Messier 104, NGC 4594 - Sombrero Galaxy

Messier 104 is known as the "Sombrero" galaxy. It is a spiral galaxy in the constellation Virgo, whose bright nucleus, unusually large central bulge, and prominent dust ring give it the appearance of a Mexican hat.

History and Discovery

The Sombrero Galaxy was discovered in 1767 by Pierre Mechain. Charles Messier made a note of this object and five others (now recognized as M 104 through M 109) in his personal list of objects, but did not include them in his original published catalog. William Herschel independently discovered the object in 1784, and noted the presence of a "dark stratum" in the galaxy's disc. M 104 was "officially" included in the Messier Catalog in 1921 by Camille Flammarion, who found Messier's personal list, and identified the Sombrero with Herschel's H I.43 and with NGC 4594.

In 1912, M 104 became the first galaxy for which a large redshift was found, by Vesto Slipher of Lowell Observatory. M 104's redshift corresponds to a recession velocity of about 1100 km/sec - too fast for it to be an object inside our Milky Way galaxy. Slipher's observations were among the first key pieces of evidence for of the expansion of the universe and the Big Bang Theory. Slipher also observed rotation in the spectrum of the Sombrero, making it the first galaxy in which rotation was observed.

Amateur Observation

The Sombrero Galaxy is located near the southern boundary of Virgo, 11.5° west of Spica and 5.5° northeast of η Corvi. It is a 9th-magnitude spiral with apparent dimensions of 8' x 4', easily visible in amateur telescopes. This galaxy's most striking feature is the massive dust band that cuts across its nucleus. Although the galaxy is visible with binoculars or a 4-inch telescope, at least an 8-inch telescope is needed to distinguish the bulge from the disk, and a 10- to 12-inch telescope is needed to see the dark dust lane.

M 104 is an unbarred spiral galaxy is of type Sa-Sb, with a big bright core and well-defined spiral arms. We view it from just 6 degrees south of its equatorial plane. Very deep photographs show that the galaxy has a faint, extended halo.

Physical Properties

At least two methods have been used to measure the distance to the Sombrero Galaxy; their results average 29.3 million light years, with an uncertainty of 1.6 million. It has a linear diameter of 130,000 light-years. The symmetric dust ring that encloses the central bulge also contains most of the galaxy's cold hydrogen gas, and is the primary site of star formation within the galaxy.

M 104 has a mildly active nucleus, as evidenced by visible emission lines and radio emission. Based on the stellar dynamics in its core, a research group demonstrated in the 1990s that a supermassive black hole, 1 billion times the mass of the Sun, is present in the center of the Sombrero Galaxy.

The Sombrero has a relatively large number of globular clusters; observations have produced population estimates in the range of 1200 to 2000. This is high compared to the Milky Way and other galaxies with small bulges, but similar to other galaxies with large bulges.

The Sombrero Galaxy lies within a cloud of galaxies that extends to the south of the Virgo Cluster. Previously accepted to be a member of the Virgo-Coma Cluster, it is currently unclear as to whether the Sombrero Galaxy is part of a formal galaxy group.