Emission-line ratios, SED, and SDSS spectrum of SDSS J015804.75-005221.8, the host galaxy of PS16dtm. Panels (a), (b), and (c) show various line ratio diagnostic diagrams involving the line ratios [O III]/Hβ, [N II]/Hα, [O I]/Hα, and [O III]/[O II] (Baldwin et al. 1981; Kewley et al. 2006). The red filled circle marks the position of PS16dtm’s host galaxy. The shaded regions are 2D histograms representing the locations of SDSS galaxies with ﹩0.04\lt z\lt 0.1﹩ measured using line fluxes from the MPA-JHU data release (http://wwwmpa.mpa-garching.mpg.de/SDSS/DR7/). Darker regions indicate a higher density of galaxies. The extreme star formation classification line (solid green; Kewley et al. 2001), the pure star formation line (dashed green; Kauffmann et al. 2003), and the Seyfert-LINER classification line (blue; Kewley et al. 2006) are shown. (d) The best-fit photometry and spectrum of the Prospector-α model (blue) are compared to the observed photometry (gray). The 16th and 84th percentiles of the model spectrum are shaded in light blue. The 16th, 50th, and 84th percentiles of the marginalized star formation history are in the inset panel. The residuals between the observations and the best-fit photometry are shown in the lower panel. (e) SDSS spectrum of the host galaxy with an inset zoomed in on Hα. The inset shows that there are significant residuals when fitting Hα with only a narrow component fixed at the instrumental resolution (dashed red for fit, dashed black for residuals), indicating the presence of an additional broad component, a signature of a Seyfert 1 AGN. The residuals resulting from including a broad (∼1680 km s⁻¹) component are also shown (solid green).