Arp 227, a pair of interacting galaxies

Arp 227, a pair of interacting galaxies

Image Credit: Jean-Charles Cuillandre, CFHT (, Giovanni Anselmi, Coelum Astronomia ( and Jean Baptiste Faure (

Arp 227 consists of two galaxies in the constellation Pisces: the large (250,000 light-years across) lenticular galaxy NGC 474 (also known as UGC 864) located about 93 million light-years away, and the spiral galaxy NGC 470 at about 95 million light-years away. They lie at a separation of about 160,000 light-years.

Astronomers detected two additional members of the group which indicates that the pair constitutes the dominant members of a loose group. Evidence also suggests that Arp 227 is an evolving group in the early phase of its evolution and that its drivers are the accretion of faint galaxies and the ongoing large-scale interaction between NGC 470 and 474.

There is a tidal tail of gas and dust that connects NGC 474 to NGC 470, showing that the two are currently undergoing interaction. The low X-ray luminosity of NGC 470 seems to be a characteristic of dynamically young systems.

NGC 474 is a classic shell galaxy. These are usually the result of a merger though there’s no evidence of this in this case. All stars in it have a common motion. Normally if there’s a recent merger there are two families of stellar motion evident in the galaxy. That isn’t the case here.

The origin of the faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with NGC 470. But if NGC 470 caused the distortions in 474 why isn’t it similarly distorted as its mass appears less it should be even more torn up? This remains unknown.

Alternately the shells could be caused by a merger with a smaller galaxy (or galaxies) in the past billion years, producing an effect analogous to ripples across the surface of a pond.

This image dramatically highlights the increasing consensus that the outer halos of most large galaxies, including our Milky Way Galaxy, are not really smooth but have complexities induced by frequent interactions with — and accretions of — smaller nearby galaxies.

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