Photometric redshifts for galaxies in the Spitzer Extragalactic Representative Volume Survey (SERVS)

Abstract

We present photometric redshifts for ∼4 million galaxies detected in the Spitzer 3.6 or 4.5 μm bands of the Spitzer Extragalactic Representative Volume Survey (SERVS). The 18 deg 2 area of SERVS covers five extragalactic fields with a variety of ancillary, optical, and near-infrared (IR) photometry. We evaluate the accuracy of our photometric redshifts with ∼90.000 publicly available, spectroscopic redshifts. Overall, we find an average of ∼0.038 for the normalized median absolute deviation (σ NMAD), a measure similar to the standard deviation yet more robust against outliers, and outlier fraction η of 3.7 per cent for the sources with the widest wavelength coverage. On the example of the XMM field, we quantify the quality and reliability of photometric redshifts as a function of (1) the number of photometric bands available in the fitting, (2) i AB magnitude, (3) Infrared Array Camera (IRAC) 3.6 μm magnitude, (4) spectroscopic redshift, and (5) the survey origin of the spectroscopic redshift. The best results are achieved when the photometry available for the fitting covers rest-frame optical and near-IR wavelengths. σ NMAD and η are smallest for the brightest objects. Similarly, we find σ NMAD and η to be smaller on average at z < 1.5 than at z > 1.5. Photometric redshifts derived without the two IRAC filter bands, but detections in all other bands are slightly underestimated. Approximately 76 per cent of SERVS sources with at least five available filter bands lie between redshift 0 and ∼1.5. We find a tail of high-redshift galaxies, i.e. ∼7 per cent of all objects with at least five available filter bands for the fitting lie at z > 3. We discuss ways to improve upon the photometric redshifts for SERVS galaxies in the future.

The Role of Environment in Galaxy Evolution in the SERVS Survey. I. Density Maps and Cluster Candidates

“…5 There are many choices here. The fact that we see comparable σ N M AD and no significant systematic biases between our photometric redshifts and those derived for the same dataset but using different models by Pforr et al (2019) suggest our results are likely robust against photometric redshift code systematics. 6 We tested using a random subsample of galaxies in our sample to avoid using the biased spectroscopic subsample.…”

The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1

“…Pforr et al (2019) quote robust scatter and outlier statistics calculated using the same definitions as in this paper for their full spectroscopic test sample (and the subset with the best available photometry), finding σ NMAD = 0.042 (0.037) and OLF = 0.105 (0.028) in the EN1 field and σ NMAD = 0.067 (0.03) and OLF = 0.205 (0.048) in LH. As the photoz estimates in Pforr et al (2019) are primarily designed for galaxies (with no AGN templates included in the fitting), we can compare these values with the statistics for the galaxy population presented in Table 3. It is clear that the results presented in this work represent a substantial improvement in overall photo-z quality.…”

“…Additionally, Pforr et al (2019) present template based photo-z estimates for 18 deg 2 of multi-wavelength optical photometry in the Spitzer Extragalactic Representative Volume Survey (SERVS; Mauduit et al 2012) over five fields, including subsets of the EN1 and LH fields. Pforr et al (2019) quote robust scatter and outlier statistics calculated using the same definitions as in this paper for their full spectroscopic test sample (and the subset with the best available photometry), finding σ NMAD = 0.042 (0.037) and OLF = 0.105 (0.028) in the EN1 field and σ NMAD = 0.067 (0.03) and OLF = 0.205 (0.048) in LH. As the photoz estimates in Pforr et al (2019) are primarily designed for galaxies (with no AGN templates included in the fitting), we can compare these values with the statistics for the galaxy population presented in Table 3.…”

“…In particular, the availability of NIR and mid-IR photometry is crucial for providing reliable estimates for the AGN population at higher redshifts (a key area of scientific interest for the LoTSS Deep Fields). Furthermore, the additional colour information provided by forced photometry across all available photometric bands is vital, offering huge improvements in photo-z estimates or SED fitting when compared to the cross-matched catalogues employed by Pforr et al (2019, see also Nyland et al 2017.…”

A wide-area GMRT 610-MHz survey of ELAIS N1 field

“…We have combined the spectroscopic redshift compilations from the Spitzer Data Fusion 3 and HELP 4 projects to collect spectroscopic redshifts. Where a spectroscopic redshift was not available, we have used photometric redshifts from (in order of preference) the HSC (Tanaka et al 2018), SWIRE (Rowan-Robinson et al 2008, 2013, HELP (Duncan et al 2018a,b), SERVS (Pforr et al 2019) and SDSS (Beck et al 2016) projects. The redshift distribution is illustrated in Figure 8.…”