2018ApJ...854..111X

We present the analysis of the physical properties of the interstellar medium in the nearby early-type galaxy NGC 3665, based on the far-infrared photometric and spectroscopic data as observed by the Herschel Space Observatory. The fit to the spectral energy distribution reveals a high dust content in the galaxy, with a dust-to-stellar mass ratio of M_dust/M_* ∼ 1.1 x 10^–4 that is nearly three times larger than the mean value of local S0+S0a galaxies. For the ionized regions (H II regions), the electron density (n_e) is around 49.5 ± 11.9 cm^–3 based on the [N II] 122 µm/[N II] 205 µm ratio. For the photodissociation regions, the heating efficiency ranges from 1.26 x 10^–3 to 1.37 x 10^–3 based on the ([C II]+[O I] 63 µm)/L_TIR, which is slightly lower than other local galaxies; the hydrogen nucleus density and the strength of the far-UV radiation field are n ∼ 10⁴ cm^–3 and G₀ ∼ 10^–0.25, respectively. The above results are consistent with the presence of weak active galactic nuclei and a low level of star-forming activity in NGC 3665. Our results give strong support to the "morphological quenching" scenario, where a compact, massive bulge can stabilize the amount of cool gas against star formation.