Photometric redshifts in the D3_24 field

Summary: Photometric redshifts have been generated for the galaxies in the D3_24 images described here.
Photometry: SExtractor was run on the D3_24_I band image to produce a catalog and a segmentation image. The SExtractor parameter file that was used can be found here to produce a catalog and a segmentation image. The x and y positions and the segmentation images were passed on to mgwynphot. This program does photometry through a circular aperture (1 arcsecond radius) which excludes pixels belonging to neighbouring galaxies as determined by the SExtractor segmenation image. The same aperture is used in all bands. Note that although this method produces fairly accurate colours -- the small aperture means less sky noise -- the magnitudes are not a good estimate of the total magnitude of the galaxy.
Photometric Redshifts: 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):
    Filterzeropoint
    gAB4.63e-12
    rAB2.83e-12
    iAB1.89e-12
    zAB1.40e-12
  • 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 two figures at right show z_phot vs z_spec for the D3_24field. The upper one show the results using all five ugriz filters (where available). The lower one (not quite as satisfactory) shows the results only using the griz filters. The heavy red line shows the z_phot=z_spec relation. The lighter lines show +/- 10% in (1+z). A green cross highlights one of the galaxies.

Inset into the lower right corner of each figure is a plot of the spectral energy distribution (flux vs. wavelength) of this galaxy. In this inset, the black dots with error bars show the ugriz-band data converted to an SED and the blue line shows the best fitting template, determined by the photometric redshift chi-squared minimization process. The black 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 insets.)

The match between z_phot and z_spec is quite good for the ugriz data and fairly good when only the griz bands are used. The match is significantly better than the pre-Elixir version of these graphs as shown on this webpage, as predicted by simulations.
The catalogs
There are two catalogs. The first (D3.24.zed) contains photometric redshifts. The second (D3.24.abs) also contains absolute magnitude information The catalogs are in ASCII format; the first line contains the column headings:
  • x and y refer to the stacked images.
  • RA, Dec are computed using the WCS of the stacked images and should be good to 0.3 arcseconds RMS.
  • uMega, gMega, rMega, iMega and zMega are the 1 arcsecond aperture magnitudes discussed above.
  • 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.
  • ABS_uMega, ABS_gMega, ..., ABS_Ic are the absolute magnitudes in the MegaCam ugriz and Johnson-Cousins UBVRI systems. These are set to zero when the photometric redshift is undefined.
The catalog is password protected:

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