Sloan Digital Sky Survey III – SDSS-III

Sound waves which propagated in the primordial plasma of the universe have left the imprint of a characteristic distance in the correlation function of matter. By measuring this standard scale at various distances, one can determine how the universe grew and how this growth has been affected by dark energy. It is possible to derive from this constraints on the dark energy equation of state.
The BOSS survey measures the position of millions of galaxies, and thus the associated correlation function. It also measures the spectrum of distant quasars ; neutral hydrogen leaves its mark in the spectra of those quasars, the so-called Lyman alpha forest, and we can measure the BAO scale at large redshift this way, and therefore go back even further in the past of the expansion of the Universe.
The French participation group to SDDS-III has focused on the measurement of BAO in the Lyman alpha forest of quasars.

The French participation group played a major role in the development of the use of the Lyman alpha forest in cosmology.
We published the first detection of the BAO scale in the forest of quasars and played an important role in all subsequent research the SDSS collaboration performed on the subject.
The French group was also responsible for the publication of a catalog of quasars at each public SDSS data release.

The main impact of this project was to create in France a recognized expertise on baryon oscillations. This allowed some of the team to participate in the SDSS-IV project, which is currently taking data, and play an important role in construction of the DESI project, scheduled for ~ 2018.

It is difficult to distinguish publications from the two successively funded ANR projects. In the period of the second project, the French team SDSS has authored more than 90 articles directly related to the project.

We were co-authors of the articles accompanying the «data releases« as well as of the major articles of the APOGEE and BOSS surveys, because of our involvement in the project.

Among the significant papers specifically published by the French team are:
Busca et al. 2013, the first measure of BAO signal in Lyman-alpha forest and the first direct evidence that the expansion of the universe was slowing down in the past.
The publication of a catalog of quasars, accompanying each «data release«
Palanque et al. 2013 & 2015, in which small-scale matter fluctuations are studied in the individual spectra of quasars.
Delubac et al. 2015, an update on the constraints from BAO in the Lyman-alpha forest.

For the APOGEE survey, important papers include:
Hayden et al, 2015: Analysis of metallicity and abundance of alpha elements of stars in the galactic disk
Fernandez-Trincado, Robin, et al, 2015: Discovery of a halo star with the typical chemical signatures of globular clusters

A French Participation Group (FPG) to the Sloan Digital Sky Survey-III has been formed in 2009 and has been supported by ANR for the 2009-2012 period. Since SDSS-III is a six-year long project, it was known from the beginning that additional funding would be required. This is why we are asking now for funding for the last two years of the survey (2013-2014), and for an extra year for data analysis. The FPG is strongly involved in the Baryonic Oscillation Spectroscopic Survey (BOSS) part of the SDSS-III but after the Besançon Observatory joined the group, we are now also involved in the galactic part of the SDSS-III surveys: SEGUE and APOGEE.

Evidence has accumulated over the last years that the expansion of the Universe is accelerating. It is one of the major topics in cosmology today, and could have implications for astronomy, particle physics, general relativity, and string theory.
One of the techniques allowing to measure precisely the cosmic expansion history over a wide span of time is the “baryon acoustic oscillations” (BAO) method, pioneered by the SDSS-I project. It is especially attractive for its simplicity and reduced systematic. Sound waves that propagate in the hot plasma of the early Universe imprint a characteristic scale on the clustering of dark matter, galaxies and the intergalactic gas. By measuring this scale at different distances, one can determine how the Universe has grown and how that growth has been affected by dark energy. More specifically, the main goal of this proposal regarding the BOSS survey is to measure, through the BAO method, the equation of state (EOS) of dark energy and its evolution with redshift.
It is proposed in BOSS to map the 3-D large scale spatial distribution of 1.5 million luminous red galaxies and of the neutral hydrogen seen in absorption in the spectra of 160 000 distant quasars. The galaxy clustering measurement will determine the absolute distance scale with 1% precision over the last 7 Gyr ; no future experiment can substantially improve this measurement because BOSS will survey a large fraction of the available cosmic volume with a dense population of strongly clustered tracers. This BOSS galaxy clustering measurement will provide a legacy anchor point for future surveys. Clustering in the quasar absorption spectra will allow 1.4 % measurements of the distance scale and the cosmic expansion rate at a lookback time of 10 Gyr, providing a precise determination of the curvature of space and unique sensitivity to dark energy at early times.

The FPG has focused most of its effort on the part of the survey aimed at recovering the BAO signal (BOSS). However, the FPG is also involved in the two surveys focusing on our Galaxy, SEGUE and APOGEE.
Over the first four years as members of the SDSS collaboration, we have achieved a very good visibility in the collaboration and we have secured leading roles in future publications. In SEGUE, we use the Besançon Galaxy model to analyze the data and to study the metallicity distribution and the thick disc structure. In APOGEE, we participated in the preparation of the target list, and plan to study the bulge metallicity, the kinematics of the bar, and the relation between the bulge and the thick disc.

In BOSS, we lead the target selection and the preparation of the data catalogue. Our Lyman-alpha forest analysis will be published as an official SDSS-III analysis in July 2012, along with the DR9 data release. We have built a strong French community in BAOs and prepared future spectroscopic surveys like e-BOSS, BigBOSS, and Euclid. This funding will allow us to keep access to SDSS data and strengthen our role in the domain.