Pan: Arp 4

These galaxies look to be close companions — a small, bright spiral galaxy flitting around the edge of a much larger spiral with a dark and disturbed countenance. But looks can be deceiving — how close are they really? The celestial pair featured in this week’s Hubble Picture of the Week is known by the name Arp 4, and lies in the constellation Cetus (the Whale).

The designation Arp 4 comes from the Atlas of Peculiar Galaxies, compiled in the 1960s by astronomer Halton Arp. “Unusual galaxies” were selected and photographed to provide examples of weird and non-standard shapes, the better to study how galaxies evolve into these forms. Throughout its mission the Hubble Space Telescope has revolutionised the study of galaxies and shown us some fantastically unusual examples from Arp’s atlas. In that catalogue, the first few galaxies like Arp 4 are “low surface brightness” galaxies, a type of galaxy that is unexpectedly faint and hard to detect. The large galaxy here — also catalogued as MCG-02-05-050 — fits this description well, with its fragmentary arms and dim disc. Its smaller companion, MCG-02-05-050a, is a much more bright and active spiral.

The trick is that these galaxies are not actually very close. The large blue galaxy MCG-02-05-050 is located 65 million light-years from Earth; its brighter smaller companion MCG-02-05-050a, at 675 million light-years away, is over ten times the distance! Owing to this, MCG-02-05-050a is likely the larger galaxy of the two, and MCG-02-05-050 comparatively small. Their pairing in this image is simply an unlikely visual coincidence. Despite this lack of a physical relation between them, our point of view on Earth allows us to enjoy the sight of Arp 4 as an odd couple in the sky.