The ident column contains the original CFHTLS ident if only one prior catalog was used. For W1 fields, as we use several prior catalogs to cover a GALEX field, we need to produce a unique ident. Hence, the CFHTLS ident are shifted with an offset, whose value can be found in the IDOFFSET FITS keyword.

IDENT = IDENT_CAT1 + (OFFSET + IDENT_CAT2) + (2*OFFSET + IDENT_CAT3) + ...

If you need to retrieve the original CFHTLS ident for a given source, you must subtract N * IDOFFSET, where N is the index of the origin CFHTLS catalog in the OPTCAT FITS keyword. IDOFFSET is computed as the power of 10 above the maximum ident in all the source catalogs. Typically IDOFFSET = 1000000.

The OPTCAT FITS keyword contain the complete list of prior catalogs that have been given as input to EMPhot (for XMMLSS/W1 this is the list of all the CFHTLS catalogs for that region). The PRIORLST FITS keyword contain the subset of catalogs that actually have priors in the current GALEX field. The CFHTLS_CAT column can be used to retrieve the name of the original CFHTLS catalog containing this source (it refers to a FITS catalog as we converted CFHTLS ascii catalogs to FITS).

FLUX_ and MAG_ columns use the same units as the GALEX columns: fluxes in counts per second, and magnitudes computed with the formula:

where ZPMAG = 20.08 (NUV) and 18.82 (FUV).

The SNR is computed as the ratio of the flux and the flux error:

The NUV_FLAG and FUV_FLAG columns consist in a combination of the following binary flags :

• 0/1 : inside a region of the GALEX mask files.
• 0/2 : inside a region of the CFHTLS mask files.
• 0/4 : negative stamps.

For the processing, the image is divided with a grid of tiles, whose dimension is given by the TSIZE_X and TSIZE_X FITS header keywords (128 pixels for the current release). The TILE_ columns give values for the tile in which the source lies.

• TILE_LIKHD gives the log likelihood :

• TILE_QUIETS_RATE gives the ratio of undetected (unactive) sources over total number of prior. A source is considered unactive if its flux is below the minimum flux (given by the MINFLUX FITS keyword, in number of photons, 10 in this release) at least N times (given by the QUIETNB FITS keyword, 3 in this release).

The *_MIX columns result from the merge of GALEX and EMPhot dirac columns:

• We first match EMPhot catalog positions (X,Y) with GALEX original xd-mcat catalog (Tolerance = 2 arcsec).

  NOTE: We only consider GALEX sources with a match in EM catalog (with optical counterpart).

• If NUV(GALEX) < 21.75 (same for both FUV, NUV filters), we use the GALEX values:

  NUV: MAG_MIX = NUV_MAG  , MAG_ERR_MIX = NUV_MAGERR
  FUV: MAG_MIX = FUV_NCAT , MAG_ERR_MIX = FUV_NCAT_ERR
  

  Otherwise MAG_MIX is the EMPhot magnitude from the dirac method.

• Making flags (FLAG_MIX, NFLAG_MIX, IDFLAG_MIX):

  We use the SEXtractor ellipses of bright GALEX sources (from GALEX pipeline) to investigate if crowding can affect their photometry :

  • FLAG_MIX:

    • FLAG_MIX=0 : EM phot. & outside ellipses of bright sources
    • FLAG_MIX=2 : EM phot. & inside ellipses of bright sources
    • FLAG_MIX=1 : GALEX phot. & with no neighbors inside ellipse
    • FLAG_MIX=3 : GALEX phot. & with neighbors inside ellipse

  • NFLAG_MIX:

    For each bright GALEX primary source with neighbors inside its ellipse, we measure the number of neighbors within 4 range of flux ratios (F_neighbor / F_galex_primary):

    N1 = Number of  neighbors with        Fnei/Fgal < 1/10
    N2 = Number of  neighbors with 1/10 < Fnei/Fgal < 1/2
    N3 =       "        "          1/2  < Fnei/Fgal < 2
    N4 =       "        "                 Fnei/Fgal > 2
    

    • we force N1,N2,N3,N4 < 10 so that NFLAG_MIX = N1 + N2*10 + N3*100 + N4*1000
    • Both the primary GALEX source and its neighbors share the same NFLAG_MIX.
    • All neighbors have the ID of the GALEX primary source saved in IDFLAG_MIX.

• A primary GALEX source with FLAG_MIX=3 (with some neighbors) and with NFLAG_MIX<10 should have a GALEX flux not significantly affected by crowding. If NFLAG_MIX>100 then at least one neighbor has a flux comparable to the primary GALEX source. Therefore pipeline GALEX flux can be uncertain.