Deep-Sky Objects: Sombrero Galaxy (M104)

Sombrero Galaxy (M104)
Image Credit: NASA, ESO, NAOJ, Giovanni Paglioli

Long thought to be an unbarred spiral galaxy, recent images taken by the Spitzer telescope in infrared have shown that the dust lane around the galaxy is far more extensive and massive than was previously thought. In fact, the mass and extent of the dust disc is more indicative of a giant elliptical galaxy; nonetheless, the galaxy’s unusually big and bright central bulge, super massive black hole, and the dust lane/halo itself all combine to make this one of the most intensely studied galaxies in the sky.

Quick Facts:

• Constellation: Virgo
• Coordinates: RA 12h 39m 59.4s |Dec. -11° 37′ 23″
• Distance: 31.13million light years
• Object type: Giant elliptical galaxy (?)
• Redshift: 0.003416
• Apparent diameter: 9 × 4 arc minutes
• Effective diameter: 50,000 light years
• Apparent magnitude: +8.98
• Other designations: NGC 4594, UGC 293, PGC 42407

Visibility

Being as bright as it is, M104 is an easy target for binoculars and small telescopes under dark skies and with good seeing conditions. However, to get the best views, an 8-inch telescope is required to resolve the central bulge, and at least a 10 to 12-inch instrument is needed to see the dark dust lane that encloses the galaxy. Look for the Sombrero Galaxy 11.5 degrees to the west of the star Spica in the constellation of Virgo, and 5.5 degrees to the north-east of the star Eta Corvi in Corvus. The best time to view M104 from the northern hemisphere is during the northern spring.

The image below shows views of M104, with the main panel showing a composite image of the three smaller pictures on the right, one of which is an optical image (Hubble), while the other two are taken in non-optical wave lengths. In blue is Chandra’s X-ray image showing hot gas in the galaxy, while in green is a Hubble image showing the bulge of starlight partially contained within a rim of dust, with the Spitzer’s infrared view highlighting its bright gaseous glow further.

Sombrero Galaxy

Physical Properties

Large Central Bulge

Apart from the prominent dust lane around the galaxy, one other distinguishing feature of M104 is its enormously bright central bulge, or nucleus. In technical terms, the bulge is classified as a LINER (Low Ionization Nuclear Emission Region), which is central regions in galaxies where ionized gas is present, but where the gas is only weakly ionized, meaning that atoms in these regions are missing relatively few electrons. It is perhaps worth noting that the energy source in LINER-type galactic nuclei remains unknown, or more precisely, undetermined. Possible energy sources include active star forming regions, or super massive black holes. In the case of M104, no active star forming regions have been identified in the nucleus, but the nucleus does contain a super massive black hole, and most investigators now believe that this black hole is the energy source that is ionizing gas in the galaxy’s core.

Central Black Hole

The presence of a black hole in M104 has been confirmed by means of measuring the speed at which some stars orbit the galaxy’s core. Using data from Hubble and other orbiting telescopes, investigators found that given the mass of the observed stars, these stars could not move at the very high speeds they do unless they are orbiting an object that is at least 1 billion times as massive as the Sun, which incidentally, makes this black hole the most massive ever discovered in a nearby galaxy.

Radiation

Nonetheless, much about the galaxy remains unknown. One unresolved mystery in particular involves the origin of sub-millimetre radiation that was discovered independently by two groups of investigators in 2006. The radiation at a wave length of 850 micrometers does not originate in the dust disc around the galaxy, as might have been expected, nor is it synchrotron emission that is sometimes seen in radio wavelengths. The unexplained radiation is also not the result of bremsstrahlung emission from hot gas, which is radiation that is produced when charged particles are decelerated when they are deflected by other charged particles. To date, the origin of the sub-millimetre radiation remains a mystery.

However, M104 is known as a powerful source of synchrotron radiation, which is produced by oscillating electrons as they pass through strong magnetic fields. This is a common feature of active galactic nuclei, but while this type of radiation often varies markedly in most active galactic nuclei, the synchrotron radiation from the Sombrero galaxy does not vary by more than 10 to 20%.

Globular Clusters

Although M104 is only about 30% as big as the Milky Way, studies have shown that the galaxy is host to between 1,200 and 2,000 globular star clusters. While the ratio of globular clusters relative to the galaxy’s total luminosity is high when compared to the Milky Way and most other galaxies with small central bulges, the ratio compares well with other galaxies with large central bulges. In fact, the globular cluster/luminosity ratio of M104 is often used to demonstrate the fact that the number of globular clusters around any given galaxy is closely related to the size of the central bulge in that galaxy.

Galactic Group Membership

While M104 is known to be embedded in a long, filament-like arrangement of galaxies, it remains unclear whether or not the galaxy is a member of a closely related group of galaxies. One method of determining group membership, known as the “hierarchical method”, suggests that M104 is a member of a group that includes NGC 4487, NGC 4504, NGC 4802, UGCA 289, and perhaps a few outliers. However, another method known as the “percolation”, or “friends-of-friends” method, and which uses similar features shared by galaxies to determine group membership, suggest that M104 is not part of group of galaxies, and may only be associated with one other galaxy, designated UGCA 287.

However, M104 does have its own companion galaxy, which was discovered in 2009. The companion has a very high stellar population, but it is very compact and believed to be the remains of a dwarf elliptical galaxy that had lost most of its stars, gas, and dust to M104 as the result of tidal interactions with M104.

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