Lifestyles of the Galaxies Next Door
Written by Linda Vu, Spitzer Science Center
December 14, 2007
|SINGS Hubble Tuning Fork Poster
NASA/JPL-Caltech/K. Gordon (Space Telescope Science Institute) and SINGS Team
The "lifestyles" of 75 neighboring galaxies are illuminated in this poster from NASA's Spitzer Space Telescope. Scientists say this fresh perspective of our cosmic neighborhood provides valuable insights into growth process of galaxies at a glance.
Over the past four years, Spitzer snapped infrared portraits of some of our most fascinating galactic neighbors as part of the Spitzer Infrared Nearby Galaxy Survey (SINGS) Legacy project. By understanding the mechanisms that fuel and hinder star production in these nearby galaxies, SINGS astronomers hope to solve the mystery of where galaxies come from, and how they've developed throughout the universe's history.
"Once the SINGS observations were done, I began to wonder how to look at all of the galaxies and make sense of the big picture. The SINGS sample of 75 galaxies was just too many to display at once on a computer screen and still be able to appreciate the spatial details present in the images," said Dr. Karl Gordon, of the Space Science Telescope Institute, in Baltimore, Md., who is a member of the SINGS team.
Eventually, Gordon decided to create a poster with the 75 galaxies organized by shape -- using the classification system that astronomer Edwin Hubble created in 1925, soon after the physical nature of galaxies was discovered. The grouping system is called "Hubble's Tuning-Fork" because its overarching shape resembles a musical tuning-fork.
In this structure, elliptical galaxies sit on the left side of the poster, creating the tuning fork's handle. They are designated by the letter "E", and given a number from zero to seven. An "E0" galaxy looks round, while an E7 galaxy is very long and thin.
Spiral galaxies are located to the right side of the poster creating the fork's two prongs. The top prong is made up of regular spiral galaxies, and identified by the letter "S." Barred spiral galaxies make up the bottom prong, and are branded "SB." Meanwhile, letters -- "a", "b", and "c" -- indicate how tightly the spiral arms are wound. An "Sa" galaxy's arms are wound very tightly, while an "Sc" galaxy's spiral arms are very loosely wound.
"Irregular galaxies were not represented in Hubble's original diagram, so we organized them on the bottom-left side of the poster," says Gordon.
In this poster, blue colors reveal light from an older population of stars. Tints of green represent organic molecules called polycyclic aromatic hydrocarbons, while red lumps show clouds of warm dust and gas heated by radiation from newborn stars.
"One of the most striking things about putting these galaxies into the tuning-fork pattern is that you see right away, elliptical galaxies are bluer, which means that they are made up of primarily older stars. The spiral galaxies on the other hand have wisps of green and red, indicating the presence of warm dust and star formation," says Dr. Robert Kennicutt, of the Institute of Astronomy at the University of Cambridge, United Kingdom, and leader of the SINGS team.
According to Kennicutt, astronomers can infer that spiral galaxies on the right-hand side of the tuning fork are younger because most are rich in dust and actively forming stars. Stars form like raindrops in space, when dense cosmic clouds of dust and gas condense, and nuclear fusion is ignited.
He also notes that the elliptical galaxies on the left largely lack dust, indicating that they are not forming many new stars. The rich-blue color of elliptical galaxies also reveals the presence of a primarily older stellar population. In contrast to the spirals, the elliptical galaxies exhausted their gas and dust supplies billions of years ago.
"When I see this poster, I am just so amazed by Spitzer's sensitivity. This infrared view really gives you a sense of the 'lifestyles' of these nearby galaxies," says Kennicutt.
"You get a sense that galaxy classification is not a simple black and white process -- tints of red in galaxies like NGC 3265, show that not all ellipticals are void of dust and star formation, and extremely blue spiral galaxies like NGC 4826, show that some spirals do have a large population of old stars. Like people, galaxies are unique individuals."
The images in this poster are three-color composites where blue depicts the galaxies at a light wavelength of 3.6 microns, while 8.0 microns is green, and 24 microns is red.
Other members include Lee Armus, George Bendo, Caroline Bot, Brent Buckalew, Daniela Calzetti, John Cannon, Daniel Dale, Bruce Draine, Charles Engelbracht, Albert Grauer, George Helou, David Hollenbach, Tom Jarrett, Lisa Kewley, Claus Leitherer, Aigen Li, Sangeeta Malhotra, Martin Meyer, John Moustakas, Eric Murphy, Michael Regan, George Rieke, Marcia Rieke, Helene Roussel, Kartik Sheth, J.D. Smith, Michele Thornley, and Fabian Walter.