Sébastien Heinis, Stéphane Arnouts, Tamás Budavári ...
sebastien@pha.jhu.edu
NOTE: These photometric redshifts mainly give a statistical information; they should be considered with caution when used on an individual object basis.
These photometric redshifts have been computed on all MIS GALEX sources with a SDSS counterpart with the following restriction:
Distance from the field center lower than 0.5deg (fov_radius <0.5)
In the following, figures use:
fields with exposition times in NUV or FUV greater than 1000 s.
non saturated objects in SDSS bands
dereddened magnitudes according to Schlegel et al maps using:
dered_fuv = fuv_mag - 8.24*e_bv
dered_fuv =
nuv_mag - (8.20*e_bv -0.62*e_bv*e_bv)
NUV or FUV magnitudes brighter than 22
objects non masked in SDSS (BLEEDING, BRIGHT_STAR, TRAIL an HOLE) or GALEX (flag >1, and nuv_mag<16.5) masks
unless otherwise stated.
There are 592 fields with SDSS overlap, and 589 with SDSS overlap with sources within 0.5 deg of the center. Here is the list of the 589 fields, giving for each field:
Since a fraction (588/687) of the IR1.1 MIS fields do not have FUV observations, in order to be homogeneous, the present photometric redshift estimation uses 6 bands: the NUV GALEX band and the u, g, r, i, and z SDSS bands.
We use here the combination of two different methods:
A template-fitting based method, using S. Arnouts code, called LePhare (i.e. TheLighthouse ... ;-) )
We used here the following templates:
- Galaxies: 42 SEDs from Bruzual & Charlot (2003) ('42 GISSEL')
- Stars: SEDs from Pickles (1998), 4 spectra of White Dwarves (Bohlin et al, 1995) and SEDs from Michel Laget (p. comm.)
- QSOs: Synthetic spectra from Cristiani & Vio (1990) and Cristiani et al (2004)
An empirical magnitude-redshift relation, based on the work of Connoly et al (1995), using an implementation from T. Budavari (called polyfit)
We refer hereafter to these methods by lephare and polyfit respectively. We present the different steps of the method in the following sections
1. Calibration of the redshift-magnitude relation
The first step consists to calibrate a redshift-apparent magnitude relation based on the SDSS spectroscopic counterparts. We use the 6 bands (NUV, u, g, r, i, z) and the spectroscopic redshift of SDSS galaxies to fit a 3rd degree polynom:
z = f(NUV, u, g, r, i, z)
The polynom coefficients are hereafter used to derive a photometric redshift. Note that this calibration rely on the SDSS spectroscopic sample (selected with r<17.5), and that the relation derived is, strictly speaking, only valid in the same volume. We explicitly use it in a larger volume in the following.
2. Template-fitting with no assumption
We first compute photometric redshifts using lephare with redshift as a free parameter. The code use galaxy, star, and qso templates.
3. Template fitting with polyfit redshift assumption
We then compute photometric redshift using lephare fixing redshift as the one derived from polyfit. The code only uses galaxy templates.
4. Template fitting with spectroscopic redshift assumption
We then compute photometric redshift using lephare fixing redshift as the spectroscopic one. The code only uses galaxy templates.
Comparison spectroscopic – photometric redshifts
Chi square classification
Color-color plots
Fraction prediction
Counts
Redshift distribution