We have found an unexpected class of astronomical objects which have notpreviously been reported, in the Evolutionary Map of the Universe Pilot survey,using the Australian Square Kilometre Array Pathfinder telescope. The objectsappear in radio images as circular edge-brightened discs about one arcmindiameter, and do not seem to correspond to any known type of object. Wespeculate that they may represent a spherical shock wave from an extra-galactictransient event, or the outflow, or a remnant, from a radio galaxy viewedend-on.

We present new spectra obtained using Keck/KCWI and perform kinematics andstellar population analyses of the shell galaxy NGC 474, from both the galaxycentre and a region from the outer shell. We show that both regions havesimilarly extended star formation histories although with different stellarpopulation properties. The central region of NGC 474 is dominated byintermediate-aged stars (8.3 \pm 0.3 Gyr) with subsolar metallicity ([Z/H]=-0.24 \pm 0.07 dex) while the observed shell region, which hosts a substantialpopulation of younger stars, has a mean luminosity-weighted age of 4.0 \pm 0.5Gyr with solar metallicities ([Z/H]=-0.03 \pm 0.09 dex). Our results areconsistent with a scenario in which NGC 474 experienced a major to intermediatemerger with a log\((M_*/M_\odot)\sim10 \) mass satellite galaxy at least \sim 2Gyr ago which produced its shell system. This work shows that the directspectroscopic study of low-surface brightness stellar features, such as shells,is now feasible and opens up a new window to understanding galaxy formation andevolution.

Stellar shells around galaxies could provide precious insights into theirassembly history. However, their formation mechanism remains poorly empiricallyconstrained. We present MUSE@VLT data of the most prominent outer shell ofNGC474, to constrain its formation history. The stellar shell spectrum isclearly detected, with a signal-to-noise ratio of around 65 pix$^{-1}$. We usea full spectral fitting method to determine the line-of-sight velocity and theage and metallicity of the shell and associated point-like sources within theMUSE field of view. We detect six globular cluster (GC) candidates and eightplanetary nebula (PN) candidates which are all kinematically associated to thestellar shell. We show that the shell has an intermediate metallicity, [M/H] =$-0.83^{+0.12}_{-0.12}$ and a possible alpha-enrichement, [alpha/Fe] ~ 0.3.This metallicity and the number of PNe are consistent with a progenitor galaxymore massive than 10^9 Msun. The photometry of the shell suggest that it ispossibly composed of stars from the low metallicity outskirts of anintermediate mass (~1:10) companion. We show that at least two globular clustercandidates are quite young, with ages below 1.5 Gyr, which gives a lower-limitfor the merger age. We note that spectroscopic data on the center of NGC~474shows the presence of a young (~1~Gyr) stellar population. We conclude that theshell formation event is likely to be an intermediate-mass merger whichhappened around 1.5~Gyr ago and which has collaterally triggered the formationof massive, up to ~ 6 x 10^4 Msun, star cluster, and a nuclear starburst inNGC474.