Abstract
ABSTRACT We present – and make publicly available – accurate and precise photometric redshifts in the ACS footprint from the COSMOS field for objects with iAB ≤ 23. The redshifts are computed using a combination of narrow-band photometry from PAUS, a survey with 40 narrow bands spaced at $100\,\mathring{\rm A}$ intervals covering the range from 4500 to $8500\,\mathring{\rm A}$, and 26 broad, intermediate, and narrow bands covering the UV, visible and near-infrared spectrum from the COSMOS2015 catalogue. We introduce a new method that models the spectral energy distributions as a linear combination of continuum and emission-line templates and computes its Bayes evidence, integrating over the linear combinations. The correlation between the UV luminosity and the O ii line is measured using the 66 available bands with the zCOSMOS spectroscopic sample, and used as a prior which constrains the relative flux between continuum and emission-line templates. The flux ratios between the O ii line and Hα, Hβ and $\mathrm{O\,{\small III}}$ are similarly measured and used to generate the emission-line templates. Comparing to public spectroscopic surveys via the quantity Δz ≡ (zphoto − zspec)/(1 + zspec), we find the photometric redshifts to be more precise than previous estimates, with σ68(Δz) ≈ (0.003, 0.009) for galaxies at magnitude iAB ∼ 18 and iAB ∼ 23, respectively, which is three times and 1.66 times tighter than COSMOS2015. Additionally, we find the redshifts to be very accurate on average, yielding a median of the Δz distribution compatible with |median(Δz)| ≤ 0.001 at all redshifts and magnitudes considered. Both the added PAUS data and new methodology contribute significantly to the improved results. The catalogue produced with the technique presented here is expected to provide a robust redshift calibration for current and future lensing surveys, and allows one to probe galaxy formation physics in an unexplored luminosity-redshift regime, thanks to its combination of depth, completeness, and excellent redshift precision and accuracy.
Highlights
Redshift galaxy surveys can be broadly divided into two categories: spectroscopic surveys and imaging surveys
To assess the accuracy and precision of the photo-z point estimates with respect to the spectroscopic point estimates, we consider the distribution of the following quantity: z ≡/(1 + zspec)
We have applied the method to data from the COSMOS field and we have assessed its performance comparing to available public spectroscopic redshifts
Summary
Redshift galaxy surveys can be broadly divided into two categories: spectroscopic surveys and imaging surveys. Resolution spectra of the object within some wavelength coverage, which is used to identify sharp features like emission and absorption lines to nail the redshift of the object with very high precision These are expensive to obtain: they require knowing the position of the object beforehand and a large exposure time, which makes it observationally inefficient to observe faint objects over a large area. Imaging surveys are able to obtain measurements of every object in the field of view at the same time from a set of bandpass filtered images, which allows to cover large areas faster and to a greater depth This happens at the expense of getting flux measurements with very poor spectral resolution since the width of typical broad-band filters is larger than 100 nm, which makes the photometric redshift (i.e. photo-z) determination much less precise and sometimes inaccurate
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