SAGE Survey

Introduction

The Stromgren-Crawford (SC) intermediate-band photometric system was invoked in the 1960s, and was originally designed to study early-type stars. After almost three decades of development, this system has been proven to be extremely powerful in the study of atmospheric parameters of various types of stars. The parameters like effective temperature Teff, gravity log g and metallicity [Fe/H], yielded from the SC system is more efficient and accurate than those from widely-used broad band photometric system or low-resolution spectra, and even comparable to those from high-resolution spectra. This system is also useful for the determination of extinctions, stellar distances, radii, and stellar evolutionary phases. However, due to the limitation of the previous instruments and facilities, the existing stellar-parameters sky survey, such as the Geneva-Copenhagen survey (Nordström et al. 200, GCS) and Hauk & Mermilliod 1998 (HM) survey, the limiting magnitude is ~8.5 mag in V-band, mainly for the stars in the solar neighborhood 40pc.

For this reason, we propose a northern deep sky survey in the SAGE photometry system, for which the limiting magnitude is extended to ~15 mag in V-band (100 sigma), in the solar neighborhood 1 kpc.


Figure 1. The footprint of the SAGE survey is ~12,000 deg2 for the north sky Dec>-5 deg, RA <12 h and RA>18 h, b>10 deg and b<-10 deg.

The Photometric System

The SAGE survey adopts a unique photometric system, which consists of Stromgren-Crawford (SC) uSC, self-designed vSAGE, SDSS g/r/i,Hαn/Hαw and DDO51 bands. The telescopes used are Steward Bok 90-inch (2.3m) telescope, Nanshan 1-m wide-field telescope of XinJiang Astronomical Observatories of CAS and the Maidanak 1-m telescope of Uzbekistan. The survey has started since 2016 and it will last for ~3-5 years. The complete depth is >21 mag in u/v-band (5 sigma), and ~0.5 billion stars will be detected. The stellar parameters of the whole sample will be derived and a sample of extreme metal-poor stars ([Fe/H]<-5) will be discovered. The SAGE survey will not only provide accurate input catalogue for the LAMOST survey, but also provide the big data for the formation and evolution of the Milky Way, especially combined with the spatial and velocity information from LAMOST and the Gaia data.


Table 1. SAGE photometric system and filter response functions


Figure 2. The telescopes used for the observations of the SAGE Survey

The Observing Progress

The observations have been carried out since the end of 2014 and the percentage completed area of the sky coverage is as following:


Figure 3(a). The uSC/vSAGE-band coverage (updated on Nov. 24, 2017)

Figure 3(b). The g/r/i-band coverage (updated on Nov. 25, 2017)

The limiting magnitudes of SAGE survey in the signal-to-noise rarios of 100, 10 and 5 are shown in Table 2. It is noted that the limiting magnitudes in u/v bands could be deeper than 21 mag, which are the deepest in all the bands, as we mainly focus on the FGK stars.


Table 2. The limiting magnitudes of the SAGE Survey for different S/N

Science Goals

  1. Determine the foreground extinction of the all the stars, evolutionary stage, stellar atmosphere parameters, distances, etc with the 4 color indices.
  2. Search and identify the solar system nearby faint objects, including white dwarfs and brown dwarfs; determine the stellar mass function; solve the problems of the G-dwarfs, dark matter and local formation scales.
  3. The fine structure of the Milky Way, including dividing thin disk and thick disk, the nature and morphology, star streams, substructures and searching the dwarf galaxies which are falling into our Milky way.
  4. Searching for the extreme metal-poor stars (EMPs); stellar early-formation, nuclear fusion processes, stellar evolution.
  5. The metallicity of the Milky Way, including the radial or vertical distributions of the thin disk and thick disk; age metallicity relation (AMR); metallicity evolution.
  6. The Galactic open clusters and globular clusters, including the member star identification, foreground stars excluding, mass function, age, abundance, etc., the radial density distribution of stars with different mass; search the super massive stars, brown stars, even free planets.
  7. The nearby galaxies, including search and identify the giant broach stars in the dwarf galaxies and search the planetary nebular and then study the age metallicity relation (AMR), metallicity gradient and galaxy structures.
  8. Combining this photometry catalogue with the LAMOST spectroscopy survey and GAIA project data, the five-dimension of the Milky Way (R.A., Dec., distances, proper motion and radial velocities) can be determined.

The Survey Team

  1. PI: Prof. Gang Zhao
  2. National Astronomical Observatories, Chinese Academy of Sciences, China (NAOC)
  3. Xinjiang Astronomical Observatories, Chinese Academy of Sciences, China (XAO)
  4. Ulugh Beg Astronomical Institutes, Uzbekistan (UBAI)
  5. Steward Observatory, University of Arizona, USA

Useful links

  1. Observing Schedule of Steward Observatory: Sep, Oct, Nov, Dec 2017
  2. Observing Schedule of Nanshan 1-m telescope: Oct - Dec 2017
  3. Observation Script and Footprint


Please contact Zhou Fan (zfan@bao.ac.cn) if you have any questions about the project.