[Paper Review] A Map of the Northern Sky: The Sloan Digital Sky Survey in Its First Year

E. K. Grebel|arXiv (Cornell University)|May 9, 2001

Astronomical Observations and Instrumentation3 citations

TL;DR

This paper presents the Sloan Digital Sky Survey (SDSS) as a groundbreaking, large-scale optical CCD survey mapping one-quarter of the northern sky with high-precision photometry, astrometry, and spectroscopy. It highlights early scientific discoveries, including insights into galactic structure and tidal streams, and outlines the Calar Alto key project for follow-up observations, establishing SDSS as a foundational resource for diverse astrophysical research.

ABSTRACT

The Sloan Digital Sky Survey (SDSS) is the largest and most ambitious optical CCD survey undertaken to date. It will ultimately map out one quarter of the sky with precision photometry in five bands, high-quality astrometry, and spectra of all galaxies and quasars brighter than certain limiting magnitudes. The scientific potential of the SDSS is enormous and addresses a wide variety of astrophysical key questions. After a proprietary period the reduced and calibrated data are made available to the astronomical community as a whole. The SDSS is run by an international consortium involving universities and research institutions. One of the participating partners is the Max Planck Institute for Astronomy in Heidelberg. Already in its first year the SDSS has led to a number of spectacular scientific discoveries. In this paper, we will introduce the SDSS survey, discuss its scientific potential, highlight important science results, and present the Calar Alto key program for SDSS follow-up studies.

Motivation & Objective

• To present the scientific framework and initial results of the Sloan Digital Sky Survey (SDSS) in its first year of operation.
• To demonstrate the survey's unprecedented depth, homogeneity, and area coverage for studying galactic structure, stellar populations, and large-scale cosmological structures.
• To establish the Calar Alto key project as a critical follow-up initiative for deep imaging and spectroscopy of SDSS-selected targets, particularly tidal streams and low-surface-brightness galaxies.
• To address calibration challenges in SDSS photometry and spectroscopy, ensuring reliable interpretation of data for population synthesis modeling.
• To enable broad community access to calibrated SDSS data after a proprietary period, fostering widespread scientific discovery.

Proposed method

• The SDSS employs a dedicated 2.5-meter telescope at Apache Point Observatory with a 3-degree field of view, using a CCD camera to conduct multi-band imaging in five photometric filters (u, g, r, i, z).
• The survey uses a 3-inch fiber spectrograph to obtain redshifts and spectral energy distributions for galaxies and quasars down to magnitude limits of r ≈ 17.7 and i ≈ 17.0.
• Data are collected over 10,000 square degrees centered on the north Galactic cap and 225 square degrees in three great circle slices near the south Galactic cap, avoiding regions of high Galactic extinction.
• Calibration is performed using standard stars and internal consistency checks, with ongoing refinement of photometric and flux calibration prior to public release.
• The Calar Alto key project conducts deep imaging and follow-up spectroscopy of SDSS-selected tidal stream candidates to measure kinematics, velocity dispersion, and luminosity functions.
• A spectroscopic template database is being developed at Calar Alto and other observatories using stars and star clusters with known metallicities, distances, and ages to improve population synthesis modeling.

Experimental results

Research questions

• RQ1 How can the SDSS data be used to map the large-scale structure of the universe and trace the distribution of galaxies and quasars across one-quarter of the sky?
• RQ2 What insights into galactic structure, particularly tidal streams and low-surface-brightness galaxies, can be gained from the high dynamic range and depth of SDSS imaging?
• RQ3 How do the SDSS photometric and spectroscopic calibrations compare to standard longslit and fiber-based measurements, and what corrections are needed?
• RQ4 To what extent do population synthesis models based on integrated light in SDSS spectra accurately reflect the true ages and metallicities of stellar populations in galaxies?
• RQ5 How can follow-up observations at Calar Alto improve the characterization of SDSS-selected targets such as tidal streams and faint galaxies?

Key findings

• The SDSS has already produced spectacular scientific discoveries in its first year, including detailed mapping of galactic structure and tidal streams, despite being in its early operational phase.
• The survey covers approximately 10,000 square degrees in the northern sky and 225 square degrees in the south, with a total area of one-quarter of the celestial sphere.
• The SDSS imaging survey is expected to catalog around 8 × 10⁷ stars, 5 × 10⁷ galaxies, and 1 × 10⁶ quasars with high-quality photometry and astrometry.
• Spectroscopic follow-up at Calar Alto enables kinematic membership determination and velocity dispersion measurements in tidal streams, providing critical constraints for dynamical modeling.
• The SDSS calibration remains preliminary, with ongoing efforts to improve absolute photometric calibration and flux calibration of spectra, particularly for fiber-based measurements of point and extended sources.
• The development of a spectroscopic template database using stars and clusters with known physical parameters is underway to improve the accuracy of population synthesis models based on SDSS data.