Sombrero Galaxy

Lore & Background

The Sombrero Galaxy was discovered on May 11, 1781 by Pierre Méchain, who described it in a May 1783 letter to J. Bernoulli later published in the Berliner Astronomisches Jahrbuch. Charles Messier made a handwritten note about it and five other objects (now M104–M109) to his personal list, but it was not officially included in the Messier Catalogue until 1921. William Herschel independently discovered it in 1784 and noted a 'dark stratum' in the galaxy's disc, now called a dust lane. In 1921, Camille Flammarion found Messier's personal list and identified the object with NGC 4594, declaring it should be included as M104.

The galaxy's most striking feature is the dust lane crossing in front of its bulge, which is actually a symmetrical ring enclosing the bulge. Most cold atomic hydrogen gas and dust lie within this ring, which is the primary site of star formation. The nucleus is classified as a low-ionization nuclear emission-line region (LINER), likely powered by an active galactic nucleus containing a supermassive black hole. In the 1990s, a group led by John Kormendy demonstrated the presence of a black hole with a mass 1 billion times that of the Sun, though a 2016 study yielded a lower value of (6.4±0.4)×10^8 M☉.

Reader's Guide

The Sombrero Galaxy holds significance as a peculiar object of unclear classification, initially thought to be a spiral galaxy but later found by the Spitzer Space Telescope to have a halo indicative of a giant elliptical galaxy. Its large bulge, central supermassive black hole, and dust lane attract professional astronomers. It is considered by some authors to be the galaxy with the highest absolute magnitude within a radius of 10 megaparsecs of the Milky Way. The galaxy has a relatively large number of globular clusters (1,200 to 2,000), and the ratio of clusters to luminosity is high compared to galaxies with small bulges. At radio and X-ray wavelengths, the nucleus is a strong source of synchrotron radiation, varying only 10–20% in luminosity. In 2006, unidentified terahertz radiation was detected from the nucleus at 850 μm, not originating from dust, synchrotron, bremsstrahlung, or molecular gas. The galaxy lies within a complex filament-like cloud extending south of the Virgo Cluster, but its membership in a formal galaxy group is uncertain, with some methods placing it in a group and others suggesting it is isolated or part of a pair.

Did You Know?

• The Sombrero Galaxy was discovered by Pierre Méchain on May 11, 1781.
• Its dust lane is actually a symmetrical ring that encloses the galaxy's bulge.
• The central supermassive black hole has a mass estimated at 1 billion solar masses (1990s) or (6.4±0.4)×10^8 M☉ (2016).
• Unidentified terahertz radiation at 850 μm was detected from its nucleus in 2006.