Category Archives: large-scale structure

Survey the Universe?

How It Has Been Done with SDSS

The Perseus Cluster, as seen through the eyes of the Sloan Digital Sky Survey. Courtesy Robert Lupton. Click to biggify.

The universe is a strange and wonderful place.  How do I know this when I haven’t explored it all? When astronomers are still searching out the distant reaches and early history of the cosmos?  I know it from the work done by scientists using such observatories as the Hubble Space Telescope, the Spitzer Space Telescope, the Wilkinson Microwave Anisotropy Probe, and the phalanx of ground-based observatories such as the Subaru Telescope, the Gemini Observatory, the European Southern  Observatory, and many, many others.  Quite significantly, I know it from the results of a sky survey that revolutionized our view of the universe–mostly of the galaxies, galaxy clusters, and quasars — but also by looking at bodies in our own solar system.  That survey was the Sloan Digital Sky Survey, which has carried out deep multi-color surveys of more than a quarter of the sky. Data from its first surveys were used to create three-dimensional maps of nearly a billion galaxies and more than 120,000 quasars. It’s now in a new program of observations called SDSSIII that will continue until the year 2014.

All this work has been done using a 2.5-meter telescope on a mountain in southern New Mexico. A single telescope!  It’s an amazing and ongoing accomplishment.

The Sloan began operation in 2005, and I often wondered about the people who put it together. Certainly I’d heard plenty about the survey at meetings, and had met some of the Sloan planners. But, as with the Hubble Space Telescope, I really didn’t know much of the history of the project when I first signed on to work with an instrument team back in grad school.  HST piqued my curiosity, and so in 1992 and 1995, I worked on a book with co-author Jack Brandt (a former team lead for the Goddard High Resolution Spectrograph on HST, and now at the University of New Mexico) called Hubble Vision. I also did a planetarium/fulldome show called Hubble Vision about HST’s accomplishments, which I periodically update.

Working on those projects gave me a lot of insight into the people who make such instruments work, and their hopes and dreams for the outcome of their astronomy work.  Hubble’s history is replete with individuals who designed the instruments, solved the problems, recognized the errors of spherical aberration, and who have made the many,  many accomplishments possible.  Some of those same folks have been involved with the Sloan Digital Sky Survey, too.

I just finished reading a book about the Sloan Digital Sky Survey (SDSS, for short) and the folks who made it a reality. The book, called A Grand and Bold Thing, by Ann Finkbeiner, gives us a look not just at Sloan and its accomplishments, but at the dream it sprang from — beginning with the spiral bound notebooks of astronomer Jim Gunn (who first brought the idea up at a meeting in Tucson in 1987), and the further refinements of the first planning documents and taking us to the observations made by this project.  At one level,  the book does what Jack and I tried to do for Hubble: give readers a look at the PEOPLE behind the instruments and accomplishments.  Ann’s writing is clear and wonderful, and she really lets the reader see the history and growth of SDSS quite clearly, through the eyes of the astronomers who made it happen.  These are REAL people who sweated over the development and installation of SDSS, and their accomplishment is considerable.

Along the way, we also learn about the universe that SDSS (and all its observational siblings) has revealed to astronomers.  SDSS’s contributions to understanding the large-scale structure of the universe are considerably one of the most important achievements in astronomy.  Without the data that SDSS, and sibling surveys such as the 2DF and 6DF observations, astronomers might still literally be groping in the dark for an understanding of how matter is distributed throughout the universe.

Finkbeiner weaves in the story of thediscovery of the structure of the cosmos as she tells the story of the SDSS.  For me, entwining together the story of scientific discovery with the tale of the people who enabled the SDSS’s odyssey of discovery is a heady brew. You should drink it in for yourself!

The Stuff Between the Galaxies

What is It?

This computer simulation by Matt Hall (NCSA) and Eric Hallman (Univ. Colorado) shows the warm-hot intergalactic medium (blue) that runs along filaments between galaxies and galaxy clusters.  (Click to biggify.)
This computer simulation by Matt Hall (NCSA) and Eric Hallman (Univ. Colorado) shows the warm-hot intergalactic medium (blue) that runs along filaments between galaxies and galaxy clusters. (Click to biggify.)

Living in a galaxy is like living in a big city.  There’s always something going on, there’s always somebody around. Even if you live out in the country, you’re still not that far from the nearest neighbor or town or burg or hamlet. But, let’s say you lived out in the desert, hundreds of kilometers from anything. You’d be surrounded by nothing, right?

Well, not exactly. It depends on how you define “nothing.”  If you think that a lack of towns or neighbors means there’s nothing out there, that’s one way to think of it. But, you’d still have sand and plants and animals surrounding you. They’re not in your social set, but that doesn’t  mean they don’t exist.

The same thing goes for galaxies and the space between them.  That space may look empty, but it’s  not.  Our galaxy is part of a cluster of galaxies called the Local Group.  The space between our galaxy and the ones next door is filled with material even though (to us with visible-light eyes) it looks like it’s empty.

While you may have heard that there’s a dark matter halo out there surrounding the Milky Way, there’s also regular old baryonic matter.

How do we know this? Bruce Dorminey, who writes for ScienceNow Daily News, sent me an article he wrote about a detection of regular matter between galaxies that was done using x-rays.

How so?  As light travels through the intergalactic medium, it encounters “stuff” — atoms of gases in clouds.  As light from more distant objects runs through those clouds, some of it is absorbed by the material.  We can actually see the fingerprints of this absorption when we look at that light through a spectrograph.

It turns out that these clouds are likely absorbing x-rays (which are also part of the electromagnetic spectrum). David Buote of the University of California at Irvine and a group of astronomers used the Chandra X-ray observatory and the European Space Agency’s XMM-Newton observatory to look at a portion of an object called the Sculptor Wall, part of a large collection of galaxies that lie about 400 million light-years away.  They were specifically looking for the fingerprints of O VII — oxygen that has been stripped of five of its eight electrons.  This O VII is part of what astronomers call the “warm-hot intergalactic medium” — a sort of rarefied plasma that absorbs various wavelengths of light, including x-rays.  Buote and his colleagues are saying that there’s an excellent chance that their discovery will hold up and that they have found another way to probe the matter that exists in the intergalactic medium. Their research will be published in the April 20th issue of Astrophysical Journal.

Astronomers have long known that the intergalactic void wasn’t completely empty, but this new work shows us what’s filling it in some places. It has pretty important implications for how we understand what astronomers call the “large-scale structure of the universe.”  This is because we are still trying to understand just how matter is distributed — whether it’s regular matter or dark matter.  In the long term, astronomers use studies like this to model just how galaxies are formed and how the universe has evolved since it began 13.7 billion years ago.