Sky & Telescope - January 2023 - 22

Astronomy's Workhorse
LEGACY CAMERA This camera collected all the
imaging data of SDSS's first decade. The camera
read the CCDs while the sky drifted by the field of
view of the telescope in great circles, so images of
objects moved along the CCD columns at the same
rate that the CCDs were being read.
It takes great effort to make " open and
accessible data " a reality. Scientists working
within the project commit to releasing data
regularly, typically about a year after it comes
off the telescope. This time gives the team
time to do necessary processing, to check and
double-check for mistakes, to write supporting documentation
- and to benefit from early access before releasing the
data to the rest of the world.
Mapping the Universe
SDSS is probably best recognized by its maps of the distant
universe. If you've been to a planetarium, you may well
have " flown through " one of these. The videos look like how
movies imagine jumping to hyperspace, flying past endless
galaxies. Eventually, they zoom out to show all that SDSS has
surveyed: a butterfly-shape slice of the cosmos.
Across SDSS's first four phases, measuring cosmological
redshifts was a main goal. Astronomers first calculate how
quickly distant galaxies appear to be moving away from us
due to the universe's expansion; this recessional velocity is proportional
to a galaxy's distance from us (S&T: Oct. 2022, p.
12). We can then use these distances to map galaxies in 3D.
These first maps of large-scale structure showed galaxies
outlining bubbles and filaments in the local universe. Other,
smaller surveys had seen evidence for this cosmic web, but
SDSS made such a leap in scale that there was no longer any
After being retired, the original SDSS
imaging camera was moved to the
basement of the Smithsonian in
Washington, D.C., where it is stored for
its significance to scientific history.
doubt of the web's existence. Later phases of
SDSS have dwarfed that first look, measuring
galaxy distances out to a redshift of 1.1 (a
lookback time of 8.2 billion years). For galaxies
with intense beacons powered by supermassive
black holes, called quasars, the measurements
extend even further, to a redshift of 3.5 (11.9 billion years).
From these maps we learned that as the universe evolves,
it becomes clumpier: Gravity pulls matter into a vast network
over time. The detailed patterns in the network tell astronomers
about the content and expansion history of the universe.
We've even identified reliable (albeit very large!) rulers
in those patterns. The imprint of ancient soundwaves shows
up as a slightly preferred distance between any two pairs of
galaxies. SDSS scientists measured this baryon acoustic oscillation
(BAO) scale for the first time in 2005. The BOSS survey,
later extended as eBOSS, improved on this work by obtaining
millions of galaxies' and quasars' redshifts.
The BAO scale has enabled astronomers to make some of
the most precise measurements of the universe's expansion
rate over a wide range of cosmic times. The result favors the
lower of two hotly contested values for the universe's current
expansion rate (S&T: Mar. 2022, p. 14). SDSS BAO data
haven't yet solved the puzzle of the mysterious dark energy
that's accelerating this expansion, but they are an important
step toward an answer.
So Many Galaxies, So Little Time
Along the way to creating cosmic maps, SDSS's observations
gave astronomers physical information about almost 1 million
galaxies. Besides measuring their distances, we can also
" weigh " galaxies by probing the motions of their constituent
stars, and we can gauge how many baby stars they are
forming. Chemical elements leave fingerprints in the detailed
spectra that tell us about both the types and ages of stars and
about the history of how past stars have enriched the gas with
heavy elements. All of this together helps us reconstruct each
galaxy's life story.
While we can only ever see an instantaneous snapshot
in the life of an individual galaxy, from data on hundreds
of thousands, or even millions, of galaxies, we can piece
together how they change over time. That wasn't possible
before SDSS: " Big samples " of galaxies had previously numbered
up to a couple thousand.
The advance to much larger samples meant astronomers
THE TELESCOPE Sunset falls at the SDSS 2.5-meter telescope at
Apache Point Observatory in New Mexico.
22 JANUARY 2023 * SKY & TELESCOPE
had to become data scientists: Instead of examining a handful
of galaxies, they started looking for big-picture correla
Sky & Telescope - January 2023

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