Messier 66, NGC 3627

Messier 66 (NGC 3627) is a spiral galaxy in the constellation Leo. It is part of the famous Leo Triplet, a small group of galaxies that also includes M 65 and NGC 3628.

Along with its neighbor M 65, M 66 was discovered by Charles Messier in 1780. Messier missed these two objects in 1773, when a comet passed between them. Because of this, Admiral Smyth incorrectly assigned their discovery to Pierre Mechain, an error which has since propagated to many other sources.

Halton Arp included M 66 as no. 16 in his Catalogue of Peculiar Galaxies. He also assigned the number 317 to the Leo Triplet (M 65, M 66, and NGC 3628). Four supernovae have appeared in this Messier 66: SN 1973R, SN 1989B, SN 1997bs, and SN 2009hd.

Amateur Observation

M 66 appears only 35' from NGC 3628, and 21' from M 65. With a visual magnitude of 8.9 and apparent dimensions of 8' x 2.5', M 66 is the largest and brightest of the trio. M 66 is a spiral with two bright arms which loop outward from the nucleus; its arms are among the most easily seen of all spiral galaxies.

Properties and Structure

At a distance of 35 million light-years, the apparent separation between M 65 and M 66 corresponds to a distance of 190,000 light years. Their true separation is no doubt considerably greater, because it is unlikely that both galaxies lie at the same distance from us. M66 has an absolute magnitude of -21, a luminosity of 21 billion suns, and a true diameter of at least 75,000 light years.

Messier 66 has a well-developed central bulge, and is classified as type Sb. Its spiral arms are obviously deformed, probably due to encounters with its neighbors. They seem to be distorted, and displaced above the plane of the galaxy. Much dust is visible, as well as a few pink nebulae - signs of star formation - near the end of one of the arms. Gravitational interaction from its past encounter with neighboring NGC 3628 has resulted in an extremely high central mass concentration, and a clump of atomic hydrogen gas that has apparently been removed from one of the spiral arms.