Estimating stellar population and emission-line properties in S-PLUS galaxies

Abstract

We present tests of a new method to simultaneously estimate stellar population and emission-line (EL) properties of galaxies out of the Southern Photometric Local Universe Survey (S-PLUS) photometry. The technique uses the ALSTAR code, updated with an empirical prior that greatly improves its ability to estimate ELs using only the survey´s 12 bands. The tests compare the output of (noise-perturbed) synthetic photometry of the Sloan Digital Sky Survey (SDSS) galaxies to properties derived from previous full spectral fitting and detailed EL analysis. For realistic signal-to-noise ratios, stellar population properties are recovered to better than 0.2 dex in masses, mean ages, metallicities, and ±0.2 mag for the extinction. More importantly, ELs are recovered remarkably well for a photometric survey. We obtain input - output dispersions of 0.05-0.2 dex for the equivalent widths of [O II], [O III], Hβ, Hα, [N II], and [S II], and even better for lines stronger than ~5 Å. These excellent results are achieved by combining two empirical facts into a prior that restricts the EL space available for the fits. (1) Because, for the redshifts explored here, Hα and [N II] fall in a single narrow band (J0660), their combined equivalent width is always well recovered, even when [N II]/Hα is not. (2) We know from SDSS that WHα+[NII]WHα+[NII] correlates with [N II]/Hα, which can be used to tell if a galaxy belongs to the left or right wings in the classical Baldwin-Phillips-Telervich (BPT) diagnostic diagram. Example applications to integrated light and spatially resolved data are also presented, including a comparison with independent results obtained with the Multi-Unit Spectroscopic Explorer (MUSE)-based integral field spectroscopy.