Physics h/wk.

The Majestic Sombrero Galaxy

The Sombrero Galaxy (also known as M104 or NGC 4594 ) is an unbarred spiral galaxy in the constellation Virgo located 28 million light years from earth. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. The galaxy has an apparent magnitude of +9.0, making it easily visible with amateur telescopes. The large bulge, the central super massive black hole are the main attraction.

The Sombrero Galaxy was discovered in March 1767 by Pierre Méchain, who described the object in a May 1767 letter to J. Bernoulli that was later published in the Berliner Astronomisches Jahrbuch. William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disc, what is now called a dust lane.

Constellation Virgo
Right ascension 12h 39m 59.4s
Declination -11° 37′ 23″
Apparent dimension (V) 8′.7 × 3′.5
Apparent magnitude (V) 8.98

Helio Radial velocity 1024 ± 5 km/s Red shift 0.003416 ± 0.000017
Galactocentric Velocity 904 ± 7 km/s Distance 29.3 ± 1.6Mly (9.0 ± 0.49 Mpc)

This galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy. This dust lane is actually a symmetric ring that encloses the bulge of the galaxy. Most of the cold atomic hydrogen gas and the dust lies within this ring. The ring might also contain most of the Sombrero Galaxy's cold molecular gas, although this is an inference based on observations with low resolution and weak detections. Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring. Based on infrared spectroscopy, the dust ring is the primary site of star formation within this galaxy.

The nucleus of the Sombrero galaxy is classified as a low ionization nuclear emission region (LINER). These are nuclear regions where ionized gas is present, but the ions are only weakly ionized (i.e. the atoms are missing relatively few electrons). The source of energy for ionizing the gas in Liners’ has been debated extensively. Some LINER nuclei may be powered by hot, young stars found in star formation regions, whereas other LINER nuclei may be powered by active galactic nuclei (highly energetic regions that contain super massive black holes). Infrared spectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation activity. However, a super massive black hole has been identified in the nucleus (as discussed in the subsection below), so this active galactic nucleus is probably the energy source that weakly ionizes the gas in the Sombrero Galaxy.

In the 1990s, a research group led by John Kormendy demonstrated that a super massive black hole is present within the Sombrero Galaxy. Using spectroscopy data from both the CFHT and the Hubble Space Telescope, the group showed that the speed of revolution of the stars within the centre of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun, or 109 M☉, is present in the centre. This is among the most massive black holes measured in any nearby galaxies.