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The Webfooted Astronomer - November 2001


Minutes Was Chicken Little 25% Right?

By Greg Donohue

ABOUT a half dozen new people attended our last general meeting. At least one new person joined SAS that night, and along with two other members, signed the use agreement for our Rattlesnake Lake viewing site. We now have 26 members authorized to use the site.

The Poo Poo Point star party is scheduled for November 17, and our next public star party at Green Lake is November 24. And don’t forget to make your reservations for the annual SAS Banquet, coming up on January 26, 2002!

Our speaker for the evening was Dr. Scott Anderson, who serves as professor in the UW Astronomy Department. He received his B.S. in Astrophysics and Mathematics from the University of New Mexico in 1979, M.A. in Astronomy from UCLA in 1980, and Ph.D. in Astronomy from UW in 1985. Professor Anderson specializes in space-based observation of high-energy phenomena such as quasar and x-ray sources. He discussed the Sloan Digital Sky Survey (SDSS) in detail.

The original aim of the SDSS was to create a 3-D map of 1/4 of the entire astronomical sky (about 10,000 deg2), using images taken from the Northern Hemisphere and out of the plane of the Milky Way to avoid obscuring gas and dust. This data will reveal the largest structures in the universe (the filaments and voids, sheets, walls and bubbles created by large-scale galactic aggregation). Competing theories regarding the formation of the universe can then be tested against reality. The more matter there is in the universe, the more “clumpy” the topology of the universe should be. The nature of the matter (baryonic or non-baryonic) also influences the large-scale structures.

Getting this 3-D map requires two steps. First, the 2-D coordinates (right ascension and declination) are determined from digital images of the sky. Second, spectra for selected galaxies are taken at a later date. The redshift of these spectra are then used to infer the distance (the third dimension) to the galaxies.

The first SDSS digital test images were taken in May 1998. A year later, SDSS obtained its first test spectra. The testing phase completed in December 2000, and the official survey got underway last January. To date, some 1,500 square-degrees of sky have been imaged (15% of the total). The 5-year survey will create a databank with 12 terabytes of digital imaging data, and 350 gigabytes of spectroscopic data. (That’s enough to fill up the 20-gig hard drives on more than 630 PCs!) The data is immediately available to the UW, and will eventually be accessible via the Internet. The first installment of the public data was released in June. It contains the first 5% of the survey data: 500 square degrees of sky, a catalogue of 100,000 objects, perhaps 35,000 galaxies, and 50,000 spectra. This dataset is available at The completed databank will contain accurate positions and absolute brightness for 100 million galaxies, 100 million stars in the Milky Way, and spectra and distances to a million select galaxies, 100,000 quasars, and 100,000 stars in our galaxy.

Surprisingly, perhaps, the task is being accomplished with a rather modest-sized (2.5-meter) telescope located at the Apache Point Observatory. To image the required number of galaxies, one only has to go down to 18th magnitude. The telescope has a 3-degree field of view and 63 milli-arcsecond resolution and is entirely dedicated to the SDSS project.

The digital imaging equipment takes 5-color images, using a 5-column by 6-row array of CCDs, each of which is 2,048 pixels on a side. Each column of 6 CCDs is filtered to record one of the 5 SDSS wavelengths of light. The telescope’s alt-azimuth mounting requires that the imaging equipment rotate to counter the rotation of the sky. The telescope does not track the objects it is imaging. Rather, the telescope is held fixed and the objects are allowed to move across the CCD array at the sidereal rate. Normally this would lead to “trailed” images. This trailing effect is eliminated by a technique known as TDI (Time Delay and Integrate), in which the electrical charges are marched through the CCD into storage at the same rate the sky moves across the array. Once an object has drifted completely across all 5 different colored CCDs in a row, the data can be integrated into a full-color image. An all-night exposure using this technique images a 2.5-degree by 100-degree strip of sky.

Using the 2-D digital images, computer software selects all galaxies down to 18th magnitude and other objects of interest for subsequent spectroscopic analysis. The coordinates of the objects are used to precisely drill 640 holes in a large metal plate. Drilling is performed at the UW. Since only 500 holes per plate are needed for the 1-million-galaxy survey, more than 100 slots are available for taking the spectrum of the other objects such as Quasars/QSOs (Quasi-Stellar Objects) and interesting stars within the Milky Way. Individual fiber-optic wires are manually inserted into each of the 640 holes. The configuration is then mounted under the telescope, and the light for each object travels down the fiber optics to a single spectrometer.

In spectroscopy, light is spread out into its component wavelengths. Due to this spreading (and therefore dimming), taking spectra of the galaxies requires longer exposure times than the 2-D digital images. The SDSS telescope must track the set of 640 objects for 45-60 minutes in order to collect enough photons. Up to 8 plates can be exposed on a good night, and the spectra for 200,000 objects have been collected so far.

Though originally conceived as a galaxy survey, SDSS is already making important discoveries involving other types of celestial objects. The first 15% of the SDSS data has spawned 70 scientific papers so far.

For example, SDSS has identified the most distant quasars to date. Quasars had been known 35 years prior to SDSS. In that time, only 40 QSOs with redshift z>4 had been discovered. Since SDSS came online, 200 quasars with z>4 have been found, a dozen of which have z>5. The most distant quasar known was discovered by SDSS. Indeed, nine of the 10 highest-z quasars are from SDSS.

The original 1216-angstrom UV light from one of the most distant quasars has been red-shifted to 8000 angstroms (infrared), indicating that this quasar already existed when the universe was no more than 1 billion years old. Of particular importance is the fact that its spectrum contains lines for nitrogen, oxygen, silicon, and other heavy elements. Only hydrogen, helium, a little lithium, and minute traces of beryllium were created during the Big Bang. So, it is evident that stars had already produced some heavy elements when the universe was less than a billion years old. Since quasars appear to be linked to super-massive black holes at the center of galaxies, very large structures appeared early in the life of the universe.

To identify QSOs for spectra imaging, software first throws out all galaxies and other extended objects. What’s left are stars and QSOs. Stars have well behaved and predictable colors, so they are also eliminated, leaving only the QSOs.

Prior to the beginning of the Sloan survey, only a single brown dwarf had been found. Brown dwarfs are objects too big to be planets, but too small to sustain a stable hydrogen fusion reaction in their cores. Their surface temperatures are estimated at about 1,300K. They range in mass from the equivalent of a few tens of Jupiters, up to about 3% of the mass of our Sun. So far, SDSS has found about a dozen new brown dwarfs, out to a distance of 30 light-years.

The universe appears to consist mostly of dark matter, whose presence is inferred due to its gravitational effects on galaxies. Two classes of objects have been proposed to constitute this dark matter. The first class of objects, known as MACHOS (Massive Compact Halo Objects), would be made up of the normal baryonic matter (protons/neutrons/electrons) that we have all come to know and love. The other class, WIMPS (Weakly-Interacting Massive Particles) would be some exotic new form of matter that we are not yet able to detect.

Neutrinos would fall into the WIMP category. Brown dwarfs, being composed of normal baryonic matter, are good candidates for MACHOs. However, the low number of brown dwarfs detected by SDSS suggests that they are not plentiful enough to be a significant constituent of the dark matter.

In its first few months, the SDSS has already made many important contributions to several areas of astronomy, and holds the promise of many exciting discoveries to come. So it appears that Chicken Little was at least partly correct: The sky (1/4 of it anyway) is falling-falling under the intense scrutiny of the Sloan Digital Sky Survey, that is!

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