H-ATLAS: estimating redshifts of Herschel sources from sub-mm fluxes

Abstract

Upon its completion the Herschel ATLAS (H-ATLAS) will be the largest submillimetre survey to date, detecting close to half-a-million sources. It will only be possible to measure spectroscopic redshifts for a small fraction of these sources. However, if the rest-frame spectral energy distribution (SED) of a typical H-ATLAS source is known, this SED and the observed Herschel fluxes can be used to estimate the redshifts of the H-ATLAS sources without spectroscopic redshifts. In this paper, we use a subset of 40 H-ATLAS sources with previously measured redshifts in the range 0.5 < z < 4.2 to derive a suitable average template for high redshift H-ATLAS sources. We find that a template with two dust components (T c = 23.9 K, T h = 46.9 K and ratio of mass of cold dust to mass of warm dust of 30.1) provides a good fit to the rest-frame fluxes of the sources in our calibration sample. We use a jackknife technique to estimate the accuracy of the redshifts estimated with this template, finding a root mean square of ∆z/(1 + z) = 0.26. For sources for which there is prior information that they lie at z > 1 we estimate that the rms of ∆z/(1 + z) = 0.12. We have used this template to estimate the redshift distribution for the sources detected in the H-ATLAS equatorial fields, finding a bimodal distribution with a mean redshift of 1.2, 1.9 and 2.5 for 250, 350 and 500 µm selected sources respectively.

The Herschel Bright Sources (HerBS): sample definition and SCUBA-2 observations

“…All poststamps are centred at the 250 µm extraction position of the Herschel catalogue. The final frame is a fitted SED, with the best-fit template in orange, fixed β template in blue and Pearson's template in grey (Pearson et al 2013). Similar figures for the entire HerBS sample can be found in Appendix B.…”

“…The flux densities in the catalogues are not de-boosted, however the flux boosting is negligible compared to the flux uncertainty; around 1 per cent at 80 mJy, and diminishing for increasing flux density, as can be seen in Table 6 of Valiante et al (2016). We estimated the redshift of each source by fitting a source template to the 250, 350 and 500 µm flux densities (Pearson et al 2013). We selected the sources at an estimated redshift, z phot , greater than 2 and a 500 µm flux density, S 500µm , greater than 80 mJy.…”

“…We selected the sources at an estimated redshift, z phot , greater than 2 and a 500 µm flux density, S 500µm , greater than 80 mJy. The source template was a two-temperature modified blackbody from Pearson et al (2013) (see eq. 3 and Table 5 in our Section 4).…”

“…We did this template fitting for two templates: best-fit, where we varied all the parameters (T c , T h , α, and β), and fixed β where we varied all parameters except β, which we fixed to 2. We also tried keeping T c , T h , α and β fixed to the values found by Pearson et al (2013). In this case we found the set of A i that gave the minimum χ 2 fit.…”

“…found by Pearson et al (2013). We estimated the uncertainty on each parameter by incrementally changing this parameter until the minimised χ 2 changes by of one (one interesting parameter, Avni 1976).…”