Posted on by Ray Sanders

Wandering Stars Shed Light on Milky Way’s Past

Measurements of the metal content of stars in the disk of our galaxy. The bottom panel shows the decrease in metal content as the distance from the galactic center increases for stars near the plane of the Milky Way disk. In contrast, the metal content for stars far above the plane, shown in the upper panel, is nearly constant at all distances from the center of the Galaxy. Image Credit: Judy Cheng and Connie Rockosi (UCSC) and the 2MASS Survey.

[/caption]Like a worldly backpacker, many stars in the Milky Way Galaxy have made interesting journeys, and have interesting stories to tell about their past. For over a decade, the Sloan Digital Sky Survey (SDSS) has been mapping stars in our Galaxy.

This week at the American Astronomical Society meeting in Austin, Texas, astronomers from University of California – Santa Cruz presented new evidence that claims to answer many questions about stars located in the disk of our galaxy. The team’s results are based on data from the Sloan Extension for Galactic Understanding and Exploration 2 (SEGUE-2).

The SEGUE-2 data is comprised of the motions and chemical compositions of over 118,000 stars, most of which are in the disk of our galaxy, but a few stars in the survey take the “scenic” route in their orbit.

“Some disk stars have orbits that take them far above and below the plane of the Milky Way,” said Connie Rockosi (University of California – Santa Cruz), “We want to understand what kinds of stars those are, where they came from, and how they got there.”

Aside from the orbital paths of these “wandering” stars being different from most other Milky Way stars, their chemical composition also makes them unique. A team led by Judy Cheng (University of California – Santa Cruz) studied the metallicity of stars at different locations in the galaxy. By studying the metallicity, Cheng and her team were able to examine how the disk of the Milky Way disk grew over time. Cheng’s study also showed that stars closer to the center of the galaxy have higher metallicity than those farther from the galactic center. “That tells us that the outer disk of our Galaxy has formed fewer generations of stars than the inner disk, meaning that the Milky Way disk grew from the inside out,” added Cheng.

When Cheng studied the “wandering” stars, she found their metallicity doesn’t follow the same trend – no matter where she looked in the target area of the Galaxy, stars had low metal content. “The fact that the metal content of those stars is the same everywhere is a new piece of evidence that can help us figure out how they got to be so far away from the plane,” Rockosi mentioned.

What the team has yet to determine is if the stars formed with their “wandering” orbits, or if something in the past caused them to migrate to their unique paths. “If these stars were born with these orbits, they were born at the same rate all over the galaxy,” Cheng said. “If they were born with regular orbits, then whatever happened to them must have been very efficient at mixing them up and erasing any patterns in the metal content, such as the inside-out trend we see in the plane.”

Some possible reasons for this mixing include past mergers of our Galaxy and others, or possibly spiral arms sweeping through the disk. Cheng’s observations will help determine what causes stars to wander far from their birthplace. Other galaxies have shown stars in their disks as well, so solving the puzzle presented by these stars will help researchers better understand how spiral galaxies like the Milky Way form.

If you’d like to read Cheng and Rockosi’s paper “Metallicity Gradients In The Milky Way Disk As Observed By The SEGUE Survey”, you can download a copy at: http://www.ucolick.org/~jyc/gradient/cheng_apj_fullres.pdf

Source: UC Santa Cruz press release

Posted on by Nancy Atkinson

What Color is the Milky Way? White as Snow (not Milk)

An image of one of the Milky Way analogs found by Timothy Licquia and Jeffrey Newman. This galaxy, known to astronomers as SDSS J083909.27+450747.7, has properties which closely match those of the galaxy we live in. Credit: Sloan Digital Sky Survey

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What color would the Milky Way appear to alien civilizations looking at our galaxy through their telescopes? It turns out the Milky Way has approximately the right name – but for all the wrong reasons. “The true color of the Milky Way is as white as fine-grained new spring snow seen in early morning light,” said Dr. Jeffrey Newman, from the University of Pittsburgh, speaking at a press conference from the American Astronomical Society (AAS) Meeting.

Our ancestors gave our galaxy the name “Milky Way” because when they looked up and saw the band of the stars that stretches from one horizon to the other, it appears white to our human eyes. “But that’s only because our low-light vision isn’t sensitive to color,” said Newman. “There are portions of the Milky Way that are more yellow or red versus more blue, but our eyes can’t pick that up. But a sensitive instrument or photograph can.”

When we look at other galaxies, we can see them in their entirety, and can examine them for color and luminosity. Color and luminosity have been great tool for astronomy, helping us to understand stars and galaxies.

“Unfortunately we can’t get a complete picture of the Milky Way from outside, so we have had to resort to other methods,” said Newman. “Not only are we looking at Milky Way from the inside, but it’s even worse than that — our view is blocked by dust, both in clouds and diffuse dust. We can only see about 1,000 -2,000 light years in any direction, even though our galaxy is a 100,000 light years across.”

A digital all-sky mosaic of our view of the Milky Way from Earth, assembled from more than 3,000 individual CCD frames. Credit: Axel Mellinger. Click on image to view a zoomable panorama.

So if you ask, ‘what is the integrated color of the Milky Way,’ we can can’t tell from a picture like the one above, we can only tell what color the local neighborhood is.

“We have had to resort to different techniques, and rather than looking at the Milky Way directly, we look at other galaxies that should be like the Milky Way and we can determine what their color and luminosity are,” Newman said.

Newman, along with Timothy Licquia, a PhD student in physics at Pitt, used images from the Sloan Digital Sky Survey — which contains detailed properties of nearly a million galaxies — and looked for galaxies with similar properties to the Milky Way in regards to total mass and star formation rates. The Milky Way Galaxy should then fall on a plot somewhere within the range of colors of these matching objects.

While the composite color of the Milky Way is snowy-white, our galaxy appears more yellow towards the center and more blue out in the spiral arms.

Newman and Licquia determined the light color temperature of the Milky Way is 4,840 K, which closely matches the light from a standard light bulb with a color temperature of 4,700-5,000K. “It is well within the range our eye can perceive as white—roughly halfway between the light from old-style incandescent light bulbs and the standard spectrum of white on a television,” said Newman. “Our eyes treat both as white.”

The color of new snow is the whitest natural color on Earth. While milk has a more bluish color than snow, the association of our Milky Way to milk has proven to be very appropriate, given the Milky Way’s true color.

Newman even wrote a Haiku about the color:

Look at new spring snow
See the River of Heaven
An hour after dawn

The Milky Way’s color could be on either side of a standard dividing line between red and blue galaxies: relatively red galaxies rarely form new stars and blue galaxies have stars still being born. This adds to the evidence that although the Milky Way is still producing stars, it is “on its way out,” according to Newman. “A few billion years from now, our Galaxy will be a much more boring place, full of middle-aged stars slowly using up their fuel and dying off, but without any new ones to take their place. It will be less interesting for astronomers in other galaxies to look at, too: The Milky Way’s spiral arms will fade into obscurity when there are no more blue stars left.”

Source: Pitt, AAS press briefing

Posted on by Nancy Atkinson

The Sloan Digital Sky Survey: “A Grand and Bold Thing”


If you do a search of articles on Universe Today, you’ll find that a large number of our posts reference the Sloan Digital Sky Survey. SDSS is a comprehensive survey to map the sky, using a dedicated 2.5 meter telescope equipped with a 125- megapixel digital camera and spectrographs. Since 2000, SDSS has created terabytes of data that include thousands of deep, multi-color images, covering more than one-quarter of the sky. SDSS is literally changing the way astronomers do their work, and represents a thousand-fold increase in the total amount of data that astronomers have collected to date. In a new book, “A Grand and Bold Thing; An Extraordinary New Map of the Universe Ushering in a New Era of Discovery,” science journalist Ann Finkbeiner tells the story of how SDSS came about (frighteningly, the survey almost didn’t happen), delving into some of the discoveries made as a result of this survey, and sharing how even armchair astronomers are now probing the far reaches of the Universe with SDSS.

SDSS has measured the distances to nearly one million galaxies and over 100,000 quasars to create the largest ever three-dimensional maps of cosmic structure. It also spawned one of our favorite citizen science projects: Galaxy Zoo.

For three years, Ann Finkbeiner researched and interviewed astronomers to get the story behind SDSS, to tell the little-known story of this grand project, and how it soon grew into a far vaster undertaking than founder Jim Gunn could have imagined. The book is extremely readable, and Finkbeiner captures the personalities who brought the project to life. If you thought Earth-based observing was passe, this book will make you re-think the future of astronomy.

Finkbeiner is a freelance science writer who has been covering astronomy and cosmology for over two decades. She has written feature articles for Science, Sky & Telescope, Astronomy, and more, with columns for USA Today, and Defense Technology International. She is co-author of The Guide to Living with HIV Infection (Johns Hopkins University Press, 1991; sixth edition, 2006), which won the American Medical Writers Association book award. She is also author of “After the Death of a Child,” and “The Jasons,” which won the American Institute of Physics’ Science Writing Award in 2008.

Below is a Q & A with Finkbeiner about “A Grand and Bold Thing.”

Q: What made you first want to write this book?
A: I was finishing a magazine article about the Sloan Digital Sky Survey just as I was beginning the interviews for a book—The Jasons—for which no one at all wanted to talk to me. But the Sloanies I was interviewing were so happy about what they were doing, so intense about it all, and so open (they even showed me their gazillion archived emails) that writing a book about them felt like it would be a blessed relief, like leaving boot camp and going to a good block party.

I was writing the magazine article in the first place because I’d attended a talk by Jim Gunn at Johns Hopkins, and while I listened, I realized I hadn’t heard any news from him for a long time. So I afterward, I asked him why he’d gone off radar. He told me he’d been working on getting a survey going, using a little 2.5 meter telescope, and I wasn’t impressed. I thought it was an odd use of his splendid capabilities. I was impressed later, though, when he stayed off radar and I found out that other excellent scientists were doing the same. I started wondering why they were giving up their careers for a sky survey.

Q: Has perception of the project changed from the time you first started writing about it until now?
A: Between the time I first heard about it—in the late 1990’s—and the present, the perception of the project changed dramatically: today, it’s hard to overstate its importance. But astronomers’ early reactions to the survey were what mine had been: Little telescope. Not spectacular resolution. Can’t go very deep in the past. Astronomers who knew the value of a survey and Jim’s reputation for building nearly-perfect instruments were quicker to see the potential, but the project’s many, many management problems led to the community taking pot shots at the Sloanies. Then when funding agencies started refusing to give astronomers money because the Sloan was going to do their pet projects better than they would, Sloan became a dirty word. Now, astronomers say it changed the way they do their work.

Q: What do you think have been the most important benefits of the Sloan Survey’s completion?
A: The Sloan was, and still is, the only systematic, beautifully-calibrated survey of the sky and everything in it. And it’s the first survey to be digital. Astronomy before Sloan was photographic, meaning you were at a rich university that owned a telescope, you decided which objects in the sky you liked and took photographs of them, and kept them for yourself. If you wanted to use the only survey of the sky, you bought expensive photographs of it. After the Sloan, you download the objects you want to study onto your computer for free. So whether you’re an astronomer or a regular person, you can study anything you want to with some of the most trustworthy data going. And if you don’t want to learn astronomy jargon and query languages, you can go to GalaxyZoo.com and join the 300,000 people doing astronomy on the Internet using this data. The Sloan has democratized astronomy. It’s made “citizen science” real. And it’s about to become redundant because it triggered a population of other newer, bigger surveys.

Q: What do you think the story of the Sloan Survey tells us about current cosmological thought?
A: Before Sloan, cosmology was seen as a fluffy science: the universe is big, distant, and hard to observe, so the phrase “precision cosmology” would have been an in-joke. But Sloan’s data is so comprehensive and exquisite that precision cosmology is now the norm.

Before the Sloan, cosmology was fractured into many fields whose relation to each other wasn’t obvious and wasn’t being studied. Sloan found all kinds of things in all areas of astronomy: asteroids in whole families, stars that had only been theories, star streams around the Milky Way, the era when quasars were born, the evolution of galaxies, the structure of the universe on the large scale, and compelling evidence for dark energy. So after the Sloan, cosmologists began seeing the universe as a whole, as a single system with parts that interact and evolve.

Q: Work like this costs an enormous amount of money, but doesn’t yield the sort of practical results the average American can see. What is the best argument to continue funding science like this?
A: The main Sloan survey cost $85 million over 10 or 15 years. In the realm of government budgets, that’s spare change. It cost so little partly because the scientists gave their time for free—they had university salaries already. And since this free time came at the expense of their own research and personal reputations, they’re a case study in altruism. In addition, the universe is mankind’s most fundamental context; and astronomy and cosmology have, I think, some of the appeal of philosophy and religion. Put scientific intelligence together with altruism and questions of origin and place in the universe, throw in beautiful pictures, and I’d give it money in a minute.

Q: There are a lot of good stories behind the making of the Survey. What are some of your personal favorites?
A: My all-time favorite is Galaxy Zoo, which started when a couple of Sloanies needed to know which galaxies were spirals, which were ellipticals, and which were irregular. But Sloan had a million galaxies, which is a lot for any human to sort through: computers are no good at identifying shapes, humans are superb at it. So the Sloanies put the million galaxies on the internet, asked for help, and within a day, their computer server melted. There are now 300,000 Galaxy Zooites of all ages, all levels of education, from all over the world, and they’ve gone way beyond classifying shapes. Hanny van Arkel, a Dutch primary school teacher, found a strange blue object the Zooites called Hanny’s Voorwerp, and after followups with xray, ultraviolet, and radio telescopes (not to mention the Hubble Space Telescope), the Voorwerp turned out to be a place in an enormous cloud of gas which was being hit by a hard xray jet from a galactic-sized black hole. Zooites also found a new kind of greenish, round galaxy, and then found enough of them that they’re now officially called Green Pea galaxies. Green Peas turn out to be small, nearby, previously unknown galaxies in which stars are being born at a furious rate. Then Zooites went off and taught themselves serious astronomical techniques and began collecting and studying irregular galaxies; astronomers knew of 161 irregulars, the Zooites found 19,000 of them and called their project, Do It Ourselves.

I also love Jim Gunn’s professional trajectory from fame to invisibility, and while invisible, his fight-starting and progress-impeding insistence on doing everything as well as it can possibly be done. When Jim started the Sloan, he was extremely famous and highly respected. He walked away from his own research and spent the next 30 (he’s still doing it) years first putting together the collaboration, then building the camera, while also overseeing and micromanaging every detail of every piece of hardware, software, and politics. He’s a perfectionist whose motto is: “if you don’t do it right to begin with, you’ll have to do it again, no matter what the bloody cost and schedule says.” He caused no end of arguments, particularly when the “young astronomers” involved adopted the same motto. The perfectionism was finally controlled, on the surface anyway, by a remarkable project manager, but Jim and the young astronomers kept doing it right on their own time and without permission. The Sloan’s whole value today is that it’s nearly perfect, and this precision has enabled much of its most important contributions. Jim’s now nominally retired and in any case, has turned the survey over to the young astronomers who have, in their turn, turned it over to the whole astronomical community and to the public.

Q: One thing that might surprise readers is how “political” scientists sometimes have to be in working with their colleagues, other institutions, and even asking for funding. Why is this, and has it always been this way?
A: It’s been that way ever since science stopped being a gentleman’s hobby—Jim’s phrase, “gentleman astronomers in their coats and ties”—and began getting funding from foundations and the government. The amount of funding is limited and everyone has to complete for the same small, fixed pot. It’s hair-raising. The astronomical community solves this brilliantly: they find out what everybody else is doing, then they do something different and complementary, and finally they get together and tell the funders what the community’s priorities are. The result is that astronomy keeps getting funded. Meanwhile, individual astronomers are free to be competitive and dog-eat-dog, just as their human nature requires.

Q: What do you hope readers take away from this book?
A: The joy and entertainment of watching these impressively intelligent and persistent guys fumble around until they’ve done something remarkable.

Posted on by Nancy Atkinson

Spitzer Spies Earliest Black Holes

This artist's conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Image credit: NASA/JPL-Caltech

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The Spitzer Space Telescope has found what appear to be two of the earliest and most primitive supermassive black holes known. “We have found what are likely first-generation quasars, born in a dust-free medium and at the earliest stages of evolution,” said Linhua Jiang of the University of Arizona, Tucson, lead author of a paper published this week in Nature.

A quasar is a compact region in the center of a massive galaxy surrounding the central supermassive black hole.

As shown by the image we posted earlier today from the Planck mission, our galaxy – and the Universe – is littered with dust. But scientists believe the very early universe didn’t have any dust — which tells them that the most primitive quasars should also be dust-free. But nobody had seen any “clean” quasars — until now.

Spitzer has identified two — the smallest on record — about 13 billion light-years away from Earth. The quasars, called J0005-0006 and J0303-0019, were first unveiled in visible light using data from the Sloan Digital Sky Survey. That discovery team, which included Jiang, was led by Xiaohui Fan, a coauthor of the recent paper. NASA’s Chandra X-ray Observatory had also observed X-rays from one of the objects. X-rays, ultraviolet and optical light stream out from quasars as the gas surrounding them is swallowed.

“Quasars emit an enormous amount of light, making them detectable literally at the edge of the observable universe,” said Fan.

These two data plots from NASA's Spitzer Space Telescope show a primitive supermassive black hole (top) compared to a typical one. Image credit: NASA/JPL-Caltech

When Jiang and his colleagues set out to observe J0005-0006 and J0303-0019 with Spitzer between 2006 and 2009, their targets didn’t stand out much from the usual quasar bunch. Spitzer measured infrared light from the objects along with 19 others, all belonging to a class of the most distant quasars known. Each quasar is anchored by a supermassive black hole weighing more than 100 million suns.

Of the 21 quasars, J0005-0006 and J0303-0019 lacked characteristic signatures of hot dust, the Spitzer data showed. Spitzer’s infrared sight makes the space telescope ideally suited to detect the warm glow of dust that has been heated by feeding black holes.

“We think these early black holes are forming around the time when the dust was first forming in the universe, less than one billion years after the Big Bang,” said Fan. “The primordial universe did not contain any molecules that could coagulate to form dust. The elements necessary for this process were produced and pumped into the universe later by stars.”

The astronomers also observed that the amount of hot dust in a quasar goes up with the mass of its black hole. As a black hole grows, dust has more time to materialize around it. The black holes at the cores of J0005-0006 and J0303-0019 have the smallest measured masses known in the early universe, indicating they are particularly young, and at a stage when dust has not yet formed around them.

The Spitzer observations were made before the telescope ran out of its liquid coolant in May 2009, beginning its “warm” mission.

Source: JPL