Explanation:
What's happening to galaxy NGC 474? The multiple layers of emission appear
strangely complex and unexpected given the relatively featureless appearance of
the elliptical galaxy in less deep images. The cause of the shells is currently
unknown, but possibly tidal tails related to debris left over from absorbing
numerous small galaxies in the past billion years. Alternatively the shells may
be like ripples in a pond, where the ongoing collision with the spiral galaxy
just above NGC 474 is causing density waves to ripple though the galactic
giant. Regardless of the actual cause, the above image dramatically highlights
the increasing consensus that at least some elliptical galaxies have formed in
the recent past, and that the outer halos of most large galaxies are not really
smooth but have complexities induced by frequent interactions with -- and
accretions of -- smaller nearby galaxies. The halo of our own Milky Way Galaxy
is one example of such unexpected complexity. NGC 474 spans about 250,000 light
years and lies about 100 million light years distant toward the constellation
of the Fish (Pisces)
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