forming region that could possibly have been created by the merging of a small dwarf companion galaxy.
Scott H.
[EDIT: Title should read The Mysterious Nature of NGC 5144]
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