NYU Value-Added Galaxy Catalog

object_sdss_imaging.fits

This file contains the basic information about the objects in the SDSS imaging catalog. Note that the object catalog is defined such that every object is some entry in the SDSS imaging catalog. Not all such entries will have entries in the other catalogs.

What SDSS objects make it into this file?

All entries in the catalog are sources in the SDSS imaging survey selected in one of the following ways:

• They are within 2 arcsec of a Main sample, QSO, or Luminous Red Galaxy target from the target version of the reductions which was fed to the tiling software. Such entries will pass the test (VAGC_SELECT AND 1).
• They are within 2 arcsec of an SDSS plate hole (whether or not it has actually been observed). Such entries will pass the test (VAGC_SELECT AND 2).
• they pass the Main sample criteria described by Strauss et al. (2002) AJ, 124, 1810 modified in the following ways:
  • the Petrosian magnitude limit is extended to r=18 (extinction-corrected)
  • the star-magnitude cut in PSF-Model magnitudes is extended down to 0.2 (more inclusive than Main)
  • the bright fiber magnitude limit is extended to g, r, and i magnitudes of 12
  • the small, bright cut is turned off (which allows some binary stars into the catalog)
  Such entries will pass the test (VAGC_SELECT AND 4). These all make this catalog a bit more inclusive than Main (one may want to reapply these cuts using the parameters in Strauss et al. for many purposes).

What do the columns mean?

Most of the columns of this file are well-documented on the photoop pages. Just note:

1. The fluxes are in units of nMgy (nano-maggies). These are 10⁹ times the ratio of the source to the AB standard source in each band. To relate these quantities to magnitudes, an object with total flux f in nMgy has magnitude
   m = [22.5 mag] − 2.5 log ₁₀ f   .
2. The units of sizes petror50, petror90, r_dev, r_exp, and the like are in pixels (an SDSS pixel is approximately 0.396'').
3. There are quality flags IMAGE_STATUS, CALIB_STATUS, and PHOTO_STATUS, which you might want to check before using data in some cases.

There are some extra columns not explained in the photoop pages:

1. VAGC_SELECT: bitmask which indicates why this target was chosen. Bits are:
   • 0 (TILED): chosen because it is near a tiled target
   • 1 (PLATEHOLE): chosen because it is near a hole in a drilled plate (which may our may not have been observed yet)
   • 2 (MAIN): chosen because it passes the Main-like criteria described above
2. CALIBOBJ_POSITION: this points to the position in the appropriate calibObj file (based on the run and camcol) with the full calibObj output in it (included FIRST and 2MASS PSC matches).

Where is the rest of the data?

The file object_sdss_imaging.fits has a selection of the photometry outputs. For the full photometry outputs, there are files named:

$VAGC_REDUX/sdss/parameters/calibObj-[run]-[camcol].fits

NYU Value-Added Galaxy Catalog