SAGE-SMC Overview

The SAGE-SMC pro ject is a Cycle 4 legacy program on the Spitzer Space Telescope, entitled, SAGE-SMC: Surveying the Agents of Galaxy Evolution in the Tidally-Disrupted, Low-Metallicity Small Magellanic Cloud, with Karl Gordon (STScI) as the PI. The project overview and initial results are described in a paper by Gordon et al. (2010, in prep). The SMC was mapped at two different epochs dubbed Epochs 1 and 2 separated by 3 (IRAC) and 9 (MIPS) months, as this provides a 90-degree roll angle in the orientation of the detectors, which optimally removes the striping artifacts in MIPS and artifacts along columns and rows in the IRAC image data. In addition, these two epochs are useful constraints of source variability expected for evolved stars and some young stellar ob jects (YSOs). The IRAC and MIPS observations from the S3MC pathfinder survey of the inner 3 sq. deg. of the SMC (PI: Bolatto, referred to as Epoch 0) have been reduced using the same software.

The SAGE-SMC project page can be found here. Also provided with the data release is the SAGE-SMC data delivery documentation [PDF].

SAGE-SMC 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, a source must be detected in at least 60% of the observations at that wavelength, at least 32% of the observations in an adjacent band (the confirming band), and the S/N must be greater than [5, 5, 5, 7] for IRAC bands [3.6um], [4.5um], [5.8um] and [8.0um]. The 2MASS K_s band is counted as a detection. For a typical source, extracted from 2x12 sec frametime images, the minimum detection criterion amounts to being detected twice in one band (usually band 1 or 2) and once in an adjacent band (sometimes referred to as the 2+1 criterion). For the catalog, sources with neighbors within a 2" radius are excluded (culled). For the archive, sources within a 0.5" are excluded. For more details, see Section 3.3 of the SAGE-SMC Data Delivery Document [PDF].

SAGE-SMC IRAC Catalog & Archive Column Descriptions

Gator serves the following SAGE-SMC IRAC catalogs and archives:
IRAC Epoch 1 Archive (83 columns [83 delivered + 5 IRSA-created], 1,281,380 data rows)
IRAC Epoch 1 Catalog (83 columns [83 delivered + 5 IRSA-created], 1,229,683 data rows)
IRAC Combined Epoch Archive (83 columns [83 delivered + 5 IRSA-created], 2,731,354 data rows)
IRAC Combined Epoch Catalog (83 columns [83 delivered + 5 IRSA-created], 2,574,736 data rows)

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

Designation is
  SST = Spitzer Space Telescope
  I   = IRAC
  SAGE 
  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                                 
       7  muxbleed correction applied                  
      *8  hot, dead or otherwise unacceptable pixel      
       9  muxbleed > 3-sigma above the background       
      10  DAOPHOT tweak positive                        
      11  DAOPHOT tweak negative                        
     *13  confusion in in-band merge                    
     *14  confusion in cross-band merge                 
     *15  column pulldown correction                    
     *16  banding correction                             
      19  data predicted to saturate
     *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)        

      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)

      Three Source Quality Flag bits (23, 24, 25) provide the 2MASS photometric quality
      flag information, whose possible values are (from worst to best) X, U, F, E, D, C, B, 
      and A (see the 2MASS documentation).

     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
       1.2   3      4
       2     5      6
       12    7      8
       30    9     10
       100  11     12

      i.e. if 0.6 and 12 were present, but only 12 was used, then
      bits 1 and 7 will be set (fluxes present) and bit 8 will be set (12s
      used), and the mflag will be 1+64+128=193.

     for SAGE-SMC HDR mode (12/0.6 sec.), the relevant numbers work out to be

     3      short exp data used, long exp data absent
     67     short used, long present but unused
     192    long exp used, short absent
     193    long exp used, short present but unused
     195    both long and short present and used