In April BOSS passed an impressive milestone with 2 million survey-quality spectra. These spectra now probe large volumes of the Universe. The galaxy spectra alone probe over 35 billion cubic lightyears (four cubic gigaparsecs), while the BOSS quasars probe an even larger volume but much more sparsely. BOSS is on pace to successfully complete its main survey of 10,000 square degrees, which is almost one quarter of the sky, by the end of SDSS-III observing in the summer of 2014.
Officially, the prize for the two millionth object goes to a spectrum of the blank sky.
Figure: The two-millionth BOSS spectrum: an observation of a blank region of the sky (green line). Most of the emission that is seen is from the Earth’s atmosphere. The peaks in the right half of the figure are emission lines from oxygen, water, carbon dioxide, and other molecules. After subtracting the sky, the remaining signal (black line) is consistent with no detected source. The quoted redshift in light green is the BOSS pipeline fitting a quasar so faint that it is entirely consistent with the noise from the observation; this is not a real detection of an object.
Why does BOSS intentionally take observations of blank regions in the sky?
It turns out that these sky spectra are actually vital to the basic analysis of the BOSS spectra of stars, galaxies, and quasars. Most of the photons detected by BOSS are emitted by the Earth’s atmosphere. We have to subtract these photons to analyze the faint signals of distant galaxies. About 10% of BOSS fibers on each plate are used to observe blank regions of the sky. BOSS takes these blank-sky observations to help in calibration and removing the sky background.
These blank sky spectra can also have scientific utility. These blank sky spectra are the easiest targets from which to study the diffuse interstellar medium, as done by Brandt & Draine:
They are also used to search for serendipitous observations of emission-line galaxies. While the fibers are placed in locations that the SDSS-III imaging data show to be blank, galaxies that emit most of their light in just a few emission lines can occassionally appear in these spectra. Such galaxies are very uncommon today and understanding their presence at high redshift (which is necessary for their ultraviolet emission lines to be observed in the visual region of the spectrum) can help us learn about the early process of galaxy formation.