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 $M_{\rm dust}$\mathrm{/}$M_\ast$ $\sim$ 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~$\text{textsc}ii$ regions), the electron density (n${}_e$) is around 49.5 $\pm$ 11.9 cm${}^{-3}$ based on the [N~$\text{textsc}ii$],122,\mum/[N~$\text{textsc}ii$],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~$\text{textsc}ii$$]+[$O~$\text{textsc}i$],63,\mum)/$L_{\mathrm{T}\mathrm{I}\mathrm{R}}$, 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.