The PAU survey: measurements of the 4000 Å spectral break with narrow-band photometry

The D4000 spectral break index is one of the most important features in the visible spectrum, as it is a proxy for stellar ages and is also used in galaxy classification. However, its direct measurement has always been reserved to spectroscopy. Here, we present a general method to directly measure the D4000 with narrow-band (NB) photometry; it has been validated using realistic simulations, and then evaluated with PAUS NBs, cross-matched with VIPERS spectra (iAB < 22.5, 0.562 < z < 0.967). We also reconstruct the D4000 with the SED-fitting code CIGALE; the use of PAUS NBs instead of broad-bands significantly improves the SED fitting results. For D4000n, the direct measurement has ⟨SNR⟩∼4⁠, but we find that for iAB < 21 all direct D4000 measurements have SNR>3⁠. The CIGALE D4000n has ⟨SNR⟩∼20⁠, but underestimates the error by >50 per cent. Furthermore, the direct method recreates well the D4000–SFR relation, as well as the D4000–mass relation for blue galaxies (for red galaxies, selection effects impact the results). On the other hand, CIGALE accurately classifies galaxies into red and blue populations. We conclude that the direct measurement of D4000 with narrow-band photometry is a promising tool to determine average properties of galaxy samples, with results compatible with spectroscopy.
The PAU Survey is partially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under grants CSD2007-00060, AYA2015-71825, ESP2017-89838, PGC2018-094773, PGC2018-102021, SEV-2016-0588, SEV-2016-0597, MDM-2015-0509 and Juan de la Cierva fellowship and Latin American Chinese European Galaxy Formation Network (LACEGAL) and Enabling Weak Lensing Cosmology (EWC) Marie Sklodowska-Curie grant No 734374 and No 776247 with European Research Development Fund (ERDF) funds from the EU Horizon 2020 Programme, some of which include ERDF funds from the European Union. IEEC and IFAE are partially funded by the Institució Centres de Recerca de Catalunya (CERCA) and Beatriu de Pinos program of the Generalitat de Catalunya. Funding for PAUS has also been provided by Durham University (via the European Research Council (ERC) StG DEGAS-259586), ETH Zurich, Leiden University (via ERC StG ADULT-279396 and Netherlands Organisation for Scientific Research (NWO) Vici grant 639.043.512), University College London and from the European Union’s Horizon 2020 research and innovation programme under the grant agreement No 776247 EWC. The PAU data center is hosted by the Port d’Informació Científica (PIC), maintained through a collaboration of CIEMAT and IFAE, with additional support from Universitat Autònoma de Barcelona and ERDF. We acknowledge the PIC services department team for their support and fruitful discussions. PR and ZC are supported by National Science Foundation of China (grant No. 12073014). The results published have been funded by the European Union’s Horizon 2020 research and innovation programme under the Maria Sklodowska-Curie (grant agreement No. 754510), the National Science Centre of Poland (grant UMO-2016/23/N/ST9/02963) and by the Spanish Ministry of Science and Innovation through Juan de la Cierva-formacion program (reference FJC2018-038792-I). AHW is supported by an ERC Consolidator Grant (No. 770935).