The central spectra of massive star-forming galaxies

We have examined the nuclear spectra of very massive star-forming galaxies at $z\sim 0$ to understand how they differ from other galaxies with comparable masses, which are typically passive. We selected a sample of 126 nearby massive star-forming galaxies ($<100\mathrm{M}\mathrm{p}\mathrm{c}$, ${10}^{11.3}{\mathrm{M}}_{\odot }\le M_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{a}\mathrm{r}}\le {10}^{11.7}{\mathrm{M}}_{\odot }$, $1{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}<\mathrm{S}\mathrm{F}\mathrm{R}<13{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$) from the 2MRS-Bright WXSC catalogue. LEDA morphologies indicate at least 63% of our galaxies are spirals, while visual inspection of Dark Energy Survey images reveals 75% of our galaxies to be spirals with the remainder being lenticular. Of our sample 59 have archival nuclear spectra, which we have modelled and subsequently measured emission lines ([NII]$\lambda 6583$, H$\alpha \lambda 6563$, [OIII]$\lambda 5008$, and H$\beta \lambda 4863$), classifying galaxies as star-forming, LINERS, or AGNs. Using a BPT diagram we find $83\pm 6$ % of our galaxies, with sufficient signal-to-noise to measure all 4 emission lines, to be LINERs. Using the [NII]$\lambda 6583$/H$\alpha \lambda 6563$ emission line ratio alone we find that $79\pm 6$ % of the galaxies (46 galaxies) with archival spectra are LINERs, whereas just $\sim 30\mathrm{\%}$ of the overall massive galaxy population are LINERs (Belfiore et al. 2016). Our sample can be considered a local analogue of the Ogle et al. (2016, 2019) sample of $z\sim 0.22$ massive star-forming galaxies in terms of selection criteria, and we find 64% of their galaxies are LINERs using SDSS spectra. The high frequency of LINER emission in these massive star-forming galaxies indicates that LINER emission in massive galaxies may be linked to the presence of gas that fuels star formation.