The Physical Characteristics of Interstellar Medium in NGC 3665 with
  Herschel Observations
  We present the analysis of the physical properties of the interstellar medium
(ISM) in the nearby early-type galaxy NGC 3665, based on the far-infrared (FIR)
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 the dust-to-stellar mass ratio of $M_{\rm
dust}$/$M_\ast$ $\sim$ 1.1 $\times$ 10$^{-4}$ that is nearly three times larger
than the mean value of local S0+S0a galaxies. For the ionized regions
(H~\textsc{ii} regions), the electron density (n$_e$) is around 49.5 $\pm$ 11.9
cm$^{-3}$ based on the [N~\textsc{ii}]\,122\,\mum/[N~\textsc{ii}]\,205\,\mum\
ratio. For the photodissociation regions, the heating efficiency is in the
range of 1.26 $\times$ 10$^{-3}$ and 1.37 $\times$ 10$^{-3}$ based on the
([C~\textsc{ii}$]+[$O~\textsc{i}]\,63\,\mum)/$L_{\rm TIR}$, which is slightly
lower than other local galaxies; the hydrogen nucleus density and the strength
of FUV radiation field are n $\sim$ 10$^{4}$ cm$^{-3}$ and G$_0$ $\sim$
10$^{-0.25}$, respectively. The above results are consistent with the presence
of weak AGN 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 amount of cool gas against star formation.
