Low redshift catalog

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NYU-VAGC on the sky (DR4 version).
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NYU-VAGC on the sky (DR4 version).

Contents

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Description

One of the areas of the SDSS which requires special care is in the treatment of galaxies at low redshift. In order to study the property of galaxies at low redshifts and, correspondingly, at low luminosities, we have done some simple checks of the SDSS catalog in this regime, cleaned up the catalog where it was simple to do so, and put together a "low-redshift" catalog of galaxies with estimated comoving distances in the range 10 < d < 150 Mpc/h.

Details of how we produced this catalog can be found in the NYU-VAGC paper. A description of the properties, luminosity function, and selection effects in our sample is given in the low luminosity paper.

This catalog is useful in three primary ways: first, to study the lowest luminosity galaxies in the sample; second, as a sample of relatively high resolution galaxies for quantitative morphological measurements; third, as a comparison sample for high redshifts.

In the DR4 area, there are 49,968 galaxies in the low-redshift catalog. Weighted by the completeness, the effective area of the sample is 4044.6 square degrees.

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Catalog format

The DR4 catalog is stored in the file:

It contains the columns:

  1. OBJECT_POSITION: index of the object in the full NYU-VAGC
  2. ICOMB: index of the polygon containing this object in the lss_combmask file
  3. SECTOR: "sector" containing this object (region covered by a unique set of plates)
  4. VAGC_SELECT: how this object was selected to be in the NYU-VAGC (see the object_sdss_imaging documentation for explanation)
  5. FGOTMAIN: fraction of Main targets for which a classification was obtained in this object's sector
  6. RA: right ascension, J2000, in degrees
  7. DEC: declination, J2000, in degrees
  8. RUN: SDSS drift scan number
  9. RERUN: SDSS processing rerun
  10. CAMCOL: SDSS camera column
  11. FIELD: SDSS field
  12. ID: SDSS object id within the field
  13. OBJC_ROWC: CCD y position in the field (centers of pixels are half-integers)
  14. OBJC_COLC: CCD x position in the field (centers of pixels are half-integers)
  15. PLATE: SDSS spectroscopic plate
  16. FIBERID: SDSS spectroscopic fiber number
  17. MJD: date of SDSS spectroscopic observation
  18. QUALITY: eyeball quality flag (see page on quality checks)
  19. ABSMAG[8]: absolute magnitude (AB) in the ugrizJHK_s bandpasses (first five from SDSS Petrosian magnitude, last three from 2MASS XSC) K-corrected and Galactic extinction corrected using the SFD dust maps.
  20. ABSMAG_IVAR[8]: inverse variance of uncertainty in absolute magnitude (including the effects of errors in the distance)
  21. MU50R: Petrosian half-light surface brightness (magnitudes in a square arcsecond)
  22. KCORRECT[8]: estimated $K$-corrections in $ugrizJHK_s$ (magnitudes)
  23. PETROFLUX[5]: SDSS Petrosian flux in $ugriz$ (nanomaggies)
  24. PETROTHETA[5]: SDSS Petrosian radius in $ugriz$ (arcsec)
  25. PETROTH50[5]: SDSS Petrosian 50\% light radius in $ugriz$ (arcsec)
  26. PETROTH90[5]: SDSS Petrosian 90\% light radius in $ugriz$ (arcsec)
  27. EXTINCTION[5]: Galactic extinction from \citet{schlegel98a: in $ugriz$ (magnitudes)
  28. SERSIC_N[5]: \Sersic\ index estimated from radial profile in $ugriz$
  29. SERSIC_TH50[5]: 50\% light radius from \Sersic\ fit in $ugriz$ (arcsec)
  30. SERSIC_FLUX[5]: total flux from \Sersic\ fit in $ugriz$ (nanomaggies)
  31. VDISP: estimated velocity dispersion from spectrum
  32. VDISP_ERR: estimated uncertainty in velocity dispersion from spectrum
  33. CLASS: spectroscopic classification, as output by the SDSS spectroscopic pipeline (note that occasionally this is incorrect in particular, a number of galaxies in our sample are classified as stars spectroscopically because the signal-to-noise ratio of the spectrum does not allow reliable discrimation between the two).
  34. SUBCLASS: spectroscopic subclassification (e.g., stellar type), as output by the SDSS spectroscopic pipeline
  35. VMAX: maximum volume in the sample over which we could have observed this object
  36. NEDNAME: name of NED match
  37. NEDCZ: redshift from NED match
  38. ZLG: Local Group relative redshift from SDSS
  39. ZDIST: peculiar velocity corrected Local Group relative redshift from SDSS
  40. ZDIST_ERR: uncertainty in ZDIST
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Atlas images, cutouts, and PSFs

In addition, we provide atlas images and cutout images for every object. These are images with nearby objects removed and replaced with noise. These images are available in the directories 

$VAGC_REDUX/lowz/images/00h
$VAGC_REDUX/lowz/images/01h
...
$VAGC_REDUX/lowz/images/23h

where each directory contains galaxies in a particular hour of right ascension (J2000).

The atlas images contain only the flux used for the photometry performed by the SDSS; that is, they are the deblended images. The names of each atlas image are based on the IAU name of each object; eg. 

lowz-atlas-J044112.00+003202.3.fits

Each file contains ten HDUs. HDUs 0, 2, 4, 6, and 8 contain the ugriz images of each galaxy. HDUs 1, 3, 5, 7, and 9 contain the inverse variance ugriz images of each galaxy.

The cutout images are identical to the atlas images but contain all of the flux detected around each object. Thus, you can use these images if you want to segment/deblend the objects yourself. The names are similar to those of the atlas images; e.g.: 

lowz-full-J044112.00+003202.3.fits

The structure of these files is the same as the atlas files.

All of the above files have correct astrometric headers.

The PSF images are in files of the form: 

psf-J044112.00+003202.3.fits

which contains five HDUs, the estimated PSFs at the center of the object in ugriz.

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Luminosity functions

The luminosity functions calculated using the step-wise maximum likelihood (SWML) method of Efstathiou et al 1988 in the low luminosity paper are included in the same directory. They are in FITS format, with columns:

  1. ABSM: center of absolute magnitude bin (for h=1)
  2. PHI: number density per cubic Mpc (for h=1) and per unit absolute magnitude
  3. LG10PHI_ERR: estimated uncertainty in the base 10 logarithm of PHI.

There are several versions, as described in the paper. First, there is the version with no surface brightness corrections applied. They should be considered "minimal" estimates of the luminosity functions. That is, at low luminosity there are likely to be many more objects undetected by our survey. These uncorrected luminosity functions (in the paper "version 1") are:

In addition, we have the luminosity functions for galaxies with half-light surface brightnesses in the r-band brighter than 24, corrected for surface brightness incompleteness, "version 2" in the paper:

In addition, we have the luminosity functions calculated separately for the classes defined in the low luminosity paper. These are not corrected for surface brightness incompleteness.

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See also

Retrieved from "http://howdy.physics.nyu.edu/index.php/Low_redshift_catalog"