2023MNRAS.526.1874T

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 $W_{\mathrm{H}\alpha +[\mathrm{N}\,\small {\rm II}] }$ 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.