The Two Sides of the Sombrero Galaxy

While in grade school, the classic image (above) of the Sombrero Galaxy was one that perked my interest in astronomy.  The disk with the bright central core gave it a mysterious look, something I wanted to learn more about.  Located 30 million light years from Earth, it is not bright enough to see with the naked eye, but easily captured by amateur astronomers armed with small telescopes.  Although reasonably close as far as galaxies go, it still is an enigma for astronomers.

The Sombrero Galaxy was discovered on May 11, 1781 by Pierre Mechain, who was working with Charles Messier to build a catalog of nebulous objects so astronomers would not mistake for comets.  The Sombrero Galaxy was not included in the original catalog, although Messier hand wrote a description of it in his personal copy.  Mechain announced the discovery in a letter to the Berlin Royal Academy of Sciences and Arts on May 6, 1783.  Eventually, in 1921, the Sombrero Galaxy was entered into the Messier catalog and given the designation M104.

News traveled slow in the 1780’s, and the Sombrero Galaxy was discovered independently by William Herschel on May 9, 1784.  Herschel’s superior optics allowed him to view the dark dust lane in the disk that provided the Sombrero Galaxy with its moniker.  In 1800, Herschel would discover infrared light.  Two centuries later, infrared imaging would allow astronomers to make an important discovery about the Sombrero Galaxy.

During the 1800’s, spiral galaxies were referred to as spiral nebulae.  At the time, it was thought the Milky Way was the sole galaxy to exist and the spiral nebulae were located within.  Telescopes did not have the ability to resolve individual stars in galaxies outside the Milky Way.  During the early 1900’s, astronomers began to challenge this view of the universe and a discovery from the Sombrero Galaxy played a crucial role in this debate.

In 1912, Vesto Slipher of the Lowell Observatory measured the red shift of the Sombrero Galaxy.  If an object is moving away from us, its light waves become elongated, that is, the light shifts towards the red part of the spectrum.  The red shift of the Sombrero Galaxy indicated it was moving away from us at a velocity of 1,000 km/s.  Moving at such a fast rate suggested M104 resided outside the Milky Way, as this is almost twice the escape velocity of our galaxy.  From an observational standpoint, this was among the first clues that the universe was expanding and not static, as was the prevailing wisdom at the time.

On April 26, 1920, what became known in astronomy circles as The Great Debate, took place in the Smithsonian Museum of Natural History between Harlow Shapley and Heber Curtis.  Shapley proposed there was only one galaxy in the universe and that the Solar System was located far from the center of that galaxy.  Curtis countered that we were located near the center of the Milky Way, but the spiral nebulae were galaxies residing outside the Milky Way.  As is often the case with debates such as this, both were right…and both were wrong.

The observations of Edwin Hubble at Mt. Wilson Observatory throughout the 1920’s proved Curtis right in that the spiral nebulae were not gas clouds in the Milky Way, but other galaxies outside the Milky Way.  However, Shapley turned out to be right on the location of Earth residing outside the center of the Milky Way.  We tend to want to confer the status of a winner and loser with debates such as these, but remember, it is the evidence, not the person, that determines if a scientific proposal is correct or not.  You’ll want to keep this in mind with similar debates today on string theory and parallel universes.  The Sombrero Galaxy, like other spiral nebulae, was reclassified as a spiral galaxy outside the Milky Way.

The Sombrero Galaxy lies about 30 million light years from Earth.  If you happen to catch it in a telescope, the light entering your eye started its journey from the galaxy when India began to slam into the Asia continent to form the Himalayan mountains.  The continents of Earth looked like this 30 million years ago.

Credit: USGS

To put a light year in perspective, even though the Sombrero Galaxy is moving away from us at a rate equal to the distance from Paris to Copenhagen every second, it has only receded 1/3 of a light year since its red shift was first measured in 1912.  The nearest star from us is Proxima Centauri at 4.2 light years.  Even traveling at 1,000 km/s, it would take some 1,200 years to reach.  That demonstrates the challenges facing those working on methods of interstellar propulsion.

Over the past two decades, space telescopes have afforded astronomers a better understanding of the internal structure of the Sombrero Galaxy.  The Hubble, of course, is the most famous of these observatories and took this iconic image in 2003.

The Majestic Sombrero Galaxy (M104)
Source: Hubblesite.org

Seen here is the notable bright core of the Sombrero Galaxy, with its signature dust lane across the disk titled only 6 degrees from our vantage point on Earth.  At 50,000 light years, the Sombrero Galaxy is only half as wide as the Milky Way but has more than 10 times the globular clusters with 2,000.  In 1996, Hubble picked up a high rate of rotation near the core of the galaxy, confirming the 1988 ground observations by John Kormendy at the Canada-France-Hawaii Telescope.  This rotation is accelerated by the presence of a black hole with a mass one billion times that of the Sun.  It is one of the largest black holes detected in a nearby galaxy.

By and large, this is the way astronomers have seen the Sombrero Galaxy until recently viewed by other parts of the electromagnetic (EM) spectrum.

Just after the above image was taken by the Hubble, NASA launched the Spitzer Space Telescope.  Named after Lyman Spitzer, Jr., who first proposed an orbiting telescope in 1946, the Sptizer observes in infrared.  Cooler objects such as planets and dust radiate most strongly in infrared.  Certain wavelengths of infrared have the ability to pass through dusty regions without being absorbed.  This gives astronomers the ability to peer behind the curtain of opaque, dusty areas to see what lies behind.  When the Spitzer took a look at the Sombrero Galaxy, this is what it saw.

Credit: NASA/JPL-Caltech/University of Arizona/STSc

The red represents a dust ring in the spiral disk.  This is the area of a spiral galaxy where stars are born.  That was expected, what was unexpected was the blue starlight in the galactic halo.  Prior to this image, it was thought the bright halo was tenuous and small.  Instead, it is an elliptical galaxy with a spiral galaxy embedded within.  The Spitzer enabled astronomers to see stars behind the dust in the halo.  This also explained why the Sombrero has far more globular clusters than spirals normally have.  Elliptical galaxies typically have a couple thousand of these clusters.

Globular Cluster M80: A Swarm of Ancient Stars in the Milky Way
Globular cluster M80. Globular clusters are dense clusters of older stars that orbit a galaxy’s core. Credit: The Hubble Heritage Team (AURA/ STScI/ NASA)

Besides the Spitzer, the Chandra X-Ray Observatory has imaged the Sombrero Galaxy.  Objects that are very hot will radiate high energy x-rays.  Galactic dust and gas does not generally fall into this category unless it is heated up by a nearby source.  What the Chandra imaged when pointed at M104 was this.

Credit: NASA/UMass/Q.D.Wang et al

With the x-ray image, the spiral disk disappears as it consists of cool dust.  The point sources in this image are high energy stars and background quasars.  The Chandra also picked up halo of hot gas that extends 10,000 light years beyond the spiral disk.  It is thought this gas is disbursed via a galactic wind originating with supernova activity throughout the galaxy.

If you want to look at M104 yourself, May is the optimal month to do so.  Best to seek a dark sky location far from city lights, and while it is too dim to be seen with the naked eye, a pair of binoculars or a small telescope is sufficient to bring it into view.  An 8-10 inch telescope will start to resolve features such as the dust lane.  Below is an image of where M104 will be located on May 14th from my hometown Buffalo, NY.  At 10:30 PM, it will be directly due south in the constellation Virgo.

M104 as seen with Starry Night.
M104 as seen with Starry Night.

It is always a challenge to target a deep space object such as M104, but like anything else, you’ll get better and better with more experience.  Do not get discouraged if unable catch it on the first try and good hunting!

*Image on top of post is the Sombrero Galaxy taken at Mt. Palomar’s 200-inch Hale Telescope.  Credit:  Mt. Palomar/Caltech.

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