January 11, 2005

A new galaxy atlas — Sloan Digital Sky Survey findings comprise new compendium of galaxy families

SAN DIEGO — In a poster whimsically titled "A whole lot of nearby galaxies," New York University astronomers David W. Hogg and Michael Blanton presented a new atlas of galaxies today at the winter meeting of the American Astronomical Society in San Diego.

The atlas shows about 4,000 galaxy images taken from the Sloan Digital Sky Survey (SDSS), the most ambitious astronomical survey project ever undertaken. With more than 200 researchers from 14 institutions around the world, the SDSS is mapping in detail about one-quarter of the sky, determining the positions and brightnesses of hundreds of millions of celestial objects. It will also measure the distances to about a million galaxies and quasars.

Combining computer-aided data analysis with manual, subjective human visual classification, the new galaxy compendium is based on ages, masses, and other physical properties. In time, this may become the largest and most useful visual atlas of galaxies ever produced.

The atlas uses quantitative physical parameters measured from the galaxy images and spectra, and presents the images for easy visual classification of the galaxies into spirals and ellipticals. This two-pronged approach allows researchers to understand the relationship between the two classification methods.

"Some properties of galaxies are straightforward to measure with computer analysis of the SDSS data," explains Blanton, "while others can only be reliably determined by human inspection. We are trying to understand the relationships between these two different kinds of properties."

This visual inspection for the purposes of galaxy classification started with Edwin Hubble in the 1920s and led to an understanding that galaxies fall into two broad types, spirals and ellipticals. Within these there are physically important subclasses.

The spirals, including the Milky Way, have young stars arranged in a large, thin, rotating disk with the stars, gas and dust arranged in spiral patterns. The ellipticals have older stars arranged in a dense, spheroidal blob.

Both kinds of galaxies are clearly visible in Hogg and Blanton's atlas, with the spirals tending to be bluer, less dense, and in regions of space with relatively few companion galaxies. The ellipticals tend to be redder, more dense and surrounded by many other galaxies.

Hogg and Blanton originally made the charts — dense grids of small color pictures of galaxies — as a research and data visualization tool for quality-checking the SDSS data set, but quickly found that the charts were interesting and beautiful, and yielded real insight into galaxy properties, making them appropriate for presentation to the whole scientific community. The color pictures — using a technique pioneered by SDSS astronomer Robert Lupton — convey far more information than the traditional rendering of astronomical images. This color rendering technique, combined with the very high quality of the SDSS images, make this galaxy atlas more scientifically powerful, and much more beautiful, than its predecessors.

Hogg explains that: "We spend a lot of time searching data for patterns that might lead to new directions for research. We have generally found that when we make a useful and informative presentation of the data for these investigations, that presentation is also beautiful and scientifically compelling on its own."

The full, technical description of the American Astronomical Society presentation is available as a PDF file at http://cosmo.nyu.edu/hogg/aas205/aas205.pdf.


  • David W. Hogg, New York University, david.hogg@nyu.edu, 646-236-3287
  • Michael R. Blanton, New York University, mb144@nyu.edu, 212-992-8791
  • James Devitt, New York University, Office of Public Affairs, james.devitt@nyu.edu, 212-998-6808
  • Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey, sdsspio@aol.com, 312-320-4794