NGC 3646

Lore & Background

NGC 3646 has a structure consisting of three parts: an inner hub, a rapidly rotating spiral, and an irregular outer ring surrounding the spiral with an angular feature at one point of the ring. Burbidge et al. estimated that their measurements of motion in the outer ring were not consistent with stable circular orbits, but Afanas'ev et al. argued that this conclusion was erroneous, based on incorrect measurements. Instead, Afanas'ev et al. found a galaxy rotation curve that places the galaxy among the most rapidly rotating and massive systems, describing it as one of the largest and most luminous spirals in the local universe.

NGC 3646 forms an isolated pair with a smaller companion galaxy, NGC 3649. The high rate of star formation in NGC 3646, the low rate in its companion, and the unusual shape of the outer ring in NGC 3646 may have resulted from interactions between these two galaxies. Four supernovae have been observed in NGC 3646: SN 1989N (Type II, mag. 14.5) discovered on 30 June 1989; SN 1999 cd (Type II, mag. 17.9) discovered on 14 May 1999; SN 2020abqw (Type II, mag. 19.2155) discovered on 5 December 2020; and SN 2021abqs (Type II, mag. 19.7848) discovered on 17 October 2021.

Reader's Guide

NGC 3646 is significant as a galaxy whose classification and dynamics have been subject to scientific dispute. Its description as both a strange spiral and a ring-shaped galaxy, along with conflicting analyses of its outer ring motion, highlights the challenges in interpreting galactic structure. The work of Burbidge et al. and Afanas'ev et al. illustrates an ongoing debate about whether the outer ring's motion is consistent with stable circular orbits or indicates a more extreme rotational state. If Afanas'ev et al. are correct, NGC 3646 ranks among the most rapidly rotating and massive spiral systems in the local universe, making it a key object for studying galaxy dynamics and evolution. Its interaction with the companion NGC 3649, suggested by the high star formation rate in NGC 3646 and the unusual ring shape, provides a natural laboratory for understanding how galaxy pairs influence each other's morphology and star formation. The four recorded supernovae, all Type II, add to its observational history and offer opportunities for studying stellar explosions in a dynamic environment.

Did You Know?

• It has been described both as a strange spiral galaxy and as a ring-shaped galaxy.
• Afanas'ev et al. argued that the galaxy is one of the most rapidly rotating and massive spirals in the local universe.
• Four Type II supernovae have been observed in NGC 3646 between 1989 and 2021.