2021A&A...645A..49G

Galaxies with nuclear bars are believed to efficiently drive gas inward, generating a nuclear starburst and possibly an active galactic nucleus. We confirm this scenario for the isolated, double-barred, luminous infrared galaxy ESO 320-G030 based on an analysis of Herschel and ALMA spectroscopic observations. Herschel/PACS and SPIRE observations of ESO 320-G030 show absorption or emission in 18 lines of H₂O, which we combine with the ALMA H₂O 4₂₃-3₃₀ 448GHz line (E_upper∼400K) and continuum images to study the physical properties of the nuclear region. Radiative transfer models indicate that three nuclear components are required to account for the multi-transition H₂O and continuum data. An envelope, with radius R∼130-150pc, dust temperature T_dust≃50K, and N_H2∼2x10²³cm^–2, surrounds a nuclear disk with R∼40pc that is optically thick in the far-infrared (τ_100µm∼1.5-3, N_H2∼2x10²⁴cm^–2). In addition, an extremely compact (R∼12pc), warm (≃100K), and buried (τ_100µm>5, N_H2≥5x10²⁴cm^–2) core component is required to account for the very high-lying H₂O absorption lines. The three nuclear components account for 70% of the galaxy luminosity (SFR∼16-18M_☉/yr). The nucleus is fed by a molecular inflow observed in CO 2-1 with ALMA, which is associated with the nuclear bar. With decreasing radius (r=450-225pc), the mass inflow rate increases up to M_inf∼20M/yr, which is similar to the nuclear star formation rate (SFR), indicating that the starburst is sustained by the inflow. At lower r, ∼100-150pc, the inflow is best probed by the far-infrared OH ground-state doublets, with an estimated M_inf∼30M/yr. The inferred short timescale of ∼20Myr for nuclear gas replenishment indicates quick secular evolution, and indicates that we are witnessing an intermediate stage (<100Myr) proto-pseudobulge fed by a massive inflow that is driven by a strong nuclear bar. We also apply the H₂O model to the Herschel far-infrared spectroscopic observations of H₂¹⁸O, OH, ¹⁸OH, OH⁺, H₂O⁺, H₃O⁺, NH, NH₂, NH₃, CH, CH⁺, ¹³CH⁺, HF, SH, and C₃, and we estimate their abundances.