NGC 5474
A distorted dwarf spiral galaxy and closest companion to M101.
NGC 5474 is a peculiar dwarf spiral galaxy in the constellation Ursa Major, located approximately 6.98 Mpc from Earth. It is a satellite companion to the larger Pinwheel Galaxy (M101) and is the closest of M101's four satellite galaxies, with a projected distance of 90 kpc from M101's photometric center. The galaxy is notable for its strong distortion resulting from a past interaction with M101, leaving its disk offset relative to its nucleus.
Quick Facts
- Constellation
- Ursa Major
- Names
- UGC 9013, PGC 50216
Facts from the source article.
M101 Interaction
Approximately 200 million years ago, NGC 5474 collided with M101, passing through the west side of M101's outer disk and distorting the morphology of both galaxies. The interaction was strongly asymmetric due to M101 being roughly an order of magnitude more massive, subjecting NGC 5474 to significant tidal forces while M101 showed comparatively modest perturbations. The photometric center of NGC 5474 has been dramatically offset from its kinematic center, though a weak, lopsided spiral arm structure remains. No significant tidal tails are present around either galaxy, a result explained by a dual retrograde geometry of the interaction. This configuration is analogous to other dwarf-dominant pairs such as the Large Magellanic Cloud and Milky Way system. Although the strongest phase of interaction has passed and no close encounter is projected within the next billion years, the galaxies remain gravitationally connected through a shared neutral hydrogen bridge and asymmetric envelope. The gas bridge is oriented in the expected direction of M101's tidal force, and the lack of high-to-intermediate velocity clouds around NGC 5474 suggests M101 is siphoning gas from the dwarf. The morphological change is ongoing; because the stellar disk revolves opposite to its orbital revolution, the distortion may diminish after several galactic rotations, but the galaxy is unlikely to fully return to its original form.
Star Formation
NGC 5474 outshines the typical dwarf galaxy threshold by 1.5 magnitudes, despite generally low star formation rates among such galaxies. Its irregular spiral arms do not inherently restrict star formation; well-defined and patchy spirals form stars at comparable rates, differing only in whether that activity is concentrated in small zones or scattered across the disk. However, a shortage of neutral hydrogen may slow NGC 5474's star formation.
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