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The oddest member of the Leo Triplet

NGC 3628 is a spiral galaxy and a member of a small, but conspicuous group of galaxies located about 35 million light-years away, toward the constellation of Leo (the Lion). The other distinguished members of this family, known collectively as the Leo Triplet, are two well-known prominent spiral galaxies, Messier 65 and Messier 66 (not seen on the image), which were both discovered in 1780 by famous French comet hunter Charles Messier. NGC 3628 is the faintest of the trio and escaped Messier’s observations with his rather small telescope. It was discovered and catalogued by William Herschel only four years later.

NGC 3628 hides its spiral structure because it is seen perfectly edge-on, exactly as we observe the Milky Way on a clear night. Its most distinctive feature is a dark band of dust that lies across the plane of the disc and which is visibly distorted outwards, as a consequence of the gravitational interaction between NGC 3628 and its bullying companions. This boxy or “peanut-shaped” bulge, seen as a faint X-shape, is formed mainly of young stars and gas and dust, which create the bulge away from the plane of the rest of the galaxy through their powerful motions. Because of its appearance, NGC 3628 was catalogued as Arp 317 in the Atlas of Peculiar Galaxies, published in 1966, which aimed to characterise a large sample of odd objects that fell outside the standard Hubble classification, to aid understanding of how galaxies evolve.

The depth of the image reveals a myriad of galaxies of different shapes and colours, some of which lie much further away than NGC 3628. Particularly noticeable is the fuzzy blob just in the centre of the image, which is a diffuse satellite galaxy. A number of globular clusters can be seen as fuzzy reddish spots in the halo of the galaxy. Also visible as bright spots near the lower edge of the image (the two blue star-like objects below the satellite galaxy) are two quasars, the central engines of distant and very energetic galaxies, billions of light-years away.

This image has been taken with the FORS2 instrument, attached to one of the ESO Very Large Telescope’s Unit Telescopes. It is a combination of exposures taken through different filters (B, V and R), for a total exposure time of just below one hour. The field of view is about 7 arcminutes across, which is why this large galaxy does not fit into the image.