NASA-Sloan Atlas

The NASA-Sloan Atlas is a catalog of images and parameters of local galaxies derived from Sloan Digital Sky Survey imaging, and with the addition of Galaxy Evolution Explorer (GALEX) data for the ultraviolet part of the spectral energy distribution. The primary documentation can be found at the NASA-Sloan Atlas website.

The version of the NASA-Sloan Atlas used by MaNGA for its targeting is v1_0_1. This version contains some new features relative to the previous publicly released version (v0_1_2). These include an expanded redshift range (to z = 0.15),
and the calculation of elliptical Petrosian aperture photometry.

Summary File

The summary catalog file for v1_0_1: nsa_v1_0_1.fits (2.5 Gbytes; datamodel) is included in our data release. A table is available in CAS corresponding to this data (nsa).

The SDSS images are based on the DR13 recalibration using PanSTARRS-1. The object detection, deblending, and other details regarding the image analysis can be found in Blanton et al. (2011). Broadly speaking, this process is better tuned for large, bright galaxies than the standard SDSS processing. Nevertheless, it is still susceptible to some similar failure modes (over or underdeblending).

The elliptical Petrosian aperture photometry is a new addition since that paper, and is considered the most reliable photometry available in this catalog. The definition of the Petrosian radius used for the apertures is based on the SDSS r-band and is identical to the usual SDSS definition except using elliptical instead of circular apertures. The position angle and axis ratio of the ellipses used are determined from the second moments of the light distribution at around the 90% light radius (as determined using the circular Petrosian fluxes). Based on simulations, these magnitudes avoid the dependence on ellipticity of the circular Petrosian magnitudes and avoid the biases associated with the single-component Sérsic fits released in v0_1_2. They do have the expected dependence of Petrosian magnitudes on profile shape, leading to an underestimate of the total flux for pure de Vaucouleurs galaxies.

We have also applied aperture corrections to the photometry, to account for the variation in point spread function between the bandpasses, particularly for GALEX. We do so by using the measured curve-of-growth to predict the aperture correction for an ideal elliptical galaxy, and applying this correction to the real data. For GALEX, these corrections can be of order 30% to 50% for galaxies with half-light radii around an arcsec; for SDSS they are always negligible.

All absolute magnitudes in the NASA-Sloan Atlas are given with H0 = 100 h km s-1 Mpc-1, so should be interpreted as M – 5 log10 h.

Detection and measurement images

Structure of directories

The full suite of results can be found in the atlas directory on the SAS. There are a number of input files and intermediate files here.

The most useful files are inside the detect subdirectory. Within this directory tree, for each galaxy in the summary file, there is a subdirectory with all of the images used to produce the parameter measurements. In these subdirectories are both calibrated mosaic images on which the analysis is done, and individual deblended atlas images just with the light from the analyzed galaxy, and a number of associated other files.

The structure of the detect directory tree is as follows:

  • detect
    • galex (original GALEX images)
    • sdss (original SDSS images)
    • 2mass (original 2MASS images, not used in analysis)
    • v1_0
      • [RA]h
        • [mp][Dec]
          • [IAUNAME]
            • atlases
              • [PID]

This tree is constructed to break up the sky into hour-wide RA ranges (“00h”, “01h”, …, “23h”) and 2 degree-wide Dec ranges (where, e.g., “m04” is between -4 and -6 deg Dec, and “p12” is between +12 and +14 deg Dec, etc.). Within each RA/Dec bin, each directory is named by an object’s IAUNAME, which is the object IAU-style name (e.g. “J123456.78+012345.6”). IAUNAME and also SUBDIR (the directory path to the object) are given for each object in the summary file. PID is the parent ID associated with the object, and is also specified for each object in the summary file. Note that although many disjoint parent objects may be detected by the NSA, only the parent object identified to contain the galaxy of interest is analyzed.

Mosaic Images

The IAUNAME directory contains several types of file related to the mosaic images analyzed for the NSA. These are the full, calibrated images which the NSA analysis is applied to. These are files that users would need to deblend and analyze the same images with different methods.

Atlas Images and Measurements

The PID directory contains files related to the particular parent identified with the galaxy of interest. They also contain the deblended child images containing just the light from the object of interest.

These are files that users would need to analyze the parent images with different methods, or to dig deeply into how the NSA decided to deblend and analyze a particular galaxy. Some of the file names include AID, which indicates which deblended child the file refers to; only one AID is analyzed, which is specified in the summary file.

For the undeblended parent object, there exists:

For the deblended child there are the following files:

Summary of other subdirectories

There are several subdirectories other than “detect” under the top-level NSA directory. These directories have intermediate files and metadata that is not generally useful. The files are fully documented in the data model. The directories are:

  • catalogs: Input catalogs for the NSA process, and the results of combining them into a single general input.
  • derived: Derived quantities (K-corrections, distances, final redshifts, etc). The majority of useful information is propagated to the summary file.
  • measure: Rolled-up results of the image measurement processes. The majority of useful information is propagated to the summary file.
  • misc: Miscellaneous files used (currently only the velocity field grid used for distances).