The Physical Characteristics of Interstellar Medium in NGC 3665 with Herschel Observations*

Abstract

Abstract 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 * ∼ 1.1 × 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 × 10−3 to 1.37 × 10−3 based on the ([C ii]+[O i] 63 μm)/ , 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 G 0 ∼ 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.