SAGE Overview

The Spitzer Space Telescope Survey of the Large Magellanic Cloud Legacy Project Surveying the Agents of a Galaxy's Evolution (SAGE) traces the life cycle of observable matter that drives the evolution of a galaxy's appearance. When complete, SAGE is expected to reveal over 6 million sources including ~150,000 evolved stars, ~50,000 young stellar objects and the diffuse interstellar medium with column densities > 1.2×10²¹ cm ^-2. The data will provide fundamental insights into the physical processes of the interstellar medium, the formation of new stars and the injection of mass by evolved stars and their relationships on the galaxy-wide scale of the Large Magellanic Cloud.

The SAGE project page can be found here. The survey is described in Meixner et al. (2006). (AJ, 132, 2268). Also provided with the data release is the SAGE data delivery documentation [PDF / PS].

SAGE (v2) IRAC Catalog & Archive Criteria

In comparison to the catalog, the archive has more source fluxes (fewer nulled wavelengths) and some more sources but these additions have more uncertainty associated with them. For the catalog, the S/N must be greater than [6, 6, 6, 10] for IRAC bands [3.6um], [4.5um], [5.8um] and [8.0um] for the flux to appear in the corresponding wavelength column. Whereas for the archive, the S/N must be greater than [5, 5, 5, 5] for IRAC bands [3.6um], [4.5um], [5.8um] and [8.0um]. For the catalog, sources with neighbors within a 2" radius are excluded (culled). For the archive, sources within a 0.5" are excluded. The difference in criteria for the catalog and archive creation is more complex and described in Section 3.2 of the SAGE Data Delivery Document [PDF / PS].

SAGE (v2) IRAC Catalog & Archive Column Descriptions

Gator serves the SAGE Winter 2008 IRAC Archive Epoch1 (88 columns [81 delivered + 5 IRSA-created], 8,785,435 data rows), IRAC Catalog Epoch1 (88 columns [81 delivered + 5 IRSA-created], 8,522,654 data rows), and MIPS 24 um Catalog Epoch1 (34 columns [29 delivered + 5 IRSA-created], 76,238 data rows). SAGE IRAC column descriptions can be found below. SAGE MIPS 24 um catalog column descriptions can be found here.

         Archive=> SSTISAGE1A JHHMMSS.SS+-DDMMSS.S           
         Catalog=> SSTISAGE1C JHHMMSS.SS+-DDMMSS.S            

Designation is
  SST = Spitzer Space Telescope
  I   = IRAC
  SAGE= LMC Survey project
  1   = epoch 1; 2=epoch 2; nothing if epoch 1+2
  C   = highly reliable Catalog
  A   = more complete Archive
  J   = 2000.0 epoch
  HHMMSS.SS   Right ascension (hr, min, sec) of source
  sDDMMSS.S   Declination (deg, min, sec) of source

  0    no sources in the Archive within 3" of the source
  1    sources in the Archive between 2.5" and 3" of the source
  2    sources in the Archive between 2.0" and 2.5" of the source
  3    sources in the Archive between 1.5" and 2.0" of the source
  4    sources in the Archive between 1.0" and 1.5" of the source
  5    sources in the Archive between 0.5" and 1.0" of the source
  6    sources in the Archive within 0.5" of the source.

¹ Source Quality Flag (sqf)

  
       bt = bit in sqf

       0  OK - no problems encountered
      bt            Flag                                 
       1  poor pixels in dark current                   
       2  flat field applied using questionable value  
       3  latent image                                 
       4  sat star correction                          
       7  muxbleed correction applied                  
      *8  hot, dead or otherwise unacceptable pixel      
       9  muxbleed > 3-sigma above the background       
      10  allstar tweak positive                        
      11  allstar tweak negative                        
     *13  confusion in in-band merge                    
     *14  confusion in cross-band merge                 
     *15  column pulldown correction                    
     *16  banding correction                             
      17  stray light                                    
      19  nonlinear correction not applied               
     *20  saturated star wing region                     
      21  pre-lumping in in-band merge                  
      22  post-lumping in cross-band merge              
     *30  edge of frame (within 3 pixels of edge)        
      31  truth list	(for simulated data)    

      For 2MASS data, the following flags from the 2MASS PSC 
      are mapped into bits 3,4,20, and 9 in the source quality flag

         -------------                 ---
         "p" persistence         latent (bit 3)
         "c" photom confusion    corr sat star wing (bit 4)
         "d" diffraction spike   uncorrectable sat star (bit 20)
         "s" electronic stripe   muxbleed (bit 9)

     bit values:
     bt 1 => 1; 2 => 2; 3 => 4; 4 => 8; 5 => 16; 6 => 32; 7 => 64; 8 => 128  
     bt 9 => 256; 10 => 512; 11 => 1024; 12 => 2048; 13 => 4096; 14 => 8192; 15 => 16384; 
     bt 16 => 32768; 17 => 65536; 18 => 131072; 19 => 262144; 20 => 524288;
     bt 21 => 1048576;   22 => 2097152;
     bt 30 => 536870912; 31 => 1073741824

     *The flags to possibly cull on. (Note that flag 8,20, and 30 have 
      already been removed from Catalog or nulled.)
	          

² Flux calculation method Flag (mf)

       mf indicates by bit whether a given exposure time was present, and
       whether that exposure time was used in the final flux.

          present  used
       exp  bit    bit
       0.6   1      2
       2     3      4
       12    5      6
       30    7      8
       100   9     10

      i.e. if 0.6 and 12 were present, but only 12 was used, then
      bits 1 and 5 will be set (fluxes present) and bit 6 will be set (12s
      used), and the mflag will be 20+24+25=49

     for SAGE HDR mode, the relevant numbers work out to be

     3      short exp data used, long exp data absent
     19     short used, long present but unused
     48     long exp used, short absent
     49     long exp used, short present but unused
     51     both long and short present and used