Sombrero Galaxy, located in the constellation of Virgo was added to Messier catalogue many years after Messier's death, actually a good 140 years after the last edition of the catalogue that Messier published himself. Of course not without any reason...
From discovery to becoming Messier object
Sombrero Galaxy, like many other Messier objects was not discovered by Messier himself, but by his friend and assistant, Pierre Méchain on the 11th of May, 1781. Probably too late for get it added it to the last edition of the catalogue that Messier published himself in the same year. That edition contained 103 objects, meaning that M103 is the last object that Messier published in his catalogue. In that year Méchain discovered five more objects, apart from the Sombrero Galaxy, and none of them were included in the last edition. Messier was aware of all these new discoveries of Méchain, he even wrote a note about the Sombrero Galaxy in his copy of the 1781 edition of the catalogue right on the day of its discovery, and later he wrote about the other five objects too. Unfortunately he never published a newer edition of the catalogue after 1781. In 1921 Nicolas Camille Flammarion found Messier's handwritten notes about the Sombrero Galaxy and suggested to add it to the Messier Catalogue as M104. There were three more additions to the catalogue in 1947, 1960 and 1967, when the Messier Catalogue has reached its final form we know today, containing the 110 objects.
The appearance of the Galaxy
The almost edge on spiral galaxy is located in the constellation of Virgo, at about 31 million light-years from us. It is smaller than our galaxy the Milky Way, its diameter is approximately 82000 light-years. Some of its distinctive features are the well defined dust lane in front of the bright galactic core and the extent halo around the bulge. These properties together create the visual appearance resembling a sombrero.
Its morphological classification is SA(s)a, which means the galaxy is an unbarred spiral galaxy with smooth, tightly wound spiral arms, having a large and bright central bulge. At least when observed in the spectrum of visible light. In infrared the galaxy looks completely different. In images created by the Spitzer infrared space telescope a much larger halo can be found and the galaxy looks more like it was elliptical, not spiral.
The core region
The core region of the Sombrero Galaxy contains weakly ionised gases, making it a LINER galaxy. LINER (Low Ionisation Nuclear Emission Region) is the way astronomers call this type of galaxies. The source of ionisation in the core region of LINER galaxies is either radiation from hot young stars, or an active galactic nucleus (AGN) containing a super massive black hole. There are a couple of signs indicating that it is most probably the latter in the case of M104: There is no significant star formation happening in the core, nor happened in the near past, therefore hot young stars are not present either. There is, however, a giant black hole that was discovered in the '90s by measuring the speeds of stars orbiting the core. The measurements indicated that the mass of the black hole must be at least 1 billion solar masses. The presence of AGN was also confirmed by observations made in X-ray and radio wavelengths. Strong synchrotron emission was identified in the core, which is a good indication of AGN being present.
The distinctive dust lane
Besides contributing to the unique appearance of the galaxy, the dust lane provides sufficient circumstances for star formation. It consists of interstellar gases and dust, containing the vast majority of the atomic hydrogen in the galaxy. Probably most of the molecular gases are accumulated in the dust lane too. Like in most of the regular galaxies, the dust concentration is the highest in the rotational plane of M104. The density is high enough to be able to absorb a significant amount of light originating from behind it, rendering the dust lane much darker than other parts of the galaxy.
