Photometric redshifts in the D3 and D4 fields

Dave Balam has prepared stacks combining the available data on the D3 and D4 fields. Photometry was obtained with SExtractor. SExtractor's "double image mode" makes it possible to get photometry with perfectly matched apertures in multiple images. Kron magnitudes (SExtractor's MAG_AUTO) were used. The catalogs from the different bands and different chips were combined into one master photometric catalog. The following zero-points were used (courtesy Dave Balam):
 u* = mag(dn) +25.280 -0.19 * (secZ-1)
 g' = mag(dn) +26.460 -0.12 * (secZ-1)
 r' = mag(dn) +25.978 -0.09 * (secZ-1)
 i' = mag(dn) +25.744 -0.04 * (secZ-1)
 z' = mag(dn) +24.800 -0.03 * (secZ-1)

Photometric redshifts were measured with gwynz which works with the template fitting method.

  • Galaxy spectra (described here) are redshifted and then multiplied by the MegaCam filter curves (described here) to produce spectral templates ranging in type from E/S0 to active starburst and in redshift from z=0 to z=1.5.
  • The galaxy photometry is converted into fluxes per unit Angstrom using the equation Where m is the magnitude, F is the flux and F0 is the flux zeropoint of the filter in question. This produces a coarse spectral energy distribution (SED). Following flux zeropoints were used (in units of W/m2/Angstrom):
  • The spectral templates were compared to the SED's using the following equation:

    Where Fi is the flux of the galaxy, in the i-th band, σi is the error on the flux Ti(t,z) is the template flux for type t and redshift z, and α  is a normalization factor given by:

    The best matching (smallest chi-squared) template gives the redshift and galaxy type.

Spectroscopic redshifts from the CFRS and DEEP were retrieved from the web. The assembled redshifts can be found here.

The figure at right shows the configuration. The red lines show the outlines of the MegaCam CCD's. The blue dots and green dots show the location of galaxies with spectroscopic redshifts from DEEP and CFRS respectively.

The DEEP+CFRS catalog was cross referenced with the photometric redshift catalog. For each DEEP or CFRS object, the closest and second closest CFHTLS was found. If the closest CFHTLS object was within 2 arcseconds it was deemed a good match unless there was a second object within 4 arcseconds (this second criteria was to avoid confusion in the case of two nearby galaxies).

The figure at right shows z_phot vs z_spec for the D3 field. The green line shows the z_phot=z_spec relation. A red cross highlights one of the galaxies.

Inset into the upper left corner is a plot of the spectral energy distribution (flux vs. wavelength) of this galaxy. In this inset, the red dots with errorbars show the griz-band data converted to an SED and the red line shows the best fitting template, determined by the photometric redshift chi-squared minimization process. The blue line shows the best fitting template, but this time constrained to be at the spectroscopic redshift.

(Clicking on one of the black dots will highlight it and produce a similiar figure but with the SED of the selected galaxy plotted in the inset.)

The match between z_phot and z_spec is not very good due to photometric errors and lack of spectral coverage as discussed here. this is borne out by a visual inspection of the template fits (which may be made through the clickable map above) shows that none of the template fits seem completely spurious.

No spectroscopic data is available in the D4 field is available, so no checks have been made. Since the photometric offsets seem to be different between the two fields (as discussed here) the photometric redshifts for the D4 should be regarded with suspicion.

The catalogs
The catalogs are in ASCII format; the first line contains the column headings:
 # x y RA Dec gMega err rMega err iMega err zMega err CCD z_phot type_phot a_phot 
  • x and y refer to the stacked images.
  • RA and Dec were computed using the CCD x,y coordinates and Dave Balam's astrometric solutions.
  • gMega, rMega, iMega and zMega are the Kron magnitudes (SExtractor's MAG_AUTO), with their uncertainties for the galaxy. The apertures are all determined in z-band. Only objects detected in all four bands are included (3 filter photometric redshifts are very unreliable, except for the special case of Lyman break galaxies).
  • CCD is the MegaCam chip number.
  • z_phot is photometric redshift.
  • type_phot gives the file name of the matching spectra template, as listed here
  • a_phot is the normalization factor required to match the template to the data.
The catalogs are password protected:

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