Some galaxies are plump and round. Others are a slender disk like our spiral Milky Way. But it seems that the Sombrero galaxy is both. Scientists have drawn this conclusion from new observations made using NASA's Spitzer Space Telescope. The galaxy, which is a round elliptical galaxy with a thin disk embedded inside, is one of the first known to exhibit characteristics of the two different types. Scientists think these findings will lead to a better understanding of galaxy evolution, a topic that is still poorly understood.
Formally known as NGC 4594, the Sombrero galaxy is located 28 million light-years away in the constellation Virgo. The galaxy was discovered in 1767 by Pierre Mechain. From Earth, we can see the thin edge of its flat disk and a central bulge of stars, making the galaxy resemble a wide-brimmed hat. Astronomers do not know whether the Sombrero's disk is shaped like a ring or a spiral, but they agree it belongs to the disk class.
Being an infrared telescope, Spitzer captures a different view of the galaxy than visible-light telescopes. In visible light, the galaxy appears to be immersed in a glowing halo, which scientists had thought was relatively light and small. But in the infrared, a different view emerges. Spitzer sees old stars through the dust and reveals the halo has the right size and mass to be a giant elliptical galaxy.
It is tempting to think the giant elliptical swallowed a spiral disk. However, astronomers say this is highly unlikely because that process would have destroyed the disk structure. One scenario they propose is that a giant elliptical galaxy was inundated with gas more than nine billion years ago. Early in the history of our universe, networks of gas clouds were common, and they sometimes fed growing galaxies, causing them to bulk up. The gas would have been pulled into the galaxy by gravity, falling into orbit around the center and spinning out into a flat disk. Stars would have formed from the gas in the disk.
The above scenario poses lots of questions. For example: How did the disk shape survive inside a massive elliptical? And how unusual is this process of formation? Researchers say the answers could help them piece together how other galaxies evolve. Another galaxy, called Centaurus A, appears also to be an elliptical galaxy with a disk inside it. But its disk does not contain many stars. Astronomers speculate that Centaurus A could be at an earlier stage of evolution than the Sombrero and might eventually look similar.
The findings also answer a mystery about the number of globular clusters in the Sombrero galaxy. Globular clusters are spherical nuggets of old stars. Ellipticals typically have a few thousand, while spirals contain a few hundred. The Sombrero has almost 2,000, a number that makes sense now but had puzzled astronomers when they thought it was only a disk galaxy.
The details on this study are being published in the Monthly Notice of the Royal Astronomical Society. The paper was posted online April 23rd. To read the paper, click on the following link: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2012.20925.x/abstract
And now, the Spitzer mission particulars…
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit these links: spitzer.caltech.edu and www.nasa.gov/spitzer
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