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SINGS: Answering Astronomy's 'Chicken or Egg' Question

NGC 4579
Spiral galaxy NGC 4579 as seen by SINGS.
Credit: NASA/JPL-Caltech/R. Kennicutt (U. of Ariz./Inst. of Astr., U. of Cambridge) and the SINGS Team

Before archeologists could unlock the histories of ancient Egypt, they first had to unravel the ancient civilization's language and learn to read the mysterious hieroglyphs etched onto the walls of tombs and temples, on tablets, obelisks, and sculptures. In other words, there were multiple mysteries to be solved before vital questions about the civilization could be answered.

Similarly, to answer astronomy's ultimate on-going "chicken or egg" question -- "Did the material comprising galaxies come together to form stars? Or did stars form in their own small clusters first and eventually merge with other star clusters to form galaxies?" -- astronomers must first solve the mystery behind how stars form from clouds of gas and determine which galactic parameters fuel and hinder star formation.

In the case of the Egyptian archeologists, the Rosetta stone, which translated the hieroglyphs into Greek, proved to be a vital key to unlocking the civilization's language. Likewise, astronomers on the Spitzer Infrared Nearby Galaxies Survey (SINGS) Legacy team hope that a sample of 75 local galaxies may be the Rosetta stone for answering questions that will eventually solve astronomy's "chicken or egg" mystery.

"We are essentially laying the foundation for astronomers to study the evolution of galaxies and how modern structure of the universe came to be," said SINGS Principal Investigator, Dr. Robert Kennicutt.

The most logical solution to answering any "what came first" question is to go back in time and witness the event. Because of the properties of how light travels, distance equals time in space, and astronomers can theoretically answer this question by building large telescopes, which look further into space and back in time.

The properties of light dictate that the farther away the object is from Earth, the longer it takes for its light to travel to Earth. Thus, like a snapshot in time, astronomers are seeing galaxies in the distant universe the way they looked hundreds of millions to billions of years ago. However, due to current technological limitations, astronomers can only see the most luminous galaxies in the distant universe as fuzzy points.

"When you look at objects that are distant, because of telescope and instrument limitations you can only see the most luminous 'freak' galaxies," says SINGS Scientist Dr. Lee Armus.

Although distant luminous galaxies are seen as fuzzy by even the most sensitive space telescopes, Spitzer's unprecedented infrared sensitive eyes allow astronomers to observe the structure of local galaxies in extraordinary detail.

"The biggest surprises in our survey data are the intricate structural details of the galaxies... we can see streams of gas and dust," said Kennicutt.

By understanding how stars form from clouds of gas in neighboring galaxies and determining the parameters that fuel and hinder star production in the local universe, astronomers hope to decipher the fuzzy points they currently see in the distant universe. They believe that the distant "freak" galaxies are just scaled up versions of nearby galaxies.

"It's like using mouse DNA to study human DNA," says Armus. "By studying local galaxies astronomers hope to gain insight into the star-formation process, which they believe to be universal. Although the link may not be direct, they will learn something."

Combining the Lights of Star Formation

Unlike archeologists using picks and trowels to solve the mystery of an ancient epoch, SINGS astronomers are using a variety of wavelengths ranging from ultraviolet (UV) to infrared.

Because even "nearby" galaxies are millions of light years away from Earth, astronomers can only gather detailed information about the formation of really bright stars that are 10 to 120 times the mass of the Sun. These stars are so massive that they mostly emit very high energy UV light. Also, because these stars are still very young, they are often shrouded by the cloud of gas and dust that they formed in. This cloud blocks most of the new star's light from our view and gives scientists a limited picture of the star formation process.

Thus, for a larger perspective on the conditions that initiate star formation, the SINGS project weaves together Spitzer's infrared data with UV information from NASA's Galaxy Evolution Explorer space telescope (GALEX), visible light data from the ground-based Kitt Peak telescope in Arizona, and near-infrared information from the 200-inch, ground-based Hale telescope at Palomar Observatory in California.

"If it wasn't for dust, you wouldn't need anything other than UV and visible light to observe star formation," says Kennicutt. "Dust can absorb anywhere from half to 99% of the UV and visible light radiation that stars put out, and knowing that you are seeing less than what is really there is a big limitation."

Since the clouds of dust re-radiate absorbed UV and visible starlight in the infrared, Spitzer provides the missing puzzle piece. By comparing infrared data with UV and visible-light data, astronomers have a better understanding of how much starlight is being absorbed by dust and can decipher the mass of the star. This information also gives astronomers a better grasp on the number of stars forming in the region.

"If you want to estimate the amount of star formation in a galaxy, you can't do it without infrared data ... this is something that infrared astronomers have long suspected, and recently confirmed through SINGS results," said SINGS Scientist, Dr. George Helou.

Besides illuminating the "missing" galactic starlight, Spitzer's wide infrared observing range is also valuable for mapping star formation in galaxies and offering insights into the role that galactic structures (spiral arms and galactic bars), gas, and dust play in the birth of stars.

With the telescope's Infrared Array Camera (IRAC), scientists can detect two things: Polycyclic Aromatic Hydrocarbon (PAH) molecules in the material surrounding infant stars and the old stars that make up the skeletal structure of a galaxy. Because PAHs light up when the radiation released by an infant star reacts with its surrounding medium, these molecules show astronomers exactly where stars are forming in the galaxy. The old stars in a galaxy are important because they give astronomers insights into the gravitational forces that may dictate how gas in the star-forming region will react to create stars.

Using Spitzer's Multiband Imaging Photometer (MIPS) scientists can measure the amount of dust in a galaxy. Because dust heats up and shines in the far-infrared as it condenses to form a star, MIPS allows astronomers to detect stars before they "turn on" visibly.

Meanwhile, Spitzer's Infrared Spectrometer (IRS) gives astronomers valuable information about the molecular environment in which stars form.

"IRAC and MIPS gives you the morphology or pictures, and tells you where things are in a galaxy ... while IRS tells you about the composition of the gas and dust," said Armus. "Every bump in the IRS graph indicates a different type of atom, molecule, or dust particle. Thus, IRS tells you a lot about the physics of what is going on even without showing you the spiral arms."

According to Helou, SINGS' sample of 75 galaxies is relatively "normal." They were chosen based on their range in metallicity, nuclear property, morphology (appearance), dust, and radiation, among other characteristics. The number 75 allowed the team to look at the most diversity with the allotted observing time.

"SINGS is the perfect Legacy project because it uses all three Spitzer instruments and it is valuable for studying a wide range of science from cosmology to dust." says Armus.

Coming Together to Create SINGS

When the Spitzer Legacy Program issued its call for proposals in June 2000, a small group of infrared astronomers interested in dissecting the star-forming regions, or interstellar medium (ISM), of galaxies began talking about the possibility of conducting observations with Spitzer.

According to Helou, Spitzer's legacy program appealed to many infrared scientists because of the substantial amount of observing time that the program offered and telescope's unprecedented infrared capabilities. "Spitzer and SINGS are perfectly matched," says Helou. "To study star formation properties of galaxies, you need a wide range of wavelengths and instrument capabilities ... Spitzer has that."

After deciding that a proposal would be submitted, the group decided that they wanted to invite Kennicutt to be the project's lead.

"Rob was very distinguished, had very well known discoveries in the field, and had experience in leading large collaborative projects," said Helou.

While Kennicutt admits that he was a little surprised by the invitation, particularly because he had been doing the majority of his research not in the infrared, but in UV and visible light, he says that he and the rest of the team realized the importance of connecting the different wavelengths.

"Before SINGS, there were a number of nearby galaxy surveys in different wavelengths, but the information wasn't connected and the data was scattered." says Kennicutt. "There were few efforts to do what we are doing."

With Kennicutt as the project's lead, the group then decided to build their team with experts that would help fulfill the project's goals and then proceeded to extend invitations.

"We needed people who were experts with Spitzer instruments, observation techniques, theoretical information, and people who have worked on star formation with different telescopes and wavelengths," said Helou.

Over time, the core group expanded to include scientists and post-docs who have "earned a position on the team" with their groundbreaking research and expertise. Today, the SINGS team consists of 27 astronomers from across the globe.

"The science that SINGS is doing will have valuable implications for the next generation of astronomers." said Kennicutt. "We are essentially laying the foundation and providing the tools to answer the universe's 'chicken or egg' question."



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