This image of the polar-ring galaxy
NGC 660, which is "swimming" in the constellation of Pisces at a distance of about 45 million light-years, is the result of about 30 hours of integration (roughly 19 hours in luminance and 11 hours in RGB colour) gathered over the course of 15 nights from mid-October to the end of December 2022. The plate scale is 0.47"/pixel, and the field of view spans about 30' x 25'.
Polar rings are rare and peculiar structures
comprised of gas, dust and stars that rotate over the poles of a host galaxy, and are found only around lenticular (S0) and elliptical galaxies (in fewer than 1% of those galaxy types). They are thought to be produced by an interaction between a pair of galaxies, the
two leading scenarios being that a ring is either created around one galaxy from material that is tidally stripped from another passing galaxy, or is formed by a merger of two galaxies that undergo an "orthogonal" head-on collision. Polar rings can be used to
probe the dark matter halo of the host galaxy, since the ring typically lies well outside the galactic disk, where it is subject to the gravitational influence of the halo.
The ring of NGC 660 is
not actually polar, but is inclined with respect to the disk by about 55 degrees, and also appears to be twisted and warped, which
suggests that it is
composed of material captured from a passing galaxy, rather than produced by a merger. The aftermath of the collision is also manifest in the vigorous
star-forming regions in the ring, and activity in the galactic nucleus, which shows prominent emission lines characteristic of a
Seyfert nucleus. Observations by the now defunct Arecibo radio observatory showed that a
spectacular radio outburst was emitted from a compact region near the galaxy core between about 2008 and 2012, and followup radio observations discovered
new jet-like features, which suggest that the nucleus has entered a new period of increased activity.
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