The strange case of the peculiar spiral galaxy NGC 5474. New pieces of a galactic puzzle

We present the first analysis of the stellar content of the structures and substructures identified in the peculiar star-forming galaxyNGC 5474, based onHubbleSpace Telescope resolved photometry from the LEGUS survey. NGC 5474 is a satellite of the giantspiral M 101, and it is known to have a prominent bulge that is significantly off-set from the kinematic centre of the underlying Hiand stellar disc. The youngest stars (age.100 Myr) trace a flocculent spiral pattern extending out to&8 kpc from the centre of thegalaxy. On the other hand, intermediate-age (age>500 Myr) and old (age>2 Gyr) stars dominate the off-centred bulge and a largesubstructure residing in the south-western part of the disc (SW over-density) and they are not correlated with the spiral arms. The oldage of the stars in the SW over-density suggests that this may be another signature of any dynamical interactions that have shapedthis anomalous galaxy. We suggest that a fly by with M 101, generally invoked as the origin of the anomalies, may not be sufficient toexplain all the observations. A more local and more recent interaction may help to put all the pieces of this galactic puzzle together.