Nuclear versus integrated spectroscopy of galaxies in the Herschel Reference Survey

Context: The determination of the relative frequency of active galactic nuclei (AGN) versus other spectral classes, for example, HII region-like (HII), transition objects (TRAN), passive (PAS), and retired (RET), in a complete set of galaxies in the local Universe is of primary importance to discriminate the source of ionization in the nuclear region of galaxies (e.g., supermassive black holes vs. young and old stars). Aims: Here we aim to provide a spectroscopic characterization of the nuclei of galaxies belonging to the Herschel Reference Survey (HRS), a volume and magnitude limited sample representative of the local Universe, which has become a benchmark for local and high-z studies, for semianalytical models and cosmological simulations. The comparison between the nuclear spectral classification and the one determined on the global galactic scale provides information about how galaxy properties change from the nuclear to the outer regions. Moreover, the extrapolation of the global star formation (SF) properties from the SDSS fiber spectroscopy compared to the one computed by Hα photometry can be useful for testing the method based on aperture correction for determining the global star formation rate for local galaxies. Methods: By collecting the existing nuclear spectroscopy available from the literature, complemented with new observations obtained using the Loiano 1.52 m telescope, we analyze the 322 nuclear spectra of HRS galaxies; their integrated spectroscopy is available from the literature as well. Results: Using two diagnostic diagrams (the BPT and the WHAN) we provide a nuclear and an integrated spectral classification for the HRS galaxies. The BPT and the WHAN methods for nuclei consistently give a frequency of 53-64% HII, around 21-27% AGNs (including TRAN), and 15-20% of PAS (including RET), whereas for integrated spectra they give 69-84% HII, 4-11% of AGNs and 12-20% PAS. Solely among late-type galaxies (LTGs) do the nuclear percentages become 67-77% HII, 22-27% AGNs (including TRAN), and only 1-7% of PAS. For the integrated spectra these frequencies become: 80-85% HII, 9-11% AGNs and 4-9% PAS. Conclusions: We find that the fraction of HII region-like spectra is strongly anticorrelated with the stellar mass. On the contrary the frequency of AGNs increases significantly with stellar mass, such that at M ∗ > 10 10.0 M☉ ∼ 66% of the LTGs are AGNs or TRAN. Moreover there is not a significant dependence of the frequency of AGNs as a function of environment: AGNs+TRAN above 10 9.0 M are consistent with ∼30% irrespective of their membership to the Virgo cluster, suggesting that the AGNs population is not sensitive to the environment. Finally, extrapolation of the global SF properties from the nuclear spectroscopy including aperture corrections leads to underestimates with respect to values derived from direct integrated Hα photometry.