The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Overview and Early Data

TL;DR

The paper articulates the scientific motivation and design of the SDSS-IV eBOSS program to extend BAO and RSD measurements into the redshift range 0.6–2 using four tracers: LRGs, ELGs, clustering quasars, and Ly$\alpha$ forest quasars. It details target selection, imaging inputs, tiling, and data-reduction pipelines (validated with SEQUELS data), and presents projected cosmological constraints, including sub-percent BAO distances, enhanced GR tests, and competitive neutrino and inflation bounds. The work demonstrates how eBOSS will leverage multi-tracer tomography and cross-correlation with imaging surveys to reduce sample variance and calibrate redshifts, delivering a rich dataset for cosmology and galaxy/quasar science. The anticipated outcomes include refined measurements of $d_A(z)$, $H(z)$, and $f\sigma_8(z)$, enabling sharp tests of dark energy models, gravity, neutrino properties, and early-universe physics, with broad scientific impact beyond cosmology.

Abstract

The Extended Baryon Oscillation Spectroscopic Survey (eBOSS) will conduct novel cosmological observations using the BOSS spectrograph at Apache Point Observatory. Observations will be simultaneous with the Time Domain Spectroscopic Survey (TDSS) designed for variability studies and the Spectroscopic Identification of eROSITA Sources (SPIDERS) program designed for studies of X-ray sources. eBOSS will use four different tracers to measure the distance-redshift relation with baryon acoustic oscillations (BAO). Using more than 250,000 new, spectroscopically confirmed luminous red galaxies at a median redshift z=0.72, we project that eBOSS will yield measurements of $d_A(z)$ to an accuracy of 1.2% and measurements of H(z) to 2.1% when combined with the z>0.6 sample of BOSS galaxies. With ~195,000 new emission line galaxy redshifts, we expect BAO measurements of $d_A(z)$ to an accuracy of 3.1% and H(z) to 4.7% at an effective redshift of z= 0.87. A sample of more than 500,000 spectroscopically-confirmed quasars will provide the first BAO distance measurements over the redshift range 0.9<z<2.2, with expected precision of 2.8% and 4.2% on $d_A(z)$ and H(z), respectively. Finally, with 60,000 new quasars and re-observation of 60,000 quasars known from BOSS, we will obtain new Lyman-alpha forest measurements at redshifts z>2.1; these new data will enhance the precision of $d_A(z)$ and H(z) by a factor of 1.44 relative to BOSS. Furthermore, eBOSS will provide improved tests of General Relativity on cosmological scales through redshift-space distortion measurements, improved tests for non-Gaussianity in the primordial density field, and new constraints on the summed mass of all neutrino species. Here, we provide an overview of the cosmological goals, spectroscopic target sample, demonstration of spectral quality from early data, and projected cosmological constraints from eBOSS.

Figures (13)

• Figure 1: Projections for eBOSS LRG, ELG, and quasar distance measurements on a Hubble Diagram presented in comoving distance ($\eta$) versus redshift. Current BAO measurements from BOSS, SDSS xu13aross15a, 6dF Galaxy Survey (6dFGS), and WiggleZ parkinson12a are compared to SNe Ia measurements betoule14a and Planck predictions (solid curve) obtained by marginalizing over the full likelihood function.
• Figure 2: Current RSD constraints on the growth as a function of redshift compared to the projected measurements from eBOSS. The current measurements include those discussed in Section \ref{['subsec:boss_rsd']} and those for 6dFGS beutler12a, the main SDSS sample howlett15a, 2dFGRS song09a, the SDSS LRG sample oka14a, a recent result from the BOSS CMASS sample alam15b, WiggleZ blake12a, and VIPERS delatorre13a. Various models of modified gravity are shown, each with the same background expansion, the same comoving BAO position, and amplitude of the power spectrum normalized to that of the CMB at high redshifts. The black curve shows the growth in a $\Lambda$CDM universe, assuming the Planck best fit model parameters. The yellow curve shows $\gamma = 0.5$ where $f=\Omega_M^\gamma$linder05a. The purple curve shows $\gamma = 0.6$.
• Figure 3: Field centers for eboss[1--5]. The SEQUELS area is clearly defined by white space between the boundaries of eboss4 and eboss5. The area covered here is the area that was tiled in the beginning of SDSS-IV and the approximate survey area expected to be completed in the first two years of observation.
• Figure 4: Left: The completeness of the noknock, decollided LRG sample over the eboss3 region. Right: The cumulative distribution of completeness in the noknock, decollided LRG sample. The distribution is weighted by the area of each independent sector defined by areas covered by overlapping and unique tiles.
• Figure 5: Examples of SEQUELS spectra that span the range of redshifts expected in the LRG, clustering quasar, and Ly$\alpha$ forest quasar samples. In each, the data are represented in black, the flux errors on each pixel in red, and the template in blue. A boxcar smoothing kernel of width 5 pixels has been applied to the data for illustrative effect. Each spectrum is classified with high confidence by the automated data reduction pipeline. Top Left: An LRG at $z=0.64$. Top Right: An LRG at $z=0.88$. Middle Left: A quasar at $z=1.08$ identified by the QSO_CORE selection algorithm. Middle Right: A quasar at $z=1.74$ identified by the QSO_CORE selection algorithm. Bottom Left: A quasar at $z=2.21$ identified by the QSO_CORE selection algorithm. Bottom Right: A quasar at $z=3.15$ identified by variability in the PTF imaging data.