Anne’s Picture of the Day: The Sombrero Galaxy

read more at Anne’s Astronomy News http://annesastronomynews.com/

December 19, 2012

The Sombrero Galaxy, an edge-on spiral galaxy in Virgo

Image Credit: NASA/ESA and The Hubble Heritage Team (STScI/AURA)

The Sombrero Galaxy (designated as Messier 104 and NGC 4594) is an unbarred spiral galaxy of about 50,000 light-years across and a mass of 800 billion suns, located some 29 million light-years away in the constellation Virgo. It is rushing away from us at roughly 1025 kilometers per second. This enormous velocity offered some of the earliest clues that the Universe was expanding in all directions.

It lies within a complex, filament-like cloud of galaxies that extends to the south of the Virgo Cluster. However, it is unclear whether the Sombrero Galaxy is part of this galaxy group. It might also be a member of a group that includes NGC 4487, NGC 4504, NGC 4802, UGCA 289, and possibly a few other galaxies. And, other indications lead to think that the Sombrero Galaxy is not in a group at all, or that it may only be part of a galaxy pair with UGCA 287.

The Sombrero Galaxy has a very bright active galactic nucleus, an unusually large central bulge of stars, well-defined tightly-wound spiral arms and a faint halo that extends over 60,000 light years from the center of the galaxy. However, the Sombrero’s most striking feature is a prominent dust lane that crosses in front of the bulge in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero.

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. Based on infrared spectroscopy, the dust ring is the primary site of star formation within the Sombrero Galaxy while its nucleus is probably devoid of any significant star formation activity.

Embedded in the bright core of the Sombrero is a smaller disk, which is tilted relative to the large disk. X-ray emission suggests that there is material falling into the compact core, where a supermassive black hole of 1 billion solar masses resides. This is among the most massive black holes measured in any nearby galaxies.

The Sombrero Galaxy has a relatively large number of globular clusters, namely nearly 2,000 (10 times as many as orbit our Milky Way galaxy). The ratio of the number of globular clusters to the total luminosity of the galaxy is high compared to the Milky Way and similar galaxies with small bulges, but the ratio is comparable to other galaxies with large bulges. This demonstrates that the number of globular clusters in galaxies is related to the size of the galaxies’ bulges. The ages of the clusters are similar to the clusters in the Milky Way, ranging from 10-13 billion years old.

The galaxy is easily visible with amateur telescopes, while the large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.

The Hubble Heritage Team took these observations in May-June 2003 with the Space Telescope’s Advanced Camera for Surveys. Images were taken in three filters (red, green, and blue) to yield a natural-color image. The team took six pictures of the galaxy and then stitched them together to create the final composite image.

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